Acerosodontosaurus – a Key Taxon at the Base of the Diapsida

The “Big Belly” Basal Diapsid
Acerosodontosaurus (Currie 1980, Bickelmann, Müller and Reisz 2009) Late Permian, was the fat kid on the block (=clade). Even so, this largely ignored and otherwise unspectacular taxon is key to our understanding of all higher diapsids, both marine and terrestrial. Hard to believe a sister to Acerosodontosaurus was ancestral to both hummingbirds and elasmosaurs, for instance. That’s the beauty of reptile evolution.

Acerosodontosaurus

Figure 1. Acerosodontosaurus in lateral view with a closeup of the hand and a cross section of the rib cage. This was the most robust member of the basal Diapsida. It had the largest dorsal vertebrae and deepest rib cage among its sisters. Such characters typically indicate an herbivorous diet, distinct from all sister taxa.

Acerosodontosaurus piveteaui was originally considered a younginiform. A later analysis by Bickelmann et al. (2009) nested Acerosodontosaurus with Hovasaurus and the same sisters recovered in the large reptile family tree seen here (Fig. 2). Always good to report when phylogenetic testing supports the literature.

The Questionable Quadratojugal/Rib Fragment
Bickelmann, Müller and Reisz (2009) determined that a bone considered to be quadratojugal (Fig. 1 in red) was instead a portion of a rib. Because of that new identification the authors considered Acerosodontosaurus a phylogenetic enigma in which the phylogenetic position was poorly understood, so they retested it. Bickelmann, Müller and Reisz (2009) then nested Acerosodontosaurus with Hovasaurus.

Ironically that questionable “rib fragment” fit perfectly into a reconstruction as a quadratojugal – BUT – the quadratojugal was the first bone to disappear in sister taxa – AND – the quadratojugal was considered missing in both Hovasaurus and Claudiosaurus, (which is disputed here). In any case and either way, one error in coding doesn’t change things much in a study of this size. Phylogenetically it just doesn’t matter. It just shifts the disappearance of the quadratojugal up or down the line.

Supporting the hypothesis that the quadratojugal was still present in Acerosodontosaurus and Claudiosaurus is the fact that successor taxa, including Stereosternum, Mesosaurus and ichthyosauriformes like Hupesuchus and Utatsusaurus all have a quadratojugal. Moreover, there are no other genuine rib fragments anywhere near the skull.

Basal diapsids.

Figure 2. Basal diapsids. Basal terrestrial forms in white. Younginiforms in green. Enaliosauria in blue. Mesosauria + Thalattosauria + Ichthyosauriformes in deeper blue.

Phylogenetic Nesting
Here Acerosodontosaurus nested at the base of two major clades, the marine Enaliosauria (= (Mesosauria + (Ichthyosauriformes + Thalattosauriformes)) + Sauropterygia) and the terrestrial Younginiformes, here represented by Thadeosaurus. So it’s a key taxon that should be used as an outgroup in focused studies of both clades.

Diet
In Acerosodontosaurus the large size of the dorsal vertebrae, coupled with the deep and wide dorsal ribs, coupled with the short robust limbs and lack of a canine tooth all point toward an herbivorous diet. No succeeding taxa had a canine tooth, but no succeeding taxon had the hallmark “big belly” of this possible herbivore. Alternatively, Acerosodontosaurus may have fed on small invertebrates and other slow-moving prey.

Niche
Acerosodontosaurus was considered aquatic in niche, like its sister taxa, the deep-tailed Hovasaurus and the less deep-tailed Thadeosaurus. The descendants of the former became increasingly marine. Descendants of the latter became increasingly terrestrial.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Currie PJ 1980. A new younginid (Reptilia: Eosuchia) from the Upper Permian of Madagascar. Canadian Journal of Earth Sciences 17(4):500-51.
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences 46:651-661.

wiki/Acerosodontosaurus

Reinterpreting the Skull of Effigia Using DGS

Updated March 12, 2015 with a revised mandible for Shuvosaurus. 

The Extreme Strangeness of Effigia
Effigia okeeffeae (Nesbitt and Norell, 2006) Carnian, Late Triassic, ~210 mya, ~ 2 m in length, was originally considered an early theropod dinosaur by Colbert, who collected the specimen in the late 1940s but never removed it from its jacket.

“Extreme Convergence”
A recent reassessment by Nesbitt and Norell (2006) and Nesbitt (2007) nested Effigia among the poposaurid rauisuchians based largely on the ankle, but they noted “extreme convergence in the body plans” with ornithomimid dinosaurs. They reported that the ankle of Effigia articulated in a crocodile-normal configuration, with a morphology similar to Alligator (Figure 1). The broken and missing calcaneal “heel”would have turned proximally, like that in a sister taxon, Shuvosaurus.

The pedes of Alligator and Effigia

Figure 1. The pedes of Alligator (left) and Effigia (right) demonstrating the convergence of the structure of the ankle bones (astragalus and calcaneum). In basal archosaurs the ankle is a simple hinge, but in Alligator the hinge takes a sharp turn between the astragalus and calcaneum. The astragalus and calcaneum of Effigia articulate in a crocodile-normal configuration and their morphology is similar to Alligator. Note the peg of the astragalus inserts into and rotates on the calcaneum. There are also axes of rotation at the distal tibia/fibula and at the (evidently missing) distal tarsals. 

The Calcaneal Tuber and its Distribution
Most paleontologists assert that the calcaneal “heel” is found only in rauisuchians + crocodylians (= pseudosuchians) not dinosaurs and their kin. Without the present expanded inclusion list, prior workers were not aware of the new clade, the Paraornithischia, that nested Effigia as a sister taxon to the phytodinosauria based on more parsimoniously shared traits from head to toe. The “extreme convergence” with restricted to the ankle.

Figure 2. Comparing the DGS method of skull reconstruction to traditional methods. Above: The skull of Shuvosaurus, a sister taxon. Next: Effigia traced from a photo published in Norell and Nesbitt 2006. Note the surangular extends over the mandibular fenestra, as in other dinosaurs and a predentary is present as in other poposaurids. Next: Original tracing of Effigia skull. Bottom: Original restoration of Effigia skull from Norell and Nesbitt (2004). Note the anterior extent of the surangular.

Figure 2. Comparing the DGS method of skull reconstruction to traditional methods. Above: The skull of Shuvosaurus, a sister taxon. Next: Effigia traced from a photo published in Norell and Nesbitt 2006. Note the surangular extends over the mandibular fenestra, as in other dinosaurs and a predentary is present as in other poposaurids. Next: Original tracing of Effigia skull. Bottom: Original restoration of Effigia skull from Norell and Nesbitt (2004). Note the anterior extent of the surangular.

DGS
I have never seen the skull of Effigia, only published photos. Even so, it appears that the original reconstruction by Nesbitt (2007) contains certain errors and oversimplifications that I repaired and reidentified. The DGS (Digital Graphic Segregation) method using Adobe Photoshop enabled a test of the original reconstruction and not all the original results could be verified.

Surangular/Dentar/Premaxilla
Chief among the problems in the Nesbitt and Norell (2006) reconstruction is the identification of the long bone over the mandibular fenestra as the surangular. This arose from the identification of the mandibular tip as the maxilla. Perhaps the authors did not realize that in sister taxa the anterior bone is a predentary or a pseudopredentary (if not homologous with the predentary in ornithischians). The temporal muscles for closing the jaw were also attached to the dentary in all tetrapods, which cannot happen when the dentary is restricted to the jaw tip. Here the toothless dentary is in its standard place and a predentary precedes it at the jaw tip.

A Descending Cranium
In Effigia the posterior skull descends, but that was “fixed” in the reconstruction of Nesbitt and Norell (2006). Here (Fig. 2) what you see is what you get.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf

AMNH Effigia webpage
wiki/Effigia

The Origin of Archaeopteryx – Illustrated

Added March 15, 2014: Be sure to also visit Aurornis here.

An Old Debate, Now Clearly Settled
The origin of birds has been long debated, but the debate has been over for awhile. Birds arose from theropod dinosaurs and recent finds from China have presented us with a wide variety of feathered theropods — all part of the big evolutionary bush that ultimately begat the modern birds that fill our skies, waddle across Antarctica and sprint across the plains of the southern continents.

There have been several phylogenetic analyses of bird origins and diversification. Today’s blog simplifies and focuses the process by eliminating most of the side branches, like the tyrannosaurids, the alvarezsaurids, etc.

The Old View
In the old days paleontologists hauled out Compsognathus, Archaeopteryx and Gallus (the chicken). These were all essentially correct and told the right story, but now we know more of the details.

Older view of bird evolution

Figure 1. Old yet essentially correct view of bird evolution featuring Compsognathus, Archaeopteryx and Gallus, the chicken. 

The Current View
The current view of bird evolution (below) gets a little more detailed.

Theropods to birds step by step.

Figure 2. Theropods to birds step by step. Purple taxa should not have been included. They are not related to dinosaurs. Yellow highlights refer to taxa shown and discussed below. 

The Current View and The List of 23 Increasingly Bird-Like Characters
The current view of bird relations (above) nests many more taxa along the lineage of pre-bird theropods. Each one adds at least one more bird-like character to the list. The only serious problems are pterosaurs and Lagerpeton (in purple) are not related to dinosaurs.

1. Mesotarsal ankle – the Ornithodira (includes pterosaurs)
2. Functional tridactyl footLagerpeton 
3. Fully perforated acetabulumHerrerasaurus
4. Loss of digit 5 on handTawa
5. Postaxial vertebral pneumaticity – Tawa
6. Pedal digit 1 loses contact with ankleCoelophysis
7. FurculaCoelophysis
8. Maxillary fenestraSpinosaurus
9. Strap-like scapula – Allosaurus
10. Dorsal astragalus tall/broad – Allosaurus
11. Expanded pneumatic ectopterygoid -Tyrannosaurus
12. Three tympanic systems in ear region – Tyrannosaurus
13. Promaxillary fenestra – Tyrannosaurus
14. Fused semilunate carpal – Alvarezsauria
15. Enlarged sternum – Alvarezsauria
16. Ossified sternal ribs – Therizinosauria* + Oviraptosauria* + Dromaeosauridae + Troodontidae
17. Shortened tail – Dromaeosauridae + Troodontidae + Archaeopteryx
18. Subdivided ulna – (not sure what this means, unless these are quill locations)  Dromaeosauridae + Troodontidae + Archaeopteryx
19. Retroverted pubes – Dromaeosauridae + Troodontidae + Archaeopteryx
20. Asymmetric flight feathers – Dromaeosauridae + Troodontidae + Archaeopteryx
21. Sickle claw on foot – Dromaeosauridae + Troodontidae
22. Forelimb longer than hindlimbArchaeopteryx (doesn’t appear to be true)
23. PygostyleJeholornis and higher birds

* Derived taxa had a shortened tail (#17) and retroverted pubis (#19)

But wait there’s more…

Taxa in the lineage of birds.

Figure 3. Sample taxa in the lineage of birds sans all the cousins and offshoots. From top to bottom: Tawa, Juravenator, Sinocalliopteryx, Archaeopteryx, Cathayornis, Sinornis plus enlarged skulls.

The Revised View and List of Characters
This simplified view of bird relations notes that pterosaurs and Lagerpeton were not related to dinosaurs and birds. It also takes the view that dromaeosaurids and oviraptorids were probably derived from Archaeopteryx due to the shared trait of an elongated coracoid (analysis not done yet). Oviraptorids appear to have reshifted the pubis forward (but that’s for another blog). Here’s a new list of bird characters as they appeared in the above taxa (Fig. 3). Tyrannosaurids and Alvarezsaurids, among others, were skipped because, although their stage of evolution did add characters, in both cases the forelimbs became reduced, representing offshoots that did not ultimately evolve into birds.

Here, in this simplified and focused account, Tawa, Juravenator and Sinocalliopteryx precede Archaeopteryx. Sinornis and Cathayornis succeed Archaeopteryx. The following list of 29 characters are offered to replace the list of 23 (above).

1. Mesotarsal ankle – Gracilisuchus at the base of the Archosauria
2. Functional tridactyl foot – Trialestes (but note sauropods had a functionally pentadactyl foot, so reversals are possible)
3. Fully perforated acetabulum – Herrerasaurus

4. Loss of digit 5 on hand – Tawa
5. Postaxial vertebral pneumaticity – Tawa
6. Strap-like scapula – Tawa

7. Finger 3 shorter than 2. Juravenator
8. Loss of digit 4 on hand
Juravenator
9. Pedal digit 1 loses contact with ankle – Juravenator
10. Furcula (fused clavicles) – Juravenator
11. Maxillary fenestra – Juravenator
12. Pubis rotated beneath anterior ilium – Juravenator 

13. Dorsal astragalus tall/broad – Sinocalliopteryx
14. Promaxillary fenestra – Sinocalliopteryx
15. Fused semilunate carpal – Sinocalliopteryx
16. Ossified sternal ribs – Sinocalliopteryx
17. Protofeathers – Sinocalliopteryx
18. Smaller teeth – Sinocalliopteryx 

19. Pubes beneath or behind acetabulum – Archaeopteryx
20 Asymmetric flight feathers – Archaeopteryx
21. Forelimb nearly as long as hindlimb – Archaeopteryx
22. Elongated coracoid locked onto sternumArchaeopteryx
23. Reduced cervical ribs – Archaeopteryx
24. Chevrons parallel centra – Archaeopteryx
25. Toes beneath shoulder glenoid – Archaeopteryx
26. Anterior skull half the height of posterior skull – Archaeopteryx
27. Pedal digit 1 retroverted for perching – Archaeopteryx 

28. Sickle toe claw – Dromaeosaurids and troodontids (neoflightless birds)
29. Pygostyle – Cathayornis and higher birds, including oviraptorids by convergence
30. Toothless – Certain higher birds, including oviraptorids by convergence

The elongated and immobile coracoids of dromaeosaurids and oviraptorids indicate they were secondarily flightless, following a sister to Archaeopteryx. Alvarezasaurids and therizinosaurids appear to have been derived from sisters to Juravenator or Sinocalliopteryx (among listed taxa) and developed their bird-like characters (short tail, retroverted pubis) by convergence.

Sinocalliopteryx was an Early Cretaceous and too large to be in the lineage of Archaeopteryx, but an earlier, smaller sister would have been a suitable candidate. The development of longer forelimbs and longer coracoids was initiated here. Along with protofeathers, some early flapping may have accompanied running.

Archaeopteryx had larger forelimbs and relatively smaller hips, indicating a transition to forelimb locomotion, which included vigorous flapping assisted by flight feathers. The center of balance had shifted forward to the shoulder glenoid, which is where it is located in flying birds (and bats and pterosaurs). This was accomplished by shortening the torso. The neck could be pulled back further and the forelimbs were elongated and adorned with larger feathers.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Chiappe LM 2009. “Downsized Dinosaurs: The Evolutionary Transition to Modern Birds”. Evolution: Education and Outreach: 248–256.
Heilmann G 1926. The Origin of Birds. London: Witherby. 208 pp
Ji Q and Ji S-A 1996. On the discovery of the earliest bird fossil in China and the origin of birds (PDF). Chinese Geology 233: 30–33.
von Meyer CEH 1861. Archaeopteryx lithographica (Vogel-Feder) undPterodactylus von Solnhofen (in German). Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 1861: 678–679.
Ostrom JH 1973.
 The ancestry of birds. Nature 242 (5393): 136–136.Bibcode 1973Natur.242..136Odoi:10.1038/242136a0.
Paul GS 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press. p. 472p

wiki/Origin_of_birds

The Redevelopment of the Calcaneal Tuber in Poposaurids and Crocs

Updated April 22, 2014 to reflect the new basal archosaur position of poposaurids.

The Traditional View of Calcaneal Tuber Distribution
Most paleontologists (recently Nesbitt 2011 and references therein) assert if the calcaneum of a derived archosauriform had a tuber, the taxon was likely a “pseudosuchian“. If no tuber the taxon was likely an ornithosuchian (= “avemetatarsalian“), and most likely a dinosaur. Seems simple enough and virtually all paleontologists buy into this paradigm.

Leverage for Limb Extension
A calcaneal tuber extends more or less posteriorly to provide more leverage for foot extension. By convergence (remember that phrase!) chiniquodontid therapsids developed a calcaneal tuber that is retained by most mammals including humans. The “Achilles” tendon is attached to this posterior extension.

“Extreme Convergence”
Nesbitt and Norell (2006) and Nesbitt (2007) nested the poposaurid Effigia okeeffeae among the rauisuchians based largely on the ankle, but they noted “extreme convergence in the body plans” with ornithomimid dinosaurs. They reported that the ankle of Effigia articulated in a crocodile-normal configuration, with a morphology similar to Alligator (Figure 1). The broken and missing calcaneal “heel”would have been oriented proximally, like that of a sister taxon, Shuvosaurus (Fig. 2).

The pedes of Alligator and Effigia

Figure 1. The pedes of Alligator (left) and Effigia (right) demonstrating the convergence of the structure of the ankle bones (astragalus and calcaneum). In basal archosaurs the ankle is a simple hinge, but in Alligator the hinge takes a sharp turn between the astragalus and calcaneum with a peg from the astragalu inserting into a socket in the calcaneum. The astragalus and calcaneum of Effigia articulate in a crocodile-normal configuration and their morphologies are similar to  those of Alligator, including the peg and socket. 

The Calcaneal Tuber and its Actual Heretical Distribution
Here, according to the large reptile study, both assessments are false. Basal (and often bipedal) taxa in the croc lineage (like GracilisuchusScleromochlus and Terrestrisuchus) had little to no tuber. Similarly, basal dinosaurs (also often bipedal), like Herrerasaurus and Silesaurus, had little to no tuber. Derived crocs (typically quadrupedal) had a calcaneal tuber. Similarly, and by convergence, Lotosaurus and poposaurs  (Fig. 3) had a calcaneal tuber and sometimes a very large one. Some were bipeds. Others were not. So the pattern of development of the tuber is not one-on-one, but needs more study.

Here (Fig. 3) Effigia nested with other poposaurs as basal archosaurs based on more parsimoniously shared traits from head to toe.

A selection of basal archosaur and poposaurid pedes

Figure 1. A selection of basal archosaur and poposaurid pedes with the calcaneum highlighted in blue. PILs in red. Since Plateosaurus had four distal carpals, it appears likely that at least some of the taxa in the lower row also had distal carpals. but that data was not published.

The Dual Convergent Enlargement of the Calcaneal Tuber
There’s no controversy to the fact that in derived crocs the calcaneal tuber was and is enlarged. There’s no controversy to the fact that in most dinosaurs the calcaneum remained small. In poposaurs some had a large calcaneal tuber. Others did not.

In crocs, as the calcaneal tuber developed, the astragalus and calcaneum stayed similar to each other in size within a size ratio remaining within 40/60 to 60/40. By contrast, in basal dinos and Silesaurus the calcaneum was less than a third of the astragalus. However, in popsaurids, the calcaneum re-enlarged as the astragalus shrank, ultmately matching the ratios seen in crocs.

Shape Variation
Note the shape of the calcaneum tuber varies greatly in the poposaurids. It was small in Lotosaurus and very large in Poposaurus.

The calcaneum as a whole was wider than long in Lotosaurus. Lotosaurus was a graviportal quadruped in which metatarsal 5 was broader proximally, creating a hook shape. This lateral expansion of the metatarsus affected the size of the calcaneum which grew laterally and larger to match.  Silesaurus, despite its narrow pes and vestigial digits 1 and 5 nested basal to Lotosaurus, suggesting that a more primitive sister to Silesaurus retained five unreduced digits.

The calcaneum was longer than wide in Poposaurus. The other poposaurs remained bipedal with a narrow pes and a narrow metatarsal 5. Thus they developed different and distinct sorts of calcaneal tubers.

Too Much Emphasis on the Ankle?
According to the tree recovered by the large reptile study the traditional view placed too much emphasis on the ankle, not accepting the possibility of convergence. The heretical view is more broadly based, employing several times more taxa without emphasizing ankle traits and embracing the possibility of convergence.

And the Convergence Does Not Stop There
Traditional trees mix phytosaurs (= parasuchians) and chanaresuchids in with other archosauriforms. Here, according to the large reptile study, these two pararchosauriforms evolved separately from the euarchosauriforms. Phytosaurs had a calcaneal tuber, but their sisters the chanaresuchids, do not. Thus the calcaneal tuber of phytosaurs developed on its own and (once again) by convergence.

References
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

AMNH Effigia webpage
wiki/Effigia

Revueltosaurus Keeps Bouncing Around

Updated December 09, 2014 as Revueltosaurus now nests with Fugusuchus.

Revueltosaurus has revealed itself to us (and to me) a little bit at a time (see below). That is okay. It’s still good science. Best guesses based on limited data are just fine. Better guesses based on more data are better, of course. This doesn’t always happen (see the flap on pterosaur origins), but at least here, now based on virtually complete skeletal material, that little Triassic oddball, Revueltosaurus, may soon stop bouncing around the reptile family tree. It’s a fun and interesting story.

First, Just Teeth – a Basal Ornithischian
When Hunt (1989) first described Revueltosaurus, it was a Triassic dinosaur, based on its teeth alone, which resembled those of basal ornithischians.

Then, Some Crushed Skull Parts, a Pelvis and Scapula – a Pseudosuchian
Parker et al. (2005) redescribed Revueltosaurus as a pseudosuchian based on some skull, pectoral and pelvic bones. Unfortunately they could not place it within any known monophyletic clade.

Reconstruction of Revueltosaurus from Parker et al. 2005.

Figure 1. Reconstruction of Revueltosaurus from Parker et al. 2005.

Third, A Nesbitt Skull Reconstruction – a Basal Aetosaur
Recently Nesbitt (2011) nested Revueltosaurus at the base of the Aetosauria as a sister to Turfanosuchus, Gracilisuchus and Ticinosuchus — all taxa just ouside of the Archosauria. Here, Ticinosuchus is at the base of the aetosauria.

Skull reconstruction of Revueltosaurus traced from Nesbitt 2011.

Figure 2. Skull reconstruction of Revueltosaurus traced from Nesbitt 2011.

Finally, an Overall Reconstruction by Jeffrey Martz – a Fugusuchus Sister
A recent award-winning reconstruction by Jeffrey Martz illustrated the complete skeleton and osteoderm covering of Revueltosaurus. Captions along with this illustration indicate that an aetosaur sisterhood is still favored. Although the skull was very close to the reconstruction offered by Nesbitt, it was distinct enough to cause a phylogenetic shift to a sister to Fugusuchus, a basal erythrosuchid.

Revueltosaurus revised from a tracing by Jeffrey Martz.

Figure 4. Revueltosaurus revised from a tracing by Jeffrey Martz. The limbs and tail have been straightened just a bit.

Is Revueltosaurus a Mini-Fugusuchus?
Not quite. Likely they shared a common ancestor. The teeth of Revueltosaurus indicate a diet of plants, but the small rib cage and the reduced depth of the pubis do not support an herbivorous diet. The shorter hind limbs of Revueltosaurus indicate that it was a full-time quadruped.

Figure 1. Revueltosaurus compared to its big sister, Fugusuchus, a basal erythrosuchid.

Figure 1. Revueltosaurus compared to its big sister, Fugusuchus, a basal erythrosuchid.

The Question Is
What is real?

References
Hunt AP 1989. A new ornithischian dinosaur from the Bull Canyon Formation (Upper Triassic) of east-central New Mexico. In Lucas, S. G. and A. P. Hunt (Eds.), Dawn of the age of dinosaurs in the American Southwest 355–358.
Parker WG., et al. 2005. The Pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. In Proceedings of the Royal Society London B 272(1566):963–969.

wiki/Revueltosaurus

Nomenclature revisions (part 4)

Today’s blog will tag on the heels of “Nomenclature revisions (parts 1, 2 and 3) to highlight a number of putative clades that are in need of revision, are no longer valid or are redundant in light of the new reptile tree (which is larger than any prior attempt and encompasses all the major clades). Today we’ll restrict our scope to the Ornithosuchia.

Ornithosuchia – retained with a revision
Gauthier (1986) defined Ornithosuchia as the taxon that included extant birds and all extinct archosaurs that are closer to birds than they are to crocodiles. This definition is retained, with a revision. Gauthier’s Ornithosuchia included Ornithosuchidae + Ornithodira. Since pterosaurs were included within Ornithodira, Gauthier’s Ornithosuchia is redundant with Reptilia. Given a new node-based definition that deletes Ornithosuchidae and Pterosauria, the new Ornithosuchia is proposed to include TurfanosuchusTriceratops, their last common ancestor and all of its descendants. The outgroup is Crocodylomorpha.

Ornithodira – redundant
Gauthier (1986) defined “Ornithodira” as all forms closer to birds than to crocodiles. Here this definition is redundant with Ornithosuchia.

Sereno (1991) defined “Ornithodira” as the last common ancestor of the dinosaurs and the pterosaurs, and all its descendants. Here this definition is redundant with Reptilia.

Avesuchia – paraphyletic and redundant
Benton (1999) defined “Avesuchia/crown-group Archosauria” as the taxon comprising “Avemetatarsalia” and “Crurotarsi” (and sister taxa of “Crurotarsi” that are closer to Crocodylia than to Aves), and all their descendants. Because the definition included parasuchians, pterosaurs and Lagerpeton, here this created a paraphyletic clade redundant with Reptilia.

Avemetatarsalia – paraphyletic and redundant
Benton (1999) defined Avemetatarsalia as all “avesuchians/crown-group archosaurs” closer to Dinosauria than to Crocodylia. That definition is redundant with Ornithosuchia. Avemetatarsalia was meant to include Scleromochlus + pterosaurs + dinosauromorphs, but here that clade is paraphyletic (or redundant with Reptilia).

Dinosauriformes – no utility, paraphyletic
Novas defined Dinosauriformes as the most recent common ancestor of Marasuchus (Lagosuchus), Dinosauria and all descendants. Since Marasuchus is derived within the Theropoda here, that definition is now redundant with Dinosauria.

Benton (2004) redefined Dinosauriformes as Neornithes and all ornithodirans closer to Neornithes than to Lagerpeton. Since “Ornithodira” is now redundant with Reptilia (see above) and Lagerpeton now nests outside Euarchosauriformes, Benton’s definition has no utility.

Dinosauromorpha – no utility, paraphyletic
Sereno (1991) defined the Dinosauromorpha as all “Ornithodira” closer to Neornithes than to Pterosauria. Since the Pterosauria is far removed from the Dinosauria, this definition has no utility.

Sereno (1991) did not fix the problem when he stated the clade consisted of Passer and all species closer to Passer than to Pterodactylus, Ornithosuchus and Crocodylus. Sereno (1991) also provided a node clade definition: the last common ancestor of Lagerpeton, Lagosuchus, Pseudolagosuchus and the Dinosauria (including Aves) and all its descendants. Here Sereno’s definition is redundant with Archosauriformes. Removing the proterochampsid, Lagerpeton, from the definition creates a monophyletic clade, but one that would be redundant with Dinosauriformes (= Dinosauria). At present there are no non-dinosaur members to populate the Dinosauriformes or the Dinosauromorpha, since Crocodylomorpha is now the sister taxon of the Dinosauria.

Dinosauria – retained and expanded
Holtz and Padian (1995) defined the Dinosauria as all descendants of the most recent common ancestor of Triceratops and Passer, the sparrow. Standing firm, this definition still includes all taxa traditionally considered dinosaurs. It also adds members of the Poposauridae, PisanosaurusSilesaurus and Lotosaurus, a clade here labeled the Paraornithischia. Traditional clade names and inclusion lists for Theropoda, Sauropodomorpha and Ornithischia are retained.

Saurischia – no utility, paraphyletic
Seeley (1888) classified dinosaurs into two orders based on pelvis morphology. Here, with the Phytodinosauria, this division is polyphyletic and has lost its usefulness.

Phytodinosauria – resurrected
Bakker (1986) coined the term “Phytodinosauria” for a clade including Sauropodomorpha + Ornithischia. Here testing supports this clade.

Paraornithischia – new clade
The new clade Paraornithischia is proposed to include EffigiaLotosaurus, their last common ancestor and all of its descendants. This clade of apparent herbivores (none have sharp, serrated teeth and some are toothless) demonstrates a variety that hints at a wider radiation of undiscovered forms, all currently restricted to the Middle and Late Triassic. The clade also includes PisanosaurusShuvosaurus/Chatterjeea and Silesaurus. This clade consists only of herbivores, many of which had a predentary, paired predentaries or something like it that may have been fused to the often toothless dentaries.  While it may be tempting to consider this the clade basal to Ornithischia, at present moving this branch to the base of the Ornithischia adds four steps. More taxa will bring greater resolution.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
 Bakker RT 1986. The Dinosaur Heresies. New York: William Morrow. p. 203. ISBN 0-14-010055-5.
Benton MJ 1990.
Origin and Interrelationships of dinosaurs, In Weishampel DB, Dodson P, and Osmólska H editors. The Dinosauria. 11–30. Berkeley: U Calif Press.
Benton MJ 1999. Scleromochlus taylori and the origin of dinosaurs and pterosaurs. London: Phil Trans Roy Soc B, 354: 1423–1446.
Benton MJ, Clark JC 1988. Archosaur phylogeny and the relationships of the Crocodylia. In Benton MJ editor. The phylogeny and classification of the tetrapods, 295–338. Syst Assoc, Sp Vol 35A, Clarendon:Oxford.
Bonaparte JF 1982. Classification of the Thecodontia. Geóbios, Mém Sp 6: 99–112.
Clark JM and Hernandez RR 1994. A new burrowing diapsid from the Jurassic La Boca formation of Tamaulipas, Mexico, J Vert Paleo 14: 180–195.
Dilkes D 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Phil Trans R Soc B 353: 501–541.
Gauthier JA 1986.
Saurischian monophyly and the origin of birds, In Padian K editor. The Origin of Birds and the Evolution of Flight, 1–55. Memoirs Calif Acad Sc 8.
Gauthier J, Kluge AG and Rowe T 1988.
Amniote phylogeny and the importance of fossils. Cladistics 4: 105–209.
Gauthier J, Estes R and de Queiroz K 1988. A phylogenetic analysis of Lepidosauromorpha, In Estes R, Pregill G, editors. Phylogenetic relationships of the lizard families, 15–98. Stanford: Stanford U Press.
Gauthier JA 1994. The diversification of the amniotes. In: Prothero DR, Schoch RM editors. Major Features of Vertebrate Evolution: 129-159. Knoxville: Paleo Society.
Gauthier JA, Padian K 1989. The origin of birds and the evolution of flight, In Padian K, Chure DJ editors. The Age of Dinosaurs: Short Courses in Paleontology, No. 2. 121–133. Paleo Soc Depart Geo Sci, Knoxville: U Tenn.
Holtz TR and Padian K 1995. Definition and diagnosis of Theropoda and related taxa. J Vert Paleo 15: 35A
Krebs B 1974. Die Archosaurier. Naturwissenschaften 61: 17–24.
Laurin M 1991.The osteology of a Lower Permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101: 59–95.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Novas FE 1992. Phylogenetic relationships of the basal dinosaurs, the Herrerasauridae. Palaeontology 35: 51–62.
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. J Vert Paleo 13:287–308.
Senter P 2004.Phylogeny of Drepanosauridae (Reptilia: Diapsida). J Syst Palaeo 2: 257–268.
Sereno PC 1991. Basal archosaurs: phylogenetic relationships and functional implications. J Vert Paleo 11 (Supp) Mem 2: 1–53.
Sereno PC 2005.
The logical basis of phylogenetic taxonomy. Syst Biol 54: 595-619.
Walker AD 1964. Triassic reptiles from the Elgin area: Ornithosuchus and the origin of carnosaurs. Phil Trans R Soc London. Ser B Bio Sci 248 (744): 53–134.

Nomenclature revisions (part 3)

Today’s blog will tag on the heels of “Nomenclature revisions (part 1 and part 2) to highlight a number of putative clades that are in need of revision, are no longer valid or are redundant in light of the new reptile tree (which is larger than any prior attempt and encompasses all the major clades). Today we’ll restrict our scope to the Archosauriformes.

The Euarchosauriformes

Figure 1. The Euarchosauriformes. Click to see more.

Erythrosuchiformes – new clade
A new definition for a monophyletic clade Erythrosuchiformes is proposed to include Erythrosuchus, Triceratops, their last common ancestor and all of its descendants. Vjushkovia had been traditionally considered an erythrosuchid but here it nests outside the Erythrosuchidae.

The Rauisuchia – retained
Rauisuchia was erected by Bonaparte to represent the clade including Rauisuchidae, Prestosuchidae, Poposauridae and Chatterjeeidae. Because Chatterjeea and Poposaurus now nest as dinosaurs, this definition of the Rauisuchia now includes all dinosaurs. Redefined as a more inclusive monophyletic node-based clade, the new Rauisuchia is proposed to include Vjushkovia, Triceratops, their last common ancestor and all of its descendants. Members also include crown-clade Archosauria and Ticinosuchidae (including Stagonolepidae). The more restricted Rauisuchidae now includes Vjushkovia, Smok, the last common ancestor and all its descendants.

Ticinosuchidae – new clade
Yarasuchus
and Ticinosuchus are basal to a clade that includes Qianosuchus and the Stagonolepidae. Within the new Rauisuchia, a definition for a monophyletic Ticinosuchia is proposed to include Ticinosuchus, Triceratops their last common ancestor and all of its descendants. The more restricted Ticinosuchidae is proposed to include Ticinosuchus, Qianosuchus, their last common ancestor and all its descendants.

Archosauria – retained
Still crocs, birds, their last common ancestor and all its descendants. No change here (except no pterosaurs, of course).

Suchia – paraphyletic
Krebs (1974) defined “Suchia,” as “Crocodylotarsi,” but not Parasuchia. Even so, such a clade remains paraphyletic here. “Suchia” had been described by Benton and Clark (1988) as Crocodylomorpha + “rauisuchians” + Stagonolepididae, but not Gracilisuchus =. Here, that assemblage also constitutes a paraphyletic group.

Pseudosuchia – redundant
In the pre-cladistic era, “Pseudosuchia” generally included Stagonolepidae, the old Rauisuchia, Ornithosuchidae and some basal crocodylomorphs. Here these form a a paraphyletic clade.

Gauthier and Padian (1989) defined “Pseudosuchia” as “crocodiles and all archosaurs closer to crocodiles than to birds. Gauthier 1986 and Senter (2004) created equivalent definitions. Unfortunately, here the “Pseudosuchia,” as defined by these authors, is redundant with the Crocodylomorpha.

Crocodylomorpha – retained, redefined
Parrish (1993) cited six synapomorphies from Walker (1964) when he embedded the old Crocodylomorpha within Rauisuchia.

Benton (1990) defined Crocodylomorpha as all archosaurs closer to Eusuchia than to Ornithosuchus or Postosuchus. While it is clear that Benton meant to include a clade similar to the present one, his definition with the present tree topology would include Ticinosuchidae (including Stagonolepidae), which was not his intention.

Sereno (2005) defined Crocodylomorpha as the most inclusive clade containing Crocodylus but not Poposaurus, Gracilisuchus, Prestosuchus and Aetosaurus. The omission of Gracilisuchus excludes a basal taxon in the present Crocodylomorpha.

A new node-based definition for the new Crocodylomorpha is proposed to include Crocodylus, Pseudhesperosuchus, their last common ancestor and all of its descendants. Ticinosuchidae is the outgroup. Scleromochlus, a taxon often nested with dinosaurs and pterosaurs [17–20,23] nests here (Figures 2) within the crocodylomorpha close to Gracilisuchus.

Crocodylotarsi – redundant
Benton and Clark (1988) defined “Crocodylotarsi” as the last common ancestor of crocodiles and Parasuchia. This represented the “crocodilian line” (Parasuchia, Rauisuchidae, Stagonolopedidae, Poposauridae and Crocodylomorpha) as opposed to the “bird line” (Ornithosuchia) as defined by Parrish (1993). In the present study that definition of Crocodylotarsi is redundant with Archosauriformes and furthermore the taxon list is paraphyletic.

Crurotarsi  – redundant
Sereno (1991) defined “Crurotarsi,” as all forms closer to Crocodylus than to Passer [146]. It was meant to include rauisuchians, phytosaurs (parasuchians), stagonolepids, poposaurs, sphenosuchians, and a few other groups including Ornithosuchidae. Here the definition is redundant with Crocodylomorpha. The taxon membership list is paraphyletic and redundant with Archosauriformes.

More coming in part 4. 

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Benton MJ 1990. Origin and Interrelationships of dinosaurs, In Weishampel DB, Dodson P, and Osmólska H editors. The Dinosauria. 11–30. Berkeley: U Calif Press.
Benton MJ, Clark JC 1988. Archosaur phylogeny and the relationships of the Crocodylia. In Benton MJ editor. The phylogeny and classification of the tetrapods, 295–338. Syst Assoc, Sp Vol 35A, Clarendon:Oxford.
Bonaparte JF 1982. Classification of the Thecodontia. Geóbios, Mém Sp 6: 99–112.
Clark JM and Hernandez RR 1994. A new burrowing diapsid from the Jurassic La Boca formation of Tamaulipas, Mexico, J Vert Paleo 14: 180–195.
Dilkes D 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Phil Trans R Soc B 353: 501–541.
Gauthier JA 1986.
Saurischian monophyly and the origin of birds, In Padian K editor. The Origin of Birds and the Evolution of Flight, 1–55. Memoirs Calif Acad Sc 8.
Gauthier J, Kluge AG and Rowe T 1988.
Amniote phylogeny and the importance of fossils. Cladistics 4: 105–209.
Gauthier J, Estes R and de Queiroz K 1988. A phylogenetic analysis of Lepidosauromorpha, In Estes R, Pregill G, editors. Phylogenetic relationships of the lizard families, 15–98. Stanford: Stanford U Press.
Gauthier JA 1994. The diversification of the amniotes. In: Prothero DR, Schoch RM editors. Major Features of Vertebrate Evolution: 129-159. Knoxville: Paleo Society.
Gauthier JA, Padian K 1989. The origin of birds and the evolution of flight, In Padian K, Chure DJ editors. The Age of Dinosaurs: Short Courses in Paleontology, No. 2. 121–133. Paleo Soc Depart Geo Sci, Knoxville: U Tenn.
Krebs B 1974. Die Archosaurier. Naturwissenschaften 61: 17–24.
Laurin M 1991.The osteology of a Lower Permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101: 59–95.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. J Vert Paleo 13:287–308.
Senter P 2004.Phylogeny of Drepanosauridae (Reptilia: Diapsida). J Syst Palaeo 2: 257–268.
Sereno PC 1991. Basal archosaurs: phylogenetic relationships and functional implications. J Vert Paleo 11 (Supp) Mem 2: 1–53.
Sereno PC 2005.
The logical basis of phylogenetic taxonomy. Syst Biol 54: 595-619.
Walker AD 1964. Triassic reptiles from the Elgin area: Ornithosuchus and the origin of carnosaurs. Phil Trans R Soc London. Ser B Bio Sci 248 (744): 53–134.

Nomenclature revisions (part 2)

Today’s blog will tag on the heels of “Nomenclature revisions (part 1) to highlight a number of putative clades that are in need of revision, are no longer valid or are redundant in light of the new reptile tree (which is larger than any prior attempt and encompasses all the major clades).

Diapsida
Laurin (1991) defined the Diapsida as the most recent common ancestor of araeoscelidians, lepidosaurs and archosaurs and all its descendants. Here that definition is redundant with Reptilia because lizards are not related to archosaurs except through Cephalerpeton, the basalmost reptile.

Benton (1990) referred the term Diapsida to the clade stemming from the first amniote with a supratemporal fenestra homologous with that of Aves. That definition may be retained despite the revelation that the diapsid opening of Sphenodon was not homologous with that of Aves because lepidosauriformes were not related to the basal diapsid, Petrolacosaurus. A node-based redefinition of the new Diapsida is proposed to include Petrolacosaurus, Triceratops, their last common ancestor and all of its descendants. It is equivalent to the definition of Benton (1990).

Younginiformes
Taxa traditionally considered “younginiformes,” such as Youngina, Acerosodontosaurus \ and Thadeosaurus do not form a monophyletic group. Here they form a basal assemblage of a larger clade. The Younginimorpha is proposed to include Thadeosaurus, Triceratops, their last common ancestor and all of its descendants.

Prolacertiformes / Protorosauria
Protorosauria no longer include tanystropheids, pterosaurs and kin. Those have all been shifted to the Squamata. The clade Protorosauria has been reduced to only Prolacerta, Protorosaurus, PamelariaBoreopricea and kin, all basal taxa to the Archosaurifomes. Redefined, the new Protorosauria is proposed to include Prolacerta, Protorosaurus, their last common ancestor and all of its descendants. A more inclusive clade, the new Prolacertiformes, is proposed to include both Protorosauria + Archosauriformes. Redefined as a node-based taxon, the new Prolacertiformes is proposed to include Prolacerta, Triceratops, their last common ancestor and all of its descendants. Orovenator is the outgroup taxon.

Archosauriformes
Gauthier (1986) proposed the term “Archosauriformes” to replace the traditional Archosauria (Proterosuchus through Dinosauria). Gauthier’s Archosauriformes retained the Proterosuchidae, Parasuchidae, Proterochampsidae, Euparkeria, Erythrosuchidae, and the Pterosauria, all taxa conventionally thought to lead to and include the Dinosauria. This needs to be revised. Here pterosaurs now nest with lizards, but the other listed clades are retained. Other former outgroups are now added. These include the Choristodera and Youngina. Not all taxa had an antorbital fenestra (see below). Redefined here, the new Archosauriformes is proposed to include Champsosaurus, Triceratops, their last common ancestor and all of its descendants. A specimen of Youngina (UC 1528) nests at the base.

The Basal Division Within the Archosauriformes
The new Archosauriformes divides at its base into two major clades, the Pararchosauriformes and Euarchosauriformes. This division was previously unnoticed  in prior studies due to exclusion of several basal taxa including several specimens of Youngina.

Pararchosauriformes
The Pararchosauriformes includes Chañaresuchus, Champsosaurus, their last common ancestor and all of its descendants. This now extinct clade also includes choristoderes, parasuchians, DoswelliaCerritosaurus and Lagerpeton, among others.

Euarchosauriformes
The Euarchosauriformes includes Proterosuchus, Triceratops, their last common ancestor and all of its descendants. This clade also includes crocodilians and birds among others. It does not include proterochampsids, parasuchians, choristoderes and kin.

More to come in part 3.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Benton MJ 1990. Origin and Interrelationships of dinosaurs, In Weishampel DB, Dodson P, and Osmólska H editors. The Dinosauria. 11–30. Berkeley: U Calif Press.
Clark JM and Hernandez RR 1994. A new burrowing diapsid from the Jurassic La Boca formation of Tamaulipas, Mexico, J Vert Paleo 14: 180–195.
Dilkes D 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Phil Trans R Soc B 353: 501–541.
Gauthier JA 1986.
Saurischian monophyly and the origin of birds, In Padian K editor. The Origin of Birds and the Evolution of Flight, 1–55. Memoirs Calif Acad Sc 8.
Gauthier J, Kluge AG and Rowe T 1988.
Amniote phylogeny and the importance of fossils. Cladistics 4: 105–209.
Gauthier J, Estes R and de Queiroz K 1988. A phylogenetic analysis of Lepidosauromorpha, In Estes R, Pregill G, editors. Phylogenetic relationships of the lizard families, 15–98. Stanford: Stanford U Press.
Gauthier JA 1994. The diversification of the amniotes. In: Prothero DR, Schoch RM editors. Major Features of Vertebrate Evolution: 129-159. Knoxville: Paleo Society.
Laurin M 1991.The osteology of a Lower Permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101: 59–95.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

Nomenclature Revisions (part 1)

Today’s blog will highlight a number of putative clades that are in need of revision, are no longer valid or are redundant in light of the new reptile tree (which is larger than any prior attempt and encompasses all the major clades).

Reptilia – revision
Gauthier, Kluge & Rowe (1988) defined Reptilia as “the most recent common ancestor of extant turtles and saurians, and all of its descendants.” They separated mammals and their ancestors as outgroups to this clade. However, here mammals and their synapsid ancestors appear within the new Archosauromorpha and thus within the Reptilia as defined above. Diapsids, like Petrolacosaurus, were derived from basal synapsids, like Heleosaurus. So we’re all related.

Amniota – redundant
Gauthier, Kluge & Rowe (1988) defined Amniota as “the most recent common ancestor of extant mammals and reptiles, and all its descendants.” Here that definition is redundant with Reptilia, a term which goes back to Linneaus (1758).

Sauria – redundant
Gauthier, Kluge & Rowe (1988) defined Sauria as the most recent common ancestor of Lepidosauria and Archosauria and all of its descendants. Here that definition is redundant with Reptilia.

Sauropsida – paraphyletic
Gauthier (1994) defined Sauropsida as “Reptiles plus all other amniotes more closely related to them than they are to mammals,” based on traditional cladograms that indicated a basal split between the Synapsida and Sauropsida. Here this is a paraphyletic assemblage.

Lepidosauromorpha – retained and redefined
Gauthier, Estes and de Queiroz (1988) defined Lepidosauromorpha as Sphenodon, squamates and all saurians sharing a more recent common ancestor with them than they do with crocodiles and birds. That definition is retained here even though they erred when including “Younginiformes” in their taxon list. Here seven “Younginiformes” nest within the new Archosauromorpha. Lepidosauromorpha therefore becomes the first of the great clades dividing the Reptilia following Cephalerpeton, the basalmost known reptile. Lepidosauromorpha includes turtles, the tuatara, lizards and snakes among living examples. Diadectids, chroniosuchids, caseids, pterosaurs, among others, are extinct lepidosauromorphs.

Laurin 1991 redefined the Lepidosauromorpha as the clade originating with the most recent common ancestor of Palaeagama, Saurosternon, Paliguana, Kuehneosaurus and Lepidosauria. While that is a monophyletic clade, Laurin’s taxon list is also redundant with the list referred to Lepidosauriformes by Gauthier, Estes and de Queiroz (1988). A new node-based definition reflecting a greater presence for the new Lepidosauromorpha is proposed to include Paliguana, Thuringothyris, their most recent common ancestor and all of its descendants.

Lepidosauriformes – revision
Gauthier, Estes and de Queiroz (1988) defined the Lepidosauriformes as Sphenodon, squamates and all organisms sharing a more recent common ancestor with them than they do with younginiforms. Here that definition is equivalent to Lepidosauromorpha. Given a node-based definition, the new Lepidosauriformes is proposed to include Sphenodon, Paliguana, their last common ancestor and all its descendants. Such a clade encompasses the same taxon list that Gauthier, Estes and de Queiroz (1988) intended. Clark & Hernandez (1994) also recovered a monophyletic Lepidosauriformes originating with Saurosternon and Paliguana.

Archosauromorpha – retained
Gauthier (1986) defined Archosauromorpha as all Saurians sharing a more recent common ancestor with Archosauria than with Lepidosauria. That definition is retained. Gauthier’s taxon list included Prolacertiformes, Rhynchosauria, Trilophosaurus and Archosauriformes. Here Rhynchosauria and Trilophosaurus shift to the Rhynchocephalia (Sphenodontia) within the Lepidosauromorpha.

Benton (1990) redefined Archosauromorpha as the most recent common ancestor of Neornithes, Squamata, and all of the descendants of this common ancestor. In the present study, that definition is redundant with Reptilia.

Laurin (1991) proposed that Archosauromorpha include the most recent common ancestor of Prolacerta, Trilophosaurus, Hyperodapedon, archosaurs and all its descendants. Under the present tree topology this definition is also redundant with Reptilia.

Dilkes (1998) proposed a definition for Archosauromorpha that included Protorosaurus and all other saurians that are related more closely to Protorosaurus than to Lepidosauria. This is a suitable definition. Following Gauthier (1986), Dilkes (1998) and the results of this study, a node-based redefinition of the new Archosauromorpha is proposed to include Protorosaurus, Westlothiana, their last common ancestor and all of its descendants. That definition includes synapsids (including mammals), sauropterygians (including ichthyosaurs), mesosaurs, araeoscelidians and archosauriforms (including dinosaurs).

More coming in parts 2 through 4.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Benton MJ 1990. Origin and Interrelationships of dinosaurs, In Weishampel DB, Dodson P, and Osmólska H editors. The Dinosauria. 11–30. Berkeley: U Calif Press.
Clark JM and Hernandez RR 1994. A new burrowing diapsid from the Jurassic La Boca formation of Tamaulipas, Mexico, J Vert Paleo 14: 180–195.
Dilkes D 1998. The Early Triassic rhynchosaur Mesosuchus browni and the interrelationships of basal archosauromorph reptiles. Phil Trans R Soc B 353: 501–541.
Gauthier JA 1986.
Saurischian monophyly and the origin of birds, In Padian K editor. The Origin of Birds and the Evolution of Flight, 1–55. Memoirs Calif Acad Sc 8.
Gauthier J, Kluge AG and Rowe T 1988.
Amniote phylogeny and the importance of fossils. Cladistics 4: 105–209.
Gauthier J, Estes R and de Queiroz K 1988. A phylogenetic analysis of Lepidosauromorpha, In Estes R, Pregill G, editors. Phylogenetic relationships of the lizard families, 15–98. Stanford: Stanford U Press.
Gauthier JA 1994. The diversification of the amniotes. In: Prothero DR, Schoch RM editors. Major Features of Vertebrate Evolution: 129-159. Knoxville: Paleo Society.
Laurin M 1991.The osteology of a Lower Permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101: 59–95.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

The Base of the Archosauria and the Origin of the Dinosauria

Updated August 20, 2015 with the addition of figure 1.

Everyone knows that crocs and birds are today’s living Archosaurs. Prehistoric crocs and dinosaurs were closer to the origin of this clade, the subject of today’s blog.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 1. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Traditionally (as in Nesbitt 2011), at the base of, or just prior to the Archosauria, you’re supposed to find pterosaurs, lagosuchids, chanaresuchids and parasuchians. Gaaak! Unfortunately such results arise from too small of an inclusion set in which unrelated taxa nest by default. We talked about this earlier.

Here (Fig. 1), at the very base of the Archosauria, you’ll find Gracilisuchus (among the crocs), Herrerasaurus (among the dinos) and Trialestes and Turfanosuchus representing more primitive sisters to both.

Turfanosuchus (Young 1973, Fig. 2) had a full set of unreduced fingers and toes, which is good for a basal taxon. Nearly all other archosaurs diminished digits 1 and 5. However, Turfanosuchus represents an herbivorous offshoot because it had an elongated torso, which Gracilisuchus and Herrerasaurus lacked.

Unfortunately, Trialestes (Rieg 1963, Fig. 2) is known from incomplete remains — some of which have only been described in the literature (Clark et al. 2000). Even so, what is known is enough to be important and revealing. I think of it as a cross between Pseudhesperosuchus and Herrerasaurus.

Gracilisuchus (Romer 1972, Fig. 2) was at the base of the crocodylomorpha, but in its own subclade along with Saltopus and Scleromochlus, two expert bipeds. Gracilisuchus was more primitive, just experimenting with bipedality on shorter hind legs and longer front ones.

In anyone’s book, Herrerasaurus (Reig 1963, Fig. 2) is the most primitive dinosaur we know. Even so, while much larger than and quite distinct from Gracilisuchus, the basal dino shared few traits with the basal croc. It was already deep into being a dinosaur with few transitional traits (but see below!). More basal archosaurs, as they are discovered, will look more and more like one of these two or a blend of both.

What Sets Archosaurs Apart?
Here’s a short list:

Basal Archosaurs

Figure 2. Basal archosaurs. Click to enlarge.

Bipedal
At this stage bird and croc ancestors began experimenting with bipedality. Rising up on their hind limbs and furthermore elevating their heels, basal archosaurs became digitigrade bipeds. This freed up their hands to do other things, like flap or become vestiges. An early protoarchosaur, Turfanosuchus, went back to a quadrupedal stance, because it had gone off to the other side and become an herbivore with a big gut that found green immobile food growing at its feet. The meat-eaters of this clade stayed bipedal. Those who chose not to fight for their food did not.

Skull Width
Here’s where basal dinos developed a skull that was narrower than tall and basal crocs developed a skull that was twice as wide as tall. The descending process of the squamosal and the entire quadratojugal tended to become more gracile in dinosaurs.

Temple Change
With a wider skull basal crocs developed their distinctive temple regions with the forward leaning quadrate and quadratojugal contacting or nearly contacting the postorbital. Perhaps this sort of architecture strengthened the temple region on a wider skull.

With a narrower skull, dinosaurs developed a more gracile and more vertical squamosal and quadrate morphology.

Nasal
Note the nasal in these taxa. It wrapped laterally around the naris unlike most other reptiles.

Postfrontal Fusion
The postfrontal were fused in most Archosaurs, but in different ways. In crocs the postorbital and postfrontal fused. In dinos the frontal and postfrontal fused. Gracilisuchus did not fuse the postorbital and postfrontal. This situation is unclear in its sister, Scleromochlus.

Orbit and Frontal
The orbit is larger than the lateral temporal fenestra in this clade. At least a tip of the frontal enters the upper temporal fenestra in archosaurs.

Pterygoid
In basal members of the Archosauria there is no interpterygoid vacuity. Both pterygoids meet or virtually meet each other in a straight line at the midline.

Ribs
In Gracilisuchus, at the base of the Archosauria, the ribs were expanded (costal plates) which seems to happen whenever tetrapods change locomotion modes, as in Ichthyostega and Thrinaxodon.  Hesperosuchus retained that rib morphology, but closer sisters, like Scleromochlus,  Terrestrisuchus and Herrerasaurus did not.

Radiale and Ulnare
Trialestes had an elongated radiale and ulnare, which is trait found only in crocs back to Pseudhesperosuchus. This may mean the radiale and ulnare were secondarily reduced in Herrerasaurus.

Ankles and the Calcaneal Tuber
In basal archosaurs there isn’t much of a calcaneal tuber. In most dinosaurs it completely disappears, but in poposaurids it enlarges (which has led to phylogenetic confusion). Likewise, in crocodylomorphs the calcaneal tuber (heel) enlarges.

Toes
It appears likely that the archosaur precursor, Pseudhesperosuchus, was a digitigrade biped based on metatarsal/toe ratios and limb length ratios. That begat Gracilisuchus and a series of bipedal crocs which were functionally quadradactyl.  Turfanosuchus, which put five digitigrade toes on the ground but reduced the length of the metatarsus. Digit 1 in Trialestes retained an elongated metatarsus, but reduced metatarsal 1 in length and diameter. That set the pattern of Herresaurus and Pampadromaeus, both of which had a similar foot. These patterns set the stage for both the functionally tridactyl feet of theropods and ornithopods, but also set the stage for the functionally quadradactyl feet of poposaurids and the functionally pentadactyl feet of sauropods (Fig. 2), achieved by reducing the long metatarsals (2-4) to the sizes of the smaller and vestigial ones (1 and 5).

Sauropod pedal evolution.

Figure 3. Basal archosaur pedal evolution. Here metatarsal 1 was shorter and more gracile in basal taxa, including theropods and ornithopods, but in basal sauropodomorphs, metatarsal 1 became more robust. In sauropods all the metatarsals became reduced, especially 2-4, and the phalanges of digit 4 were reduced and lost. This demonstrates how a functionally tridactyl pes, as in Herrerasaurus, can become a functionally pentadactyl pes by losing and shrinking elements!!!

Nomenclature Problems
The shifting of taxa here from the traditional tree(s) (Nesbitt 2011) brings with it confirmation and falsification of certain clade names. These we’ll talk about next in a series of four upcoming blogs. (Hint: Say good-bye to “Ornithodira”, “Dinosauriformes” and “Dinosauromorpha”).

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Benton MJ and Clark JM 1988. Archosaur phylogeny and the relationships of the Crocodilia in MJ Benton (ed.), The Phylogeny and Classification of the Tetrapods 1: 295-338. Oxford, The Systematics Association.
Bonaparte JF 1982. Classification of the Thecodontia. Geobios Mem. Spec. 6, 99-112
Brusatte SL , Benton MJ , Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Clark JM, Sues H-D and Berman DS 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20(4):683-704.
Huene FR 1910. Ein primitiver Dinosaurier aus der mittleren Trias von Elgin. Geol. Pal. Abh. n. s., 8:315-322.
Juul L 1994. The phylogeny of basal archosaurs. Palaeontographica africana 1994: 1-38.
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Novas FE 1994. New information on the systematics and postcranial skeleton ofHerrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Romer AS 1972. 
The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287-308.
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.
Sues H-D, Nesbitt SJ, Berman DS and Henrici AC 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society Bpublished online 
Wu X-C and Russell AP 2001. Redescription of Turfanosuchus dabanenesis(Archosauriformes) and new information on its phylogenetic relationships. Journal of Vertebrate Paleontology 21(1):40-50. Online pdf.
Young CC 1973. [On a new pseudosuchian from Turfan, Sinking (Xinjiang).] Memoirs of the Institute of Vertebrate Paleontology and Paleoanthropology of the Academia Sinica, Series B 10:15-37.

wiki/Gracilisuchus
wiki/Herrerasaurus
wiki/Sanjuansaurus
wiki/Trialestes
wiki/Turfanosuchus