The forgotten clade: the REAL proximal ancestors to Dinosauria

Ignored by Baron et al. 2017, and everybody else
the Junggarsuchus clade (including Pseudhesperosuchus, Carnufex and Trialestes in order of increasing quadrupedality, Figs. 1–4) nests as the proximal ancestors to Herrerasaurus (Fig. 1) and the rest of the Dinosauria (Fig. 5) in the large reptile tree (LRT). That cladogram tests a wider gamut of taxa in greater detail than any other reptile cladogram ever published, attempting to not overlook anything. The Junggarsuchia is a sister clade to the Crocodylomorpha with both arising from a taxon near Lewisuchus (Fig. 1). Traditional paleontology (see Wikipedia) nests this largely ignored clade with the sphenosuchian crocodylomorphs (Fig. 4)… and for two good reasons!

Figure 1. Members of the Junggarsuchus clade were derived from a sister to the basal crocodylomorph, Lewisuchus and produced one line that includes Pseudhesperosuchus and Trialestes. The other line produced dinosaurs. These taxa are shown to scale. Note the evolution from a bipedal configuration to a quadrupedal stance.

Figure 1. Members of the Junggarsuchus clade were derived from a sister to the basal crocodylomorph, Lewisuchus and produced one line that includes Pseudhesperosuchus and Trialestes. The other line produced dinosaurs. These taxa are shown to scale. Note the evolution from a bipedal configuration to a quadrupedal stance.

One: Paleontologists never seem to include Dinosauria
in their smaller gamut croc analyses because they’re looking at crocs!~. So once again, taxon exclusion is holding some workers back from seeing ‘the big picture’. ReptileEvolution.com and the blog you are currently reading is all about examining ‘the big picture.’

Figure 2. Skulls of the Junggarsuchus clade not to scale. Herrerasaurus is the basalmost dinosaur.

Figure 2. Skulls of the Junggarsuchus clade not to scale. Herrerasaurus is the basalmost dinosaur, closely related to Junggarsuchus.

Two: Junggarsuchians ALSO have elongate proximal wrist bones
Elongate proximal carpals are found in both sphenosuchian crocs and derived members of the Junggarsuchus clade. Paleontolgists wrongly assumed such odd wrist bones were homologous. It’s an easy mistake to make. However, the LRT makes clear that intervening taxa, including Junggarsuchus, do not have elongate wrist bones.

Among taxa that preserve the manus,
(Fig. 3) it is Junggarsuchus that nests closest to Herrerasaurus and the Dinosauria.

Figure 3. Hands of Lewisuchus, Herrerasaurus, Junggarsuchus, Pseudhesperosuchus and Trialestes. The proximal carpals (radiale and ulnare) were elongate by convergence with a line of crocodylomorphs. This has confused paleontologists and mentally removed them from possible ancestry to the Dinosauria. Note the very short proximal carpals in Junggarsuchus.

Figure 3. Hands of Lewisuchus, Herrerasaurus, Junggarsuchus, Pseudhesperosuchus and Trialestes. The proximal carpals (radiale and ulnare) were elongate by convergence with a line of crocodylomorphs. This has confused paleontologists and mentally removed them from possible ancestry to the Dinosauria. Note the very short proximal carpals in Junggarsuchus.

Like the basal members of the Crocodylomorpha
the Junggarsuchus clade (the Prodinosauria here) transition from bipedal basal members to quadrupedal derived members, with the longest forelimbs belonging to the most derived member, Trialestes (Fig. 3). Distinct from the others and contra the original interpretation, I think Trialestes may have had a larger ulnare than radiale, to match its larger ulna.

Figure 4. Crocodylomorph manus and carpus samples including Terrestrisuchus, Erpetosuchus, Hesperosuchus and Dibothrosuchus along with Scleromochlus documenting the elongate radiale and ulnare on derived taxa. Ticinosuchus is the closest example of an ancestral/plesiomorphic manus in the LRT.

Figure 4. Crocodylomorph manus and carpus samples including Terrestrisuchus, Erpetosuchus, Hesperosuchus and Dibothrosuchus along with Scleromochlus documenting the elongate radiale and ulnare on derived taxa. Ticinosuchus is the closest example of an ancestral/plesiomorphic manus in the LRT.

Let’s not forget
PVL 4597 (Fig. 6) which was mistakenly considered a specimen of Gracilisuchus by (Lecuona and Desojo 2011), but under phylogenetic analysis in the LRT, still nests as the proximal outgroup to Herrerasaurus. It is tiny specimen, supporting the hypothesis of phylogenetic miniaturization at clade origin. And it retains a small proximally oriented calcaneal tuber, as found in other Junggarsuchians.

Figure 1. Subset of the LRT focusing on the Archosauria (Crocodylomorpha + Dinosauria and kin). Gray areas document specimens with elongate proximal carpals (radiale and ulnare).

Figure 5. Subset of the LRT focusing on the Archosauria (Crocodylomorpha + Dinosauria and kin). Gray areas document specimens with elongate proximal carpals (radiale and ulnare).

We looked at
phylogenetic miniaturization at the origin of several pterosaur clades. Well, it happens here too, at the base of the Dinosauria (Fig. 1) with PVL 4597 (Fig. 6), easily overlooked, easily mistaken for something else.

One should not ‘choose’ outgroup taxa
based on paradigm, tradition, guessing, convenience or opinion. Rather outgroup taxa should ‘choose themselves’ based on rigorous testing of a large gamut of outgroup candidates in phylogenetic analysis. To minimize selection bias, the LRT provides 858 outgroup taxa the opportunity to nest close to dinosaurs.

Figure 6. The closest known taxa to the Dinosauria, PVL 4597, is a tiny taxon (phylogenetic miniaturization) with erect hind limbs, a large and deep pelvis and a tiny calcaneal tuber.

Figure 6. The closest known taxa to the Dinosauria, PVL 4597, is a tiny taxon (phylogenetic miniaturization) with erect hind limbs, a large and deep pelvis and a tiny calcaneal tuber.

 

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.
Bonaparte JF 1969. 
Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Butler RJ. et al. 2014. New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation. BMC Evol. Biol. 14, 128.
Clark JM et al. 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.
doi:10.1671/0272-4634(2000)020[0683:ANSOHA]2.0.CO;2.
Clark JM, Xu X, Forster CA and Wang Y 2004. A Middle Jurassic ‘sphenosuchian’ from China and the origin of the crocodilian skull. Nature 430:1021-1024.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum(Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Nesbitt SJ 2011. The early evolution of archosaurs: relationship and the origin ofmajor clades. Bull. Amer. Mus. Nat. Hist. 352, 1–292.
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
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.
Zanno LE, Drymala S, Nesbitt SJ and Schneider VP 2015. Early Crocodylomorph increases top tier predator diversity during rise of dinosaurs. Scientific Reports 5:9276 DOI: 10.1038/srep09276.

wiki/Pseudhesperosuchus
wiki/Junggarsuchus
wiki/Carnufex
wiki/Herrerasaurus
wiki/Sanjuansaurus

 

What traits separate phytodinosaurs from theropods?

Yesterday we looked at the origin of dinosaurs in the context of and contra the recent Baron et al. 2017 paper. Today we’ll look at the basal split between basal phytodinosaurs, like Eodromaeus (Figs. 1, 2), with the closely related basal theropods, like Tawa (Fig. 1).

Figure 1. The theropod Tawa compared to the closely related phytodinosaur, Eodromaeus.

Figure 1. The theropod Tawa compared to the closely related smaller phytodinosaur, Eodromaeus.

Placed side-by-side to scale
Tawa and Eodromaeus are similar overall, though the plant-eaters were initially smaller. The details (below) demonstrate the initial steps toward herbivory that characterize the Phytodinosauria, distinct from the Theropoda and basal Dinosauria from which they evolved (contra Baron et al. 2017).

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

How do basal phytodinosaurs differ from the basal theropods?
Here’s the LRT list:

  1. lateral rostral shape: convex and smoothly curved (also in ancestral Herrerasaurus and Gracilisuchus);
  2. premaxilla/maxilla angle 25–45º;
  3. naris shape in lateral view almost round (not longer than tall or taller than long);
  4. postfrontal has no contact with the upper temporal fenestra;
  5. opisthotic oriented laterally without posttemporal fenestrae;
  6. palatal teeth (only on basalmost taxa);
  7. maxillary tooth depth ≤ 2x width in lateral view;
  8. last maxillary tooth at mid orbit (also in Herrerasaurus);
  9. olecranon process present (convergent in Buriolestes);
  10. metacarpals 2 and 3 align with m1.1 (except Eodromaeus);
  11. acetabulum laterally oriented (no ventral deflection, as in basal theropods);
  12. femoral head with neck and offset (appears later in theropods);
  13. penultimate manual phalanges not the longest in each series;
  14. loss of pubic boot (likely plesiomorphic because outgroups to Herrerasaurus do not have a pubic boot).

Summary and significance
Compared to the closely related theropod Tawa, the overall similar phytodinosaur Eodromaeus had a taller rounder rostrum, shorter teeth, a higher coronoid process, a longer dorsal region with more robust dorsal vertebrae, reduced gastralia, a more robust pectoral girdle and forelimb with shorter, less raptorial fingers, a deeper pubis and ischium with more robust hind limbs. The shorter teeth and larger belly together with the more robust limbs and back are traits seen in a wide variety of herbivores, even if only transitional at this early stage.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.

Basal dinosaurs: what makes them dinosaurs?

Earlier we looked at Baron et al. 2017, who heretically allied ornithischians with theropods to create a new clade ‘Ornithoscelida.’ After a week of study and several tests, I came away with the realization that the Baron et al. tree topology did not depend on taxon exclusion, based on comparable results from subsets of the large reptile tree (LRT). Reducing the list of LRT taxa to match the Baron et al. taxon list did not change the dinosaur tree topology. Rather scoring inaccuracies were the problem, and that goes back to the original matrix presented by Nesbitt 2011, which Baron et al. continued using. You might remember, that’s the study that nests pterosaurs with parasuchians with no intervening taxa, among many other odd nestings. We’ll detail those inaccuracies later this week.

One of the reasons
Baron et al. allied theropods with ornithisichians is because they nested two phytodinosaurs, Eodromaeus (Fig. 1) and Eoraptor, as basalmost theropods. So when they thought they were comparing the ‘theropod’ Eoraptor with the ornithischian, Heterodontosaurus (their figure 2), they were actually comparing two phytodinosaurs. So, of course they shared a long list of characters. To really test their assertion, Baron et al. should have tested Heterodontosaurus against a real basal theropod, like Tawa (Fig. 2). (Tomorrow we’ll compare Tawa with Eodromaeus.)

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Along the way, 
I was able to better understand what was going on at the  base of the Dinosauria and at the basal Theropod/Phytodinosaur split. For instance, as noted earlier, Herrerasaurus (Fig. 3) has a smaller number of cervicals than other dinos do, matching the number found in ancestral basal crocodylomorpha, like Lewisuchus, not silesaurids, like Silesaurus.

Figure 2. Tawa, a basal theropod. Note how closely it resembles the phytodinosaur, Eodromaeus. That's because they are closely related as basalmost dinosaurs.

Figure 2. Tawa, a basal theropod. Note how closely it resembles the phytodinosaur, Eodromaeus. That’s because they are closely related as basalmost dinosaurs.

How do basal dinosaurs differ from the basal croc ancestors?
Here’s the latest list:

  1. skull shorter than cervical series (not Herrerasaurus):
  2. skull width less than 1.2x height;
  3. posterolateral premaxilla narrower than naris (not Herrerasaurus);
  4. quadrate curls posterodorsally (not Herrerasaurus);
  5. dentary tip straight or descends (does not rise);
  6. deep canine maxillary teeth absent (not Herrerasaurus);
  7. nine or more cervicals (not Herrerasaurus);
  8. cervical ribs slender, parallel centra (Herrerasaurus unknown);
  9. 3–4 sacral ribs (not Herrerasaurus, which has 2);
  10. mid-caudal centra 3x longer than tall where known (not Herrerasaurus);
  11. clavicles poorly ossified or absent (reappear in later taxa);
  12. interclavicle poorly ossified or absent;
  13. manual unguals long with penultimate phalanges longer than proximal phalanges;
  14. tibia not shorter than femur (not Herrerasaurus);
  15. fourth trochanter sharp (also in the proximal dinosaur outgroup taxon PVL 4597);
  16. tibia not < 2x ilium length (not Herrerasaurus);
  17. advanced and simple metatarsal joint without a calcaneal tuber (also in Lewisuchus, convergent in Silesaurus);
  18. compact metatarsus;
  19. longest metatarsal: #3;
  20. metatarsal #1 < 75% of metatarsal #3 length;
  21. metatarsals 2–4 ≥ tibia (not Herrerasaurus);
  22. proximal metatarsals 1 and/or 5 reduced in diameter.

Summary and significance
Arising from the morphology of basal bipedal crocodylomorphs, many of these basal dinosaur traits document a longer, leaner bipedal morphology with an increasingly robust fulcrum (pelvis and sacrum), along with longer distal hind limb elements with more reliance on the middle digits for faster locomotion. By contrast the proximal outgroup clade, Junggarsuchus and kin, documents a return to quadrupedalism terminating with Trialestes and its long forelimbs. The increasingly elongate radiale and ulnare of the Junggarsuchus clade turn out to be convergent, rather than homologous, with those of other bipedal crocodylomorphs, like Terrestrisuchus and Hesperosuchus, a clade that also transitioned toward a quadrupedal configuration, as seen in extant crocodylomorphs. We’ll take a closer look at this clade and this transition later this week.

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined.

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.

 

Tulerpeton pes options

Updated Dec 13, 2017, with a re-nesting of Tulerpeton between Ichthyostega and Eucritta.

Earlier the pedal elements of the amphibian-like reptile Tulerpeton were moved around to produce a reasonable reconstruction. Today I offer a few more options (Fig. 1) including one with six toes. All appear to be reasonable.

Figure 1. Tulerpeton pes reconstruction options using published images of the in situ fossil.

Figure 1. Tulerpeton pes reconstruction options using published images of the in situ fossil.

Such long toes
with so many phalanges in these patterns of relative length are not found in basal tetrapods. They hint at reptiles to come, able to clamber about over obstacles.

Figure 1. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2.

Figure 2 Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2.

A little backstory
Tulerpeton curtum (Lebedev 1984, Fammenian, Latest Devonian, 365 mya) was described as, “one of the first true tetrapods to have arisen.” Here it nests between Ichthyostega and Eucritta + Seymouriamorpha. Very little other than the limbs are known. In life it would have been similar to and the size of Gephyrostegus, Urumqia and EldeceeonTulerpeton lived in shallow marine waters.

References
Coates MI and Ruta M 2001 2002. Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Lebedev OA 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407–1413.
Lebedev OA and Clack JA 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology36: 721-734.
Lebedev OA and Coates MI 1995. postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev. Zoological Journal of the Linnean Society. 114 (3): 307–348.

wiki/Tulerpeton

 

 

Teleocrater: a sister to Yarasuchus, not the earliest bird-line archosaur.

Unfortunately
another paper that was improperly vetted (refereed). I heard about this one on NPR when co-author Richard Butler was interviewed by the BBC.

Nesbitt et al. 2017
report: The relationship between dinosaurs and other reptiles is well established (1–4,) but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies.”

We’re in trouble from the opening salvo.
The large reptile tree (LRT) does not recover the same tree topology as these authors hypothesize (Fig. 1). And the sequence of dinosaurian features is not obscured in the LRT. There are plenty of fossils with transitional morphologies. Unfortunately, these authors either chose to ignore them or scored them haphazardly. Based on the theory that evolution happens with small changes the Nesbitt et al. tree topology (Fig. 1) is completely bonkers, adding unrelated taxa while excluding pertinent sisters to Teleocrater (here labeled under its new clade, Aphanosauria).

Figure 1. Aphanosauria according to Nesbitt et al. 2017. Two of these clades are unrelated to archosaurs. Marasuchus IS a dinosaur. Silesaurus is a poposaur more distantly related to dinos than crocs.

Figure 1. Aphanosauria according to Nesbitt et al. 2017. Two of these clades are unrelated to archosaurs. Marasuchus IS a dinosaur. Silesaurus is a poposaur more distantly related to dinos than crocs. Where are the crocs?

Nesbitt et al. report:
“H
ere we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch.” There is no such thing as an avian stem lineage. Avemetarsalia includes pterosaurs and dinosaurs, so it is a junior synonym for Reptilia in the LRT. The closest ancestors to dinosaurs were bipedal basal crocodylomorphs in the LRT. I don’t see them in figure 1. 
Teleocrater holotype.
NHMUK (N
atural History Museum, London, UK) PV 
R6795, a disassociated skeleton of one individual, including: cervical, trunk, and caudal vertebrae, partial pectoral and pelvic girdles, partial forelimb and hind limbs. 
Referred material.
Elements f
ound near the holotype, but from other 
individuals, which represent most of the skeleton and that are derived from a paucispecific bone bed containing at least three individuals.
Figure 2. The chimaera created by several specimens attributed to Telocrater. Even if all these piece do fit together like Nesbitt et al. indicate, Telocrater is closer to Yarasuchus and Ticinosuchus than it is to the last common ancestor of Archosauria.

Figure 2. The chimaera created by several specimens attributed to Teleocrater. Even if all these piece do fit together like Nesbitt et al. indicate, Teleocrater is closer to Yarasuchus and Ticinosuchus than it is to the last common ancestor of Archosauria. See figure 3.

The specimens that produced this reconstruction (Fig. 2)
are all associated. So there is great confidence that all of the bones are conspecific. The problem, once again, is taxon exclusion, and maybe a large dose of bad scoring (see below)

Figure 3. Telocrater to scale compared with likely sister taxa among the Ticinosuchidae in the LRT. Note the resemblance of the Telocerater maxilla to that of these sister taxa.

Figure 3. Teleocrater to scale compared with likely sister taxa among the Ticinosuchidae in the LRT. Note the resemblance of the Teleocerater maxilla to that of these sister taxa.

Oddly
the authors report that “Osteoderms are not preserved and were probably absent.” And yet their reconstruction (Fig. 2) has osteoderms in the black outline. What bias is present here?

Oddly
the authors report, Our phylogenetic analyses recovered Teleocrater in a clade containing Yarasuchus, Dongusuchus and Spondylosoma.” And yet they did not include Yarasuchus in their phylogenetic figure (Fig. 1). The latter two are know from scraps. Yarasuchus (Fig. 3) is much more complete. 

Problems with the Nesbitt et al. 2017 cladogram

  1. The outgroup for Prolacerta + Archosauriformes (Proterosuchus) is the unrelated lepidosaur, Mesosuchus.
  2. The unrelated thalattosaur, Vancleavea, nests between Erythrosuchus and the unrelated chanaresuchid, Tropidosuchus. None of these taxa even look alike!
  3. The Yarasuchus clade, and before it the Parasuchus clade gives rise to the pterosaurs DimorphodonEudimorphodon and another chanaresuchid, Lagerpeton, both purportedly in the lineage of dinosaurs. These are all actors pretending to be relatives. How is this possible that Nesbitt et al, and the referees and editors at Nature are not raising objections to this? This is total madness at the highest levels.

Need I go on???
Why is Teleocrater big news? Because the authors positioned it as an ancestor to dinosaurs. It may be, but it is buried deep, deep, deep in the lineage. Why was the relationship with Yarasuchus buried? You know why… it’s not as ‘sexy’ to the press.

Nesbitt et al. report,Aphanosauria…is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia.” Surprised to see they didn’t say, ‘pterosaurs and Tyrannosaurus rex.’ 

Folks, it’s all showmanship.
I’m sure the authors have all toasted their new paper in Nature by now. I hate seeing the subject of evolution twisted, torn and laid bare like this.

The real importance of Teleocrater
is its basal position in a clade I earlier called Ticinosuchidae, arising from basal rauisuchians, like Vjushkovia, giving rise to a wide variety of taxa like Aetosaurus and Arizonasaurus) while also giving rise to Decuriasuchus, which gave rise to poposaurs, like Turfanosuchus, and archosaurs, thus ultimately including dinosaurs.

References
Nesbitt SJ et al. (10 co-authors) 2017. The earliest bird-line archosaurs and the assemblof the dinosaur body plan. Nature doi:10.1038/nature22037. (online pdf)

1. Benton, M. J. & Clark, J. M. in The Phylogeny and Classification of the Tetrapods.
Volume 1: Amphibians, Reptiles, Birds. Systematics Association Special Volume
35A (ed. Benton, M. J.) 295–338 (Clarendon, 1988).
2. Gauthier, J. Saurischian monophyly and the origin of birds. Mem. Calif. Acad.
Sci. 8, 1–55 (1986).
3. Sereno, P. C. Basal archosaurs: phylogenetic relationships and functional
implications. Soc. Vertebr. Paleontol. Mem. 2, 1–53 (1991).
4. Sereno, P. C. The evolution of dinosaurs. Science 284, 2137–2147 (1999).

The cervical/dorsal transition in Herrerasaurus

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined.

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined in the drawing. See text for details. Drawings from Novas and Sereno 1994.

In their report on the basal dinosaur Herrerasaurus,
Sereno and Novas 1993 reported, “The cervical column (Fig. 1) was preserved in articulation with the skull. The anterior cervical vertebrae are better preserved than the posterior cervical vertebrae, and nearly all the ribs are lacking.”

“Because the cervical-dorsal transition in vertebrae or ribs is not preserved, we regard the first ten presacral vertebrae as cervical vertebrae, based on the condition in other basal dinosaurians.”

Despite their assessment, 
Novas and Sereno appear to have reconstructed Herrerasaurus with no more than six or possibly seven cervicals (Fig. 1, original).  With phylogenetic scoring at issue, a deeper look was warranted.

Novas and Sereno 1993 considered Herrerasaurus
a member of the ‘Ornithodira’ thus related to pterosaurs and Lagerpeton. That hypothesis is not supported by the present study.

By contrast,
in the large reptile tree (LRT) Herrerasaurus arises from another list of taxa, including Junggarsuchus, Pseudhesperosuchus, LewisuchusTurfanosuchus and further distantly Decuriasuchus. Bittencourt et al. 2014 identify seven cervicals in Lewisuchus (Fig. 1) with the eighth having ribs descending into the torso. Seven is a number common to crocodylomorpha* and Lewisuchus nests at its base. Turfanosuchus and Decuriasuchus each have eight cervicals, a plesiomorphic number going back at least to basal archosauriformes, like Proterosuchus and to basal diapsids, like Petrolacosaurus.

Based on available data,
Herrerasaurus had but seven cervicals as a basal dinosaur. Based on data from Tawa, Marasuchus, Eodromaeus and Eoraptor, all slightly more derived basal dinosaurs had 9 or 10.

* Among Crocodylomorpha, Scleromochlus has six cervicals, Gracilisuchus, Litargoschus and Terrerstrisuchus have eight, as do modern crocs and their relatives by convergence. Intervening taxa often have seven.

References
Bittencourt JS, Arcucci AB, Maricano CA and Langer MC 2014. Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chañares Formation, Argentina) its inclusivity, and relationships amongst early dinosauromorphs. Journal of Systematic Palaeontology. Published online: 31 Mar 201. DOI:10.1080/14772019.2013.878758
Nesbitt SJ. et al. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464(7285):95-8
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchusadmixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13
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.

wiki/Lewisuchus
wiki/Herrerasaurus

 

Ichthyostega’s toes – evidence of regeneration?

Figue 1. The pes (foot) of Ichthyostega has 7 digits. Those five that most parsimoniously match related taxa are  listed. The vestigial digit between 2 and 3 may be the result of injury and rejuvenation.

Figue 1. The pes (foot) of Ichthyostega has 7 digits. Those five that most parsimoniously match related taxa are listed. The vestigial digit between 2 and 3 may be the result of injury and imperfect or unfinished regeneration.

You might remember
earlier the basal tetrapod Ichthyostega (Fig. 1) shifted its nesting closer to Proterogyrinus (Figs. 2, 3) and Eucritta (Fig. 4) at the base of the Reptilomorpha. One of the reasons for that shift was a reexamination of the pes of Ichthyostega, which has seven digits. Which digits are homologous with the five that are found in many other higher tetrapods?

Figure 2. Proterogyrinus pes according to Holmes.

Figure 2. Proterogyrinus pes according to Holmes.

Metatarsal and phalangeal proportions 
provide clues. If the above digit identities ares used, there is a pretty close match to related taxa. Acanthostega, for instance, has eight pedal digits with metatarsal 3 about twice as long as the more medial metatarsals. Distinct from Ichthyostega, Acanthostega has only one phalanx on digit 1 and only 2 phalanges on digit 2, but in keeping with the ‘one less’ phalangeal formula, digits 3–7 stop at 3 phalanges. In Ichthyostega digits 4 and 5 each add a phalanx, approaching the pattern seen in Proterogyrinus.

Figure 3. Proterogyrinus pedes in situ (black) and restored (blue).

Figure 3. Proterogyrinus pedes in situ (black) and restored (blue).

Holmes 1984
reconstructed the pes of Proterogyrinus (Fig. 2). If one takes the data from in situ drawings provided by Holmes (Fig. 3), reconstructions of both pedes can be created to check the accuracy of the Holmes reconstruction while removing any freehand bias.

Figure 4. Eucritta in situ and reconstructed. Note the large pes in green.

Figure 4. Eucritta in situ and reconstructed. Note the large pes in green.

The pes of the related Eucrtta also bears another look.
It is more difficult to reconstruct based on the taphonomic scattering of the elements. If you’ll notice the medial three digits of Eucritta each appear to have one less phalanx, as in Acanthostega.

 Which makes one wonder about Ichthyostega.
The vestigial digit between 2 and 3 in particular gives one pause. We know that salamanders can regrow their extremities. Based on the unusual apparent binding of pedal digits 1 and 2 in Ichthyostega, along with the vestige of a digit between 2 and 3, One may wonder if that unusual morphology is the result of an accident or injury with subsequent imperfect or unfinished regeneration. Another identical Ichthyostega pes would falsify this hypothesis.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Clack JA 1998. A new Early Carboniferous tetrapod with a mélange of crown group characters. Nature 394: 66-69.
Clack JA 2007. Eucritta melanolimnetes from the Early Carboniferous of Scotland, a stem tetrapod showing a mosaic of characteristics. Transactions of The Royal Society of Edinburgh 92:75-95.
Holmes R 1984. The Carboniferous Amphibian Proterogyrinus scheelei Romer, and the Early Evolution of Tetrapods. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 306: 431-524.
Jarvik E 1952. On the fish-like tail in the ichthyostegid stegocephalians. Meddelelser om Grønland 114: 1-90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Romer AS 1970. A new anthracosaurian labyrinthodont, Proterogyrinus scheelei, from the Lower Carboniferous. Kirtlandia 10:1-16.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proc. R. Soc. Lond. B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.

wiki/Ichthyostega
wiki/Eucritta
wiki/Proterogyrinus

Diplovertebron vs. Gephyrostegus

Updated June 13, 2017 with the realization that Watson’s 1926 Diplovertebron is the same specimen as Gephyrostegus watsoni (bohemicus). 

This blog had its genesis in a reader comment
that considered the taxon, Diplovertebron congeneric with the coeval Gephyrostegus bohemicus and G. watsoni (Fig. 1), echoing earlier authors. Although there may be some confusion here (see below), and several specimens have been attributed to Gephyrostegus by various authors, the specimen illustrated and labeled by Watson 1926 (Fig. 1) is not one of them, unless it was drawn very poorly. If anyone has in situ skeletal material, please send it along for an update.

Gleaning data from several papers, provided that update. 

Part of my confusion
lies in the Wikipedia article on Diplovertebron, which states it was 60 cm in length, at least 5x larger than the one illustrated by Watson and far larger than any of its sister taxa. There may be a paper I am unfamiliar with at present that clarifies the matter.

So far, I have not found it. 60 cm may be an error.

The Westphalian (310 mya) tetrapods
include some reptile-like amphibians and some amphibian-like reptiles. This strata is 30 million years younger than the Viséan, where members from the first great radiation of reptiles can be found. Several late-survivors of earlier radiations can still be found in Westphalian strata.

Earlier G. watsoni nested among basal archosauromorpha, apart from G. bohemicus at the base of the Reptilia and separated by Eldeceeon. So the three taxa in figure 1 are separated from each other by intervening genera and therefore cannot be congeneric.

With present data, flawed though it may be
Diplovertebron nests in the large reptile tree (LRT) with Utegenia, at the base of the Lepospondyli, the clade that ultimately gives us frogs, like Rana, salamanders, like Andrias, and caecilians, like Dermophis.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. None of these are congeneric.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. to scale  None of these are congeneric. That’s because Watson’s drawing (upper left) was poorly drafted. 

Revised backstory:
Diplovertebron punctatum (Fritsch 1879, Waton 1926; DMSW B.65, UMZC T.1222a; Moscovian, Westphalian, Late Carboniferous, 300 mya) aka:  Gephyrostegus watsoniBrough and Brough 1967) and  Gephyrostegus bohemicus (Carroll 1970; Klembara et al. 2014) after several name changes perhaps this specimen should revert back to its original name as it nests a few nodes away from Gephyrostegus.

Derived from a sister to EldeceeonDiplovertebron was basal to the larger Solenodonsaurusand the smaller BrouffiaCasineria and WestlothianaDiplovertebron was a contemporary ofGephyrostegus bohemicus, Upper Carboniferous (~310 mya), so it, too, was a late survivor.

Overall smaller and distinct from Eldeceeon, the skull of Diplovertebron had a shorter rostrum, larger orbit and greater quadrate lean. The dorsal vertebrae formed a hump and had elongate spines. The hind limbs were much longer than the forelimbs. The tail is incomplete, but appears to have been short and deep.

Seven sphere shapes were preserved alongside this specimen. They may be the most primitive amniote eggs known.

Watson 1926 attempted a freehand reconstruction (see below) that was so different from this specimen that for a time it nested as a separate taxon, now deleted.

References
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–165. doi:10.1098/rstb.1967.0006
Carroll RL 1970. 
The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Jaeckel O 1902. Über Gephyrostegus bohemicus n.g. n.sp. Zeitschrift der Deutschen Geologischen Gesellschaft 54:127–132.
Klembara J, Clack J, Milner AR and Ruta M 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proceedings of teh Royal Society, London B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Gephyrostegus
wiki/Diplovertebron

 

Pushback on ‘Dawndraco’ (Pteranodon UALVP 24238)

Figure 1. Pteranoodn (Dawndraco) UALVP 24238 in situ, with Martin-Silverstone tracing applied, with mandible moved and missing parts colorized. The putative rostral tip looks more like displaced manus elements.

Figure 1. Pteranoodn (Dawndraco) UALVP 24238 in situ, with Martin-Silverstone tracing applied, with mandible moved and missing parts colorized. The putative rostral tip looks more like displaced manus elements. The crest and distal wing finger do not belong to the original specimen.

A new paper by Martin-Silverstone et al. 2017
disputes the earlier study by Kellner 2010, giving a new generic name to a well-preserved putative Pteranodon specimen, UALVP 24238, Figs. 1-3). They also write: “The re-evaluation of Pteranodon sensu lato by Kellner (2010) is troubling for pterosaur palaeontology, as so much of our understanding of pterosaur ontogeny and growth stem from Bennett’s work on Pteranodon and the conclusion that Pteranodon specimens can be divided into two closely and perhaps anagenetically related species.” Bennett’s conclusions were disputed earlier here, here and here, and are nowhere in evidence here (Fig. 2). Praise for Bennett’s work needs to be limited to those items that stand the tests of closer scrutiny and analysis, Gender and ontogenetic differences recovered in Bennett’s statistical analyses are not recovered in phylogenetic analysis.

Figure 2. The Tanking-Davis specimen compared to other forms. Specimen w and specimen z appear to be the closest to the Tanking-David specimen. Specimen 'w' = Pteranodon sternbergi? USNM 12167 (undescribed). Specimen 'z' = Pteranodon longiceps? Dawndraco? UALVP 24238. Click to enlarge.

Figure 2. The Tanking-Davis specimen compared to other forms. Specimen w and specimen z appear to be the closest to the Tanking-David specimen. Specimen ‘w’ = Pteranodon sternbergi? USNM 12167 (undescribed). Specimen ‘z’ = Pteranodon longiceps? Dawndraco? UALVP 24238. Click to enlarge.

As you can see (Fig. 2) NONE
of the known Pteranodon-grade skulls would be considered conspecific in the modern world, and few would be considered congeneric. Size and crest size differences are without a doubt phylogenetic (contra Bennett and Martin-Sivlerstone et al.) as demonstrated in the large pterosaur tree. You can’t get large or have a large crest without evolving from smaller progenitors. It may also be the case that Pteranodon, like pterosaurs in general were extremely individually variable within a genus, but we’d need a time machine or a mass fossil assemblage for that.

Moreover,
UALVP 24238 had a tiny cranium, very different from the large cranium of P. sternbergi (FHSM VP 339, Fig. 2).

Figure 3. The UALVP specimen of Pteranodon. Note the lack of taper in the rostrum along with the small size of the orbit.

Figure 3. The UALVP specimen of Pteranodon. Note the lack of taper in the rostrum along with the small size of the orbit.

From the Martin-Silverstone et al. 2017 abstract:
“The previous most comprehensive study on Pteranodon [Bennett 1991, 1992m 19994, 2001] recognized two species: P. longiceps and P. sternbergi, but complete skeletons of Pteranodon are rare. One of the best preserved (UALVP 24238) has been identified as both P. sternbergi and as a new genus and species, Dawndraco kanzai. Here, the specimen is redescribed, additional portions of the rostrum are identified for the first time, new details of the specimen’s provenance and preparation history are presented, and its taxonomic placement is discussed. Whereas the shape of the rostrum appears at first glance to distinguish it from known Pteranodon, this feature is more parsimoniously interpreted in the context of sexual dimorphism; a male has a longer and therefore more shallowly tapering rostrum. Metrics from this specimen, and from published photographs and illustrations, support the conclusion that the rostrum of UALVP 24238 is not unique, and so provides no grounds for recognition of a taxon distinct from Pteranodon sternbergi. Other putatively unique features of UALVP 24238 are examined and found unconvincing.”

The rostrum is not the key trait that separates
UALVP 24238 from P. sternbergi (Fig. 2). It’s the cranium (among comparable elements preserved). The two species are related, but not conspecific. A phylogenetic analysis would have been helpful here. A set of skull reconstructions would have made things clear. Both are lacking from the new Martin-Silverstone study.

References
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 1992. Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Bennett SC 1994. Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occassional Papers of the Natural History Museum University of Kansas 169: 1–70.
Bennett SC 2001. The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153.
Kellner AWA 2010. Comments on the Pteranodontidae (Pterosauria, Pterodactyloidea)
with the description of two new species. Anais da Academia Brasileira de Ciências 82(4): 1063-1084.
Martin-Silverstone E, Glaser JRN, Acorn JH, Mohr S and Currie PJ 2017. Reassesment of Dawndraco kanzai Kellner, 2010 and reassignment of the type specimen to Pteranodon sternbergi Harksen, 1966.  Vertebrate Anatomy Morphology Palaeontology 3:47-59.
Marsh OC 1876a. Notice of a new sub-order of Pterosauria. American Journal of Science, Series 3, 11:507-509.
Miller HW 1971. A skull of Pteranodon (Longicepia) longiceps Marsh associated with wing and body parts. Kansas Academy of Science, Transactions 74(10):20-33.

wiki/Pteranodon

Basal tetrapods revised with more taxa

Full resolution during a Heuristic search was not enough.
Full resolution with high Bootstrap scores was the goal. Reexamination of the data would hopefully get to that goal, as it did so many times before. Sometimes it takes awhile. It’s a learning process, and I learned a lot over the last several weeks, sometimes from difficult and scrappy data. Here’s the result:

Figure 1. Subset of the large reptile tree (LRT) focusing on basal tetrapods.

Figure 1. Subset of the large reptile tree (LRT) focusing on basal tetrapods.

Some interesting results here. 

  1. Large temnospondyls are now split in two  (with, as before, many former small temnospondyls joining the equally small lepospondyls).
  2. Ichthyostega, now not so primitive, nests closer to Reptilomorpha.
  3. New reconstructions are offered for some taxa, like Tuditanus and Utaherpeton.
  4. Basal diplocaulids, like Keraterapeton, were added.
  5. Two taxa known as Trematosaurus, one with a shorter rostrum, one with a longer one, are split apart on the tree. Gavial-like snouts are not monophyletic at present, but long-nouted forms do not have long snouts as juveniles. This is a well-known quagmire I may get into later.

Look for more basal tetrapods with legs, not fins in the Late Devonian.
Not sure where they are, but they are out there. Apparently there were several ventures onto land, not just one fin-to-finger transition.

In a few days
I’ll start with some of the interesting details as time allows, but basically this completes the task, the tree, and the broad strokes that hypothetically echo the origin of reptiles and the variation that followed thereafter.