Patterns in T-rex scales

The big news last night
was all about the scale patches found on a T-rex skeleton (Bell et a. 2017). This one (Fig. 1) is from the dorsal neck. It may or may not be from the midline.

Figure 1. GIF animation, 3 rounds, about 24 frames adding possible pattern overlays to the patch.

Figure 1. GIF animation, 3 rounds, about 24 frames adding possible pattern overlays to the patch. Photo from Bell et al. 2017. If this scale patch evolved from feathers only the visible branching pattern is similar. Distinct from feathers, each scale has its own base in the epidermis. The largest of these scales are each slightly longer than a centimeter in length.

The paper focused on
various aspects of theropod integument, chronicling taxa with feathers and others with scales along with the origin of scales from feathers in certain theropods. The paper also nested feathered Yutyrannus basal to tyrannosaurs. In contrast, the large reptile tree (LRT) nests feathered and winged Zhenyuanlong  basal to tyrannosaurs, Yutyrannus closer to Allosaurus.

Here
(Fig. 1) I applied colors to apparent patterns in the scale patch. Not sure what they mean. Overall this patch reminds me of a town with one main street, several side streets and dozens of single resident plots, each a little more than 1 cm on a side. Each scale has its own base in the epidermis. Were they derived from feathers? The scales provide no clue to that origin. The medial symmetry shown here may be a result of this patch lying on the midline (sagittal plane).

References
Bell et al. (6 other authors) 2017. Tyrannosaurid integument reveals conflicting patterns of gigantism and feather evolution. Biology Letters 13: 20170092. http://dx.doi.org/10.1098/rsbl.2017.0092

More PVL 4597 post-crania

Lecuona et al. 2017 
redescribe the post-crania of the basal archosaur and basal Crocodylomorph (in the large reptile tree = LRT), Gracilisuchus. They used six specimens and recovered them as basal Suchia (= aetosaurs, rauisuchians and crocodylomorphs). The LRT does not recover these three monophyletic clades in one larger monophyletic clade. So the LRT does not support the ‘Suchia’ as a clade.

Figure 1. The PVL 4597 specimen attributed to Gracilisuchus by Lecuona et al. 2017, but nesting at the base of the Dinosauria in the LRT.

Figure 1. The PVL 4597 specimen attributed to Gracilisuchus by Lecuona et al. 2017, but nesting at the base of the Dinosauria in the LRT. The pelvis of Herrerasaurus is shown on frame 2 of this simple animation. 

Unfortunately
Lecuona et al. excluded from their analysis several taxa that nest close to Gracilisuchus in the large reptile tree. These include Saltopus and Scleromochlus, which nest as sisters to Gracilisuchus in the LRT. Lewisuchus is mentioned in the text and combined as a chimaera with Pseudolagosuchus, but I don’t see the combo in the published trees. Their cladogram includes several suprageneric taxa (always to be avoided) including ‘Pterosauromorpha’ (= pterosaurs + Scleromochlus, not recovered as a clade in the LRT as a clade and how would one score such an internally varied taxon??).

Lecuona et al. nest their purported Gracilisuchus specimens together.
By contrast the LRT nests PVL 4597 at the base of the Dinosauria.

Fortunately 
the authors provide more data on the provisional dinosaur outgroup taxon, PVL 4597, so far based on hind limb traits only (Lecuona and Desojo 2011. The preserved skull in the specimen has not yet been published.

As in the LRT
the authors also find a close relationship between Turfanosuchus and Gracilisuchus. Unlike the LRT, they nest Yonghesuchus between them. The LRT nests it as a sister to Dromicosuchus.

It’s not common for a specimen to be published in bits and pieces
The original pelvis data came out 6 years ago. The new data for PVL 4597 (Fig. 1) still lacks the skull, which will be published in the future. I have written to Dr. Lecuona encouraging an expansion of the taxon list. I also hope the suprageneric taxa will be broken up into lists of genera.

References
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.
Lecuona A, Desojo JB and Pol D 2017.
New information on the postcranial skeleton of Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position. Zoological Journal of the Linnean Society zlx011 1–40.

Aetiocetus, Desmostylus and the origin of baleen

Aetiocetus
(Emlong 1966; late Oligocene, 30 mya; Fig. 1) has been the poster-child for basal baleen whales for several decades, embraced by all whale workers. That needs to change.

When Aetiocetus was first published,
Emlong wrote:“If it were not for the presence of functional teeth on this mature specimen, this cetacean could easily be placed in the order Mysticeti.” (= baleen whales). Nevertheless, Emlong placed his discovery within the Archaeoceti, based on its primitive dentition. He also noted the nares had migrated further back on the skull than in typical archaeocetes, though still anterior to the orbits.

In the pre-cladistic era
Van Valen 1968 placed Aetiocetus as a basal mysticete despite the presence of teeth.

Of course, this assumes
that baleen whales were derived from toothed whales, which they are not, according to a wider gamut study, the large reptile tree (LRT, 1011 taxa).

Figure 1. Palates of two baleen whales, one toothed whale and Neoparadoxia, a desmostylian. See text for details.

Figure 1. Palates of two baleen whales, one toothed whale (Aetiocetus) and Neoparadoxia, a desmostylian. See text for details, but note the vomer splitting the maxilla in three related taxa and the pointed premaxilla in only one.

Aetiocetus is remarkable
for having both a full set of teeth and nutrient foramina. According to cetacean workers,t he presence of nutrient foramina (tiny holes and grooves in the palatal portion of the maxilla) indicates the presence of baleen in Aetiocetus. That’s because nutrient foramina are otherwise absent in archaeocetes and odontocetes (so far). In baleen whales some of the lateral foramina are located in the alveolar grooves, where the tooth roots used to be and where the teeth are in Aetiocetus. Other nutrient foramina radiate along the roof of the mouth (palatal portion of the maxilla). What Aetiocetus was growing there, we may never know. Based on the LRT, that soft palatal tissue in Aetiocetus was not homologous with baleen, though it would have been convergent.

According to Wikipedia
Aetiocetus shares several traits with all mysticetes. The mandibular symphysis is not fused. The descending process of the maxilla becomes a toothless plate below the orbit. A wide rostrum is present.” There is no doubt that among toothed whales, aetiocetes share more traits with baleen whales than any other toothed whales. However, distinct from prior studies, the LRT permits baleen whales to nest wherever they want to. And they don’t want to nest with toothed whales.

Re: Aetiocetus traits
It should be noted that even though the mandibular symphysis is not fused, it is narrow with parallel tips, unlike the anteriorly wide jaws of mysticetes and desmostylians. As in Aetiocetus, sperm whales likewise have a wide palate, at least posteriorly,  Aetiocetes were contemporaries of basal baleen whales like Cetotherium. In the LRT cetotheres are not basal mysticetes, but gray whales are.

We looked at other mysticete traits
and the desmostylian/mysticete connection earlier here, here and here.

The LRT nests Aetiocetus
with NMV P252567 (Marx et al. 2016) and these two nest between the archaeocete Zygorhiza and the extant odontocetes, Orcinus and Physeter. Archaeocetes have teeth of several shapes. Extant odontocetes have simple cones only. The genus Aetiocetus could be phylogenetically transitional in that the species A. weltoni has different tooth types, but A. polydentatus has simple cones only. I say ‘could be’ because those species have not yet been tested in the LRT.

Perhaps overlooked by whale workers 
the desmostylian Neoparadoxia (Fig. 1) also has nutrient foramina in a toothless groove between its anteriorly directed tiny canine and four posterior teeth (premolar #3 and three molars). I am hard pressed to see foramina elsewhere in the palate from available photos. Barnes 2013 wrote: “There are two small centrally placed nutrient foramina in each palatine bone. One is near the maxillary–palatine suture, and the other is located more medial to that one.” Note, Barnes did not report foramina in the maxilla.

Like baleen whales,
the palate is wide and the premaxilla is transversely oriented in Neoparadoxia. The palate already includes a long toothless groove. We don’t know what was growing in that groove. In the heretical LRT Neoparadoxia and other desmostylians are baleen whale outgroups, so that could be where a baleen precursor was growing. The rostrum is downturned slightly. The nares are telescoped to the back creating a blowhole.

Marx et al. nested NMV P252567
within the Aetiocetidae and thought they were shedding new light on the origin of baleen. From their abstract: “Baleen is thought to have appeared in archaic tooth-bearing mysticetes during a transitional phase that combined raptorial feeding with incipient bulk filtering. Here we show that tooth wear in a new Late Oligocene mysticete belonging to the putatively transitional family Aetiocetidae is inconsistent with the presence of baleen, and instead indicative of suction feeding. Our findings suggest that baleen arose much closer to the origin of toothless mysticete whales than previously thought. In addition, they suggest an entirely new evolutionary scenario in which the transition from raptorial to baleen-assisted filter feeding was mediated by suction, thereby avoiding the problem of functional interference between teeth and the baleen rack.” Desmostylians also suck, apparently (see below). And then you don’t have to explain away tooth wear!

Geisler and Sanders 2003
did not consider anthracobunids, desmostylians and tenrecs in their phylogenetic analysis of whales, but instead relied on Sus, the pig, as an outgroup. A larger gamut analysis finds Sus is not related to whales of any sort. When you don’t have the correct outgroups, you can’t place confidence in the order of appearance of derived traits and the order of derived taxa.

Figure 3. Old, toothless Desmostylus mandible with single downturned canine compared to the empty alveolus and mandibles of the gray whale (Eschrichtius).

Figure 2. Old, toothless Desmostylus mandible with single downturned canine compared to the empty alveolus and mandibles of the gray whale (Eschrichtius).

Santos, Parham and Beatty 2016
described an old toothless (save for one canine) Desmostylus mandible (Fig. 2). It bears comparison to the mandible of Eschrichtius, the gray whale, which has an anterior alveolus for an absent canine. Feeding strategies were probably similar. Wikipedia reports, “The (gray) whale feeds mainly on benthic (= sea floor) crustaceans, which it eats by turning on its side …and scooping up sediments from the sea floor.”  Desmostylians were also sea floor feeders, but with their shovel-like jaws, did not have to turn on their sides. Chiba et al 2015, suggests desmostylians used suction to pull in their food, and did not chew it.

Figure 4. Comparison of several desmostylian mandibles with that of Eschrichtius, the gray whale. As this lineage of desmostylians get larger, they more closely match the mandible of the gray whale.

Figure 3. Comparison of several desmostylian mandibles with that of Eschrichtius, the gray whale. As this lineage of desmostylians get larger, they more closely match the mandible of the gray whale. Notice the increasing extent of toothlessness and the gradual lengthening of the mandible. Drawings from Chiba et al. 2015.

Chiba et al. 2015
were kind enough to include a set of desmostylian mandibles to scale (Fig. 3) which, as they grew phyllogenetically larger, more closely approximated the much larger mandible of the gray whale, Eschrichtius, an extant basal baleen whale. That Sanjussen specimen, in particular, comes as a confirming relegation! (Unfortunately this was completely lost on Chiba et al.)

Hypothetical evolution of a feeding strategy
If desmostylians were not vegetarians, but fed by digging their anterior tusks into crustacean-laden sediments, then forcing out the excess water before swallowing, they were essentially doing the same sort of benthic feeding as the gray whale.

But what about those odd-looking desmostylian teeth that disappear in old age?
When individual desmostylians were younger and smaller, their benthic prey would have been relatively larger and teeth may have been necessary for crushing the hard shells before swallowing them. On the other hand, older and larger specimens did not need molars because they could swallow their tiny prey without chewing. In desmostylians even a little bit of baleen would have improved the filtering ability. That humble genesis would ultimately evolve to become the giant strips of baleen found in giant mysticetes as the teeth disappeared phylogenetlcally (Fig. 3), not side-by-side with sharp teeth, as imagined in Aetiocetus.

Figure 4. Baleen highlighted in this gray whale skull. Brighter green is below the gum line.

Figure 4. Baleen highlighted in this gray whale skull. Brighter green is below the gum line.

Next steps in this feeding strategy hypothesis:
Baleen whales, other than the gray whale, have left their benthic feeding grounds for the open seas where some fill their expandable throats with sea water and sieve for prey as that volume is forced out past their lips. Others. like the right whale, use their tongue to force out the sea water through deeper baleen filters. Still others probably perform a combination of the two, tongue and throat.

Time to clear out a few Wikipedia misconceptions

  1. Wikipedia reports, “Desmostylians are the only known extinct order of marine mammals.” — not supported by the LRT. Mysticeti are a clade within Desmostylia.
  2. Wikipedia reports, “The Desmostylia, together with Sirenia and Proboscidea (and possibly Embrithopoda), have traditionally been assigned to the afrotherian clade Tethytheria” — not supported by the LRT. Desmostylia are related to anthracobuinds, Hippopotamus and mesonychids in order of increasing distance. Not sure why this isn’t obvious.
  3. Wikipedia reports, “Aetiocetus is a genus of extinct basal mysticete, or baleen whale.” — not supported by the LRT. Aetiocetus nests between archaic toothed whales and extant toothed whales in the LRT.

Next time someone runs a cladistic analysis on whales,
please use the cetacean taxa and outgroups recovered by the LRT to check their validity. If they are again excluded from future studies the present LRT hypothesis of relationships will never have a chance to be verified or become the consensus. Let’s not let pigs stay the ancestors of whales when we already know better!

References
Barnes LG 2013. A new genus and species of Late Miocene Paleoparadoxiid (Mammalia, Desmostylia) from California. Contributions in Science 521:51-114.
Chiba K et al. 2015. A new desmostylian mammal from Unalaska (USA) and the robust Sanjussen jaw from Hokkaido (Japan), with comments on feeding in derived desmostylids. Historical Biology 28(1-2): 289 DOI: 10.1080/08912963.2015.1046718
Cope ED 1872. Descriptions of some new Vertebrata from the Bridger Group of the Eocene. Proceedings of the American Philosophical Society 12:460-465
Ekdale EG and Berta A 2015. Vascularization of the gray whale palate (Cetacea, Mysticeti, Eschrichtius robustus): Soft tssue evidence for an alveolar source of blood to baleen. The Anatomical Record Advances in Integrative Anatomy and Evolutionary Biology. February 2015; DOI: 10.1002/ar.23119
Emlong D 1966. A new archaic cetacean from the Oligocene of Northwest Oregon. Bulletin of the Museum of Natural History, University of Oregon. 3: 1–51.
Geisler JH and Sanders AE 2003. Morphological evidence for the phylogeny of Cetacea. Journal of Mammalian Evolution. 10: 23–129. doi:10.1023/A:1025552007291
Gray JE 1864. “Eschrichtius“. Annals of the Magaztine Natural History. 3 (14): 350.
Kimura T and Ozawa T 2002. A new cetothere (Cetacea: Mysticeti) from the early Miocene of Japan. Journal of Vertebrate Paleontology 22(3):684-702
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Reinhart RH 1959. A review of the Sirenia and Desmostylia. University of California Publications in Geological Sciences 36(1):1–146.
Santos G, Parham J and Beatty B 2016. New data on the ontogeny and senescence of Desmostylus (Desmostylia, Mammalia). Journal of Vertebrate Paleontology. doi: 10.1080/02724634.2016.1078344
Van Valen L 1968. Monophyly or diphyly in the origin of whales. Evolution. 22 (1):37–41.

wiki/Balaenoptera
fossilworks/Isanacetus
wiki/Eschrichtius
wiki/Paleoparadoxia
wiki/Neoparadoxia

Saltopus preserves early archosaur skin and scales

Basal Crocodylomorpha

Figure 1. Basal Crocodylomorpha, including Gracilisuchus, Saltopus, Scleromochlus and Terrestrisuchus

The basal archosaur
(crocs + dinos) Saltopus (von Huene 1910; Late Triassic; ~210 mya, ±60 cm long; Figs. 1–3) poorly preserves bones, but also preserves some scaly skin.

Figure 2. Saltopus skin and scales surrounding the right pelvis.

Figure 2. Saltopus skin and scales surrounding the right pelvis. Not all bones nor all scales are traced here. Benton and Walker 2011 reported no evidence for osteoderms. The bones are hard to delineate and segregate from scales because here they are covered with fossilized desiccated skin. Photo from Benton and Walker 2011 who trace the femoral head extending beneath the pelvis. 

Saltopus nests well within
the Crocodylomorpha and, along with Scleromochlus (Fig. 1), present examples of basal archosaur skin and scales. Even more basal, Gracilisuchus (Fig. 1) had dorsal scutes derived from ancestors going back to the Early Triassic Euparkeria.

Figure 2. Saltopus pelvis latex peel from Benton and Walker 2011. They found two large sacrals. Using DGS I found four sacrals, the same length as the dorsals and causals. Sister taxa have four sacrals.

Figure 2. Saltopus pelvis latex peel from Benton and Walker 2011. They found two large sacrals. Using DGS I found four sacrals, the same length as the dorsals and causals. Sister taxa have four sacrals.

Gracilisuchus and basal dinosaurs
had only two sacral vertebrae, but basal bipedal crocs, like Scleromochlus, double that number. Benton and Walker 2011 traced two sacrals in a latex cast of the sacral area, but each sacral was twice as long as proximal dorsals and causals. Here four sacrals are tentatively identified in the latex peel, all about as long as proximal non-sacral vertebrae.

Dinosaur skin
can be scaly, or naked with feathers, or a combination of the two. Dinosaur scales may be different than croc or lizard scales in that at least some dinosaur scales, like those on the metatarsus of theropods appear to be derived from former feathers.

References
Benton MJ and Walker AD 2011. Saltopus, a dinosauriform from the Upper Triassic of Scotland. Earth and Environmental Science Transactions of the Royal Society of Edinburgh: 101 (Special Issue 3-4):285-299. DOI:10.1017/S1755691011020081
von Huene FR 1910. Ein primitiver Dinosaurier aus der mittleren Trias von Elgin. Geol. Pal. Abh. n. s., 8:315-322.

wiki/Saltopus

Is Phascolotherium a basal mammal? Perhaps not…

Yesterday we looked at the origin of mammals and noted the Rowe 1988 considered the fossil mandible Phascolotherium bucklandi (Middle Jurassic; Owen 1838; Fig. 1) one of the earliest known mammals. Unfortunately, the mandible specimen does not have enough traits to nest Phascolotherium in the large reptile tree (LRT, 1011 taxa) with complete resolution.

Nevertheless,
that doesn’t stop one from visually comparing Phascolotherium to more complete taxa.

Figure 1. Phacolotherium compared to the tritylodontid Jeholodens.

Figure 1. Phacolotherium compared to the tritylodontid Jeholodens. The smaller Jeholodens image is to scale with the much larger Phacolotherium specimen.

There’s a pretty good match for Phascolotherium
with Jeholodens  (Ji et al. 1999); non-mammalian cynodont/tritylodontid/mammaliaform known from the Middle Cretaceous. The Jeholodens mandible is smaller than Phascolotherium, It has an unerupted 4th molar, which would indicate immaturity if it was a mammal. Only mammals do not replace molars.

Of Jeholodens
Ji et al. 1999 reported, “The postcranial skeleton of this new triconodont shows a mosaic of characters, including a primitive pelvic girdle and hindlimb but a very derived pectoral girdle that is closely comparable to those of derived therians. Given the basal position of this taxon in mammalian phylogeny, its derived pectoral girdle indicates that homoplasies (similarities resulting from independent evolution among unrelated lineages) are as common in the postcranial skeleton as they are in the skull and dentition in the evolution of Mesozoic mammals.”

There’s a postscript
Take another look at the mandible of Jeholodens (Fig. 1). Note the giant incisor 1 and the robust jaw articulation. Where else do we see this combination in small mammals? In Multituberculata and Haramiyidae, but both nest with rodents, plesiadapids and carpolesteids in the LRT. Traditional cladograms nest Multituberculata and Haramiyidae either before Mammalia or between monotremes and therians as very basal mammals. I have long wondered, if this was so, which basal pre-mammals or mammals look most like multituberculates and might therefore be most closely related? Could it be Jeholodens? So it was time for a test. Shifting the multituberculates to Jeholodens currently adds 34 steps to the LRT. Let’s see what happens when multis are re-scored with prejudice toward Jeholodens

So what happened?
The multis and haramiyids did not shift. The tree topology did not change. Apparently any resemblance between Jeholodens and these two clades must have been by convergence. Or the amount of convergence is overtaking the true relationship. Since everything in science is provisional we’ll keep testing.

References
Ji Q, Luo Z and Ji S 1999. A Chinese triconodont mammal and mosaic evolution of the mammalian skeleton. Nature 398:326-330. online.
Owen R 1838
. On the jaws of the Thylacotherium prevostii (Valenciennes) from Stonesfield. Proceedings of the Geological Society of London 3, 5–9.

The Origin of Mammals: Rowe 1988

Rowe 1988
provided a list of skeletal traits found in mammals not found in their proximal outgroups. Here they are broken down into digestible categories. Noteworthy are the many traits associated with improvements and refinements to hearing and smelling. Noticeable by their absence are any dental traits.

Facial

  1. Premaxilla internasal process absent (external nares confluent
  2. Ethmoid and maxillary turbinals ossified
  3. Internasal septum ossified
  4. Ossified quadratojugal absent
  5. Sclerotic ossicle absent

Palatal

  1. Ectotympanic horizontal (former reflected lamina rotates from vertical
  2. Squamosal suspensorial notches absent – sites of former connections to quadrate and quadratojugal
  3. Cribiform plate (ethmoid ossifies below olfactory bulbs)
  4. Pterygoid transverse process vestigial (muscles now fill the gap)
  5. Tegmen tympani (thin plate of bone spread over the cochlear capsule forming a new side wall for the cranium)
  6. Hyoid arch evolves to become petrosal bridge

Occipital

  1. Occipital condyles expanded upwards and laterally, far apart from one another
  2. Hindbrain greatly expanded overlies fenestrae vestibuli
  3. Paroccipital process directed ventrally (no longer sloping ventrolaterally)
  4. Pneumatic mastoid process (no longer solid)
  5. Styloid process – no longer a separate bone, the stylohyal fuses the otic capsule, joining the paroccipital process
  6. Craniomandibular joint positioned anterior to fenestra vestibuli (hearing organ opening)

Mandible

  1. Craniomandibular joint formed only by squamosal and dentary
  2. Meckelian sulcus (trough) enclosed – when open it held the post dentary elements
  3. Coronoid bone vestigial or absent – the dentary takes over
  4. Splenial vestigial or absent
  5. Articular, prearticular, surangular and angular suspended from the skull (as tiny ear bones and ectotympanic bulla respectively)

Axial

  1. Proatlas not ossified
  2. Atlas intercentrum and neural arches fused to form one ring-like bone, vertebra #1.
  3. Atlas rib absent (actually fused to the atlas)
  4. Axis prezygopophyses absent
  5. Postaxial cervical ribs fused to vertebrae

Appendicular

  1. Styloid processes on dstial ends of radius, tibia and fibula
  2. Patella present along with patellar facet on femur
  3. Entocuneiform–Hallucial (distal tarsal 1 and m1.1) articulation saddle-shaped permitting greater mobility
  4. Secondary ossifications on long bones and girdles – ossified joints
  5. Flexor sesamoids

Under this guidance
and prior to the use of software in cladisitic analysis Rowe 1988 indicated that
“Morganucodontidae, Kuehneotherium, Dinnetherium, Sinoconodon and Haramiyidae can no longer be considered mammals.” In Rowe’s tree Multituberculata nest between monotremes and metatherians. (Contra Novacek 1997, who nested that clade outside the Mammalia.)

The LRT does not agree with parts of this topology
In the LRT haramiyidans nest with multituberculates, both with rodents. There are no pre-rodent, pre-placental or pre-mammal taxa with such derived traits. Attempts to put a cynodont-like middle ear on the multituberculate Megaconus are largely the product of hope, bias and imagination, not data.

Remember
Living monotremes have tiny ear bones below and internal to the mandible, distinct from placentals and marsupials that have tiny ear bones just posterior the jaw joint. This indicates that monotremes had a separate, but convergent (parallel) evolutionary history with regard to the tiny ear bones. In the LRT. Kuehneotherium nests at the base of the monotremes and thus within Mammalia, at its base. Based on the very derived character of all monotremes, including Kuehneotherium, the very first mammals had a much earlier origin.

According to Rowe 1988
Phascolotheriium bucklandi
(Middle Jurassic, Owen 1838, Clemens et al. 1977) is the oldest known mammal.  Amphitherium (de Blainville 1838) is from the same strata. Both were discovered within the first few decades of modern British paleontology. Unfortunately there are not enough traits in Phascolotherium to nest it in the LRT without massive loss of resolution.

References
Butler P M Clemens, W. A. (2001). Dental Morphology of the Jurassic Holotherian Mammal Amphitherium, with a Discussion of the Evolution of Mammalian Post-Canine Dental Formulae. Palaeontology. 44 (1): 1–20.
Novacek MJ 1997. Mammalian evolution: An early record bristling with evidence. Current Biology 7(8):pR489–R491. DOI: http://dx.doi.org/10.1016/S0960-9822(06)00245-4 
Owen R 1838.
On the jaws of the Thylacotherium prevostii (Valenciennes) from Stonesfield. Proceedings of the Geological Society of London 3, 5–9.
Rowe T 1988.
Definition, diagnosis, and origin of Mammalia. Journal of Vertebrate Paleontology 8(3):241-264.

Liaoningvenator: Bird-like troodontid? Or troodontid-like bird?

Shen et al. 2017 describe
a new troodontid, Liaoningvenator curriei (DNHM D3012; Dalian Natural History Museum; Figs. 1-2; Early Cretaceous), they nest Liaoningvenator outside of the Aves (birds).

Figure 1. Liaoningvenator has a long neck and short torso. It nests as a secondarily flightless bird in the LRT, rather than as a troodontid.

Figure 1. Liaoningvenator has a long neck and short torso. It nests as a secondarily flightless bird in the LRT, rather than as a troodontid.

From the abstract:
“A new troodontid, Liaoningvenator curriei gen. et sp. nov., is described based on a complete skeleton from the Early Cretaceous Yixian Formation of Beipiao City, Liaoning Province. It bears the following characteristics of Troodontidae: numerous and more closely appressed maxillary and dentary teeth; the teeth markedly constricted between the roots and crowns; the nutrient foramina in groove on the external surface of dentary; distal caudal vertebrae having a sulcus on the dorsal midline rather than a neural spine. Unlike other troodontids, Liaoningvenator exhibits a sub-triangular ischial boot in lateral view and slender ischial obturator process; transition point in caudal vertebrae starts from the seventh caudal vertebra. A phylogenetic analysis recovers Liaoningvenator and Eosinopteryx as sister taxa that belong to the same clade.”

Figure 2. Troodontid-like birds and bird-like troodontids shown together to scale.

Figure 2. Troodontid-like birds and bird-like troodontids shown together to scale. Note the robust hind limbs  in the secondarily flightless birds, Jianianhualong and Liaoningvenator.

By contrast,
the large reptile tree (LRT, 1011 taxa) nests Liaoningvenator with Jianianhualong as a large flightless basal sapeornithid bird—and all birds nest within the Troodontidae. Size-wise Liaoningvenator is midway between the smaller Archaeopteryx recurva (Fig. 2) and the larger Jianianhualong. So this might be a transitional taxon between the two.

Unrelated
Eosinopteryx (Fig. 2) continues to nest outside of Aves (birds). Distinct from Eosinopteryx, Liaoningvenator has a much shorter torso and much longer neck, as in other birds. Like Jianianhualong metarsal 4 is longer than 3 in Liaoningvenator, among many other traits (see below). Shen et al. did not mention Jianianhualong, probably because the two taxa were published within a few weeks of each other. You might remember earlier Xu et al. 2017 also nested Jianianhualong with the non-avian troodontids. Shen et al. included Sapeornis in their phylogenetic analysis. Not sure why they nested apart in the LRT.

A reconstruction of the Liaoningvenator skull
(Fig. 2) has a large openings and gracile bones. What Shen et al. identified as a maxillary foramen is identified here as the base of the naris. The in situ tail curls anteriorly and several caudal vertebrae are visible over the torso.

From the Shen et al. diagnosis:
“A new troodontid dinosaur bears the following unique combination of characters including autapomorphies indicated with an asterisk and new characters indicated with a double asterisk: prominent slender triradiate postorbital*; deltopectoral crest distinctly extended to the half of the humeral shaft*; no posterior process on the dorsodistal end of ischium**; slender obturator process of ischium**; manual phalanx I-1 longer than metacarpal II**, the length ratio of phalanx I-1 to metacarpal II about 1.49**; the width of metatarsus distally distinctly decrease**; transition point in caudal series starts from the seventh caudal vertebra**.

Troodontid or not?
The large flightless basal birds share a long list of traits in common with troodontids and a few that show they are distinct. Here is a list of the differences between bird-like troodontids, like Sinornithoides and Anchiornis, and the troodontid-like sapeornithid birds, like Jianianhualong and Liaoningvenator.

Liaoningvenator bird traits not shared with non-avian troodontids:

  1. Ventral aspect of premaxilla > 1/3 preorbit length
  2. Ascending process of premaxilla extends beyond naris and contacts frontals (nasal separated)
  3. Lacrimal deeper than maxilla
  4. Major axis of naris 30-90º
  5. Posterolateral premaxilla absent (also in Xiaotingia and Eosinopteryx)
  6. Nasals not longer than frontals (also in Xiaotingia and Eosinopteryx)
  7. Antorbital fenestra without fossa
  8. Manual mc2 and 3 do not align with joints on digit 1
  9. Metatarsal 5 not shorter than pedal digit 5

Shifting
Liaoningvenator and Jianianhualong to Sinornithoides adds 14 steps.

Paul 2002
considered the possibility of secondarily flightless (neoflightless) birds, unfortunately without the benefit of a phylogenetic analysis. Paul wrote: “Reversal normally associated with loss of flight is observed in ornithomimids, therizinosaurs and dromaeosaurs.” The LRT found possibly volant bird-like taxa associated with therizinosaurus (Rahonavis), Ornitholestes (microraptorids) and troodontids (birds), but not ornithomimids (related to Compsognathus) and dromaeosaurs (related to Shuvuuia).

Paul wrote:
“The less sharply flexed, broad coracoids of flightless birds recapitulate the dino-avepod condition. The loss of any sternal keel and shortening of the arms area also normal reversals for flightless birds. The semilunate carpal block and arm folding mechanism…are sometimes lost in flightless birds.”

References
Paul G 2002. Dinosaurs of the Air. Johns Hopkins Press
Shen C-Z, Zhao B, Gao C-L, Lü J-C and Kundrat 2017. A New Troodontid Dinosaur (Liaoningvenator curriei gen. et sp. nov.) from the Early Cretaceous Yixian Formation in Western Liaoning Province. Acta Geoscientica Sinica 38(3):359-371.
Xu X, Currie P, Pittman M, Xing L, Meng QW-J, Lü J-C, Hu D and Yu C-Y 2017. Mosaic evolution in an asymmetrically feathered troodontid dinosaur with transitional features. Nature Communications DOI: 10.1038/ncomms14972.

Pseudhesperosuchus fossil photos

Earlier I used
Greg Paul and José Bonaparte drawings of the basal bipedal croc Pseudhesperosuchus Bonaparted 1969) for data on this taxon. The specimen has some traits that lead toward the secondarily quadrupedal Trialestes. Together they are part of a clade that is closer to basal dinosaurs than traditional taxa paleontologists have been working with.

The drawings were great,
but I wondered what the real material looked like…and more importantly, what was real and what was not.

A recent request to
the curators at Miguel Lillo in Argentina was honored with a set of emailed jpegs from their museum drawers (Fig.1), for which I am very grateful. These were traced in line and color and reassembled with just a few unidentified parts left over (Fig. 2).

Figure 1. GIF movie of the skull of Pseudhesperosuchus showing the original drawing, the fossil and DGS tracings of the bones.

Figure 1. GIF movie of the skull of Pseudhesperosuchus showing the original drawing, the fossil and DGS tracings of the bones.

Pseudhesperosuchus jachaleri (Bonaparte 1969 Norian, Late Triassic ~210mya, ~1 m in length, was derived from a sister to Junggarsuchus and  Lewisuchus and was at the base of a clade that included Trialestes on one branch and the Dinosauria on the other branch.

Figue 1. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Figue 2. A new reconstruction of the basal bipedal croc, Click to enlarge. Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Much larger and distinct from Lewisuchus,
the skull of Pseudhesperosuchus had a smaller antorbital fenestra, an arched lateral temporal fenestra, a deeper maxilla and a large mandibular fenestra. The seven cervicals were attended by robust ribs.

The scapula and coracoid were each rather slender and elongated. An straight interclavicle was present. The forelimbs were long and slender. The radiale and ulnare were elongated, a croc trait. Only three metacarpals and no digits are known.

The ilium was relatively small, but probably longer than tall and not perforated. The femur remained longer than the tibia. The tarsus, if that astragalus is identified correctly, included a simple hinge ankle joint. Only two conjoined partial metatarsals are known.

There is a small box
full of little sometimes interconnected squares among the Pseudhesperosuchus material (Fig. 2, aqua colored). I’m guessing that those are osteoderms, and if so, were probably located along the back. These would have helped keep that elevated backbone from sagging in this new biped.

The improvements in the Pseudhesperosuchus data
changed a few scores, but did no change the tree topology. The large reptile tree (LRT) can be seen here.

It’s good to see what Pseudhesperosuchyus really looked like,
— or at least get a little closer to that distant ideal. Size-wise and morphologically, this largely complete specimen is closer to the basal dinosaur outgroup than any other currently included in the LRT. And yet it is also distinctly different as it shares several traits with Trialestes unknown in any dinosaur. As a denizen of the Late Triassic, Pseudhesperosuchus represents a radiation that occurred tens of millions of years earlier, probably in the Middle Triassic. None of this clade survived into the Jurassic, as far as we know.

References
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.