Akidolestes, perhaps not a platypus mimic, but a platypus ancestor and basalmost mammal

Updated June 3, 2017 with a relabeled Ornithorhynchus pelvis. I should have known the original was in error. 

First of all,
the large reptile tree nests Akidolestes (Fig. 1;  Li and Luo 2006; Yixian Formation, Early Cretraceous. 124 mya; NIGPAS* 139381A, B) and the platypus, Ornithorhynchus, as the two basalmost mammals, contra Li and Luo 2006, which considered their similar traits convergent. (*Nanjing Insitute of Geology and Palaeontology, Nanjing, China)

Second,
Cretaceous mammals are most often described and nested by their tooth shapes because often that’s all the evidence we have of them.

Third,
the large reptile tree includes very few tooth shape characters, and none that describe the great variety in mammal molar cusps.

Fourth,
the large reptile tree has not tested the closest sisters to Akidolestes (according to Li and Luo 2006), such as Zhangheotherium yet.

That being said, 
I’m not the first paleontologist to wonder about monotreme phylogeny (Rich et al., 2005). Wikipedia reports, “The precise relationships between extinct groups of mammals and modern groups, such as monotremes, are somewhat uncertain.”

Figure 1. Akidolestes plate on counter plate on original tracing with color tracing added. Note the fish and the fur. The tail and beak are not broad. Images from Li and Luo 2006.

Figure 1. Akidolestes plate on counter plate on original tracing with color tracing added. Note the fish and the fur. The tail and beak are not broad. Images from Li and Luo 2006.

Akidolestes cifellii
(Early Cretaceous, Li and Luo 2006; Figs. 1, 2) was described as a symmetrodont therian (Class Mammalia > Clade Trechnotheria > Family Spalacotheriidae) with some monotreme-like postcranial features (Fig. 2). Essentially they report that Akidolestes was not related to Ornithorhynchus, but was a platypus mimic in certain regards. Here Akikdoletes cifelli nests as a sister to Ornithorhynchus and the evidence is compelling, at least, so far…

Figure 2. The homologies of the hind limb of Akidolestes and Ornithorhynchus that Li and Luo considered as 'functional convergence'.

Figure 2. Illustraiion by Li and Luo 2006. The homologies of the hind limb of Akidolestes and Ornithorhynchus that Li and Luo considered as ‘functional convergence’. Note the prepubes, the pfp = parafibular process and poison spur. This suite of traits was seen as convergent in otherwise unrelated taxa by Li and Luo. They did not note the poison spur in their illustration text, but did so in their main text.

Missing from the Li and Luo tree 
is the taxon Chiniquodon, one of the last mammal ancestors known from head to tail. But even if Chiniquodon is deleted from the LRT, the topology does not change.

Figure 3. Chiniquodon in situ, plate, counter plate, with selected bones colorized and manus + plate reconstructed. There are only 3 sacrals here, not 5. Note the parafibular sesamoid.

Figure 3. Chiniquodon in situ, plate, counter plate, with selected bones colorized and manus + plate reconstructed. There are only 3 sacrals here, not 5. Note the parafibular sesamoid.

The teeth of Akidolestes
are shown in this Li and Luo closeup, colorized here (Fig. 4) for clarity. Note the flat, but not laterally expanded rostrum with dorsal nares (Fig. 5).

Figure 4. Rostrum of Akidolestes from Li and Luo with bones and teeth colorized

Figure 4. Rostrum of Akidolestes from Li and Luo with bones and teeth colorized

 

Figure 5. Reconstructed skull of Akidolestes traced from the lo-rez published images. Compared to Ornithorhynchus, Akidolestes has a larger orbit with a complete circumorbital series, a full arcade of teeth, and a relatively narrow cranium

Figure 5. Reconstructed skull of Akidolestes traced from the lo-rez published images. Compared to Ornithorhynchus, Akidolestes has a larger orbit with a complete circumorbital series, a full arcade of teeth, and a relatively narrow cranium

 

The reconstructed skull of Akidolestes
(Fig. 5) has a long, but not distally expanded rostrum, a relatively narrow cranium, a full set of teeth and a larger orbit with a complete circumorbital series, The canines are not long, the molars have complex cusps. If this is indeed a sister to Ornithorhynchus, the great expansion of the braincase seen in extant monotremes, and many therians apparently is convergent based on the narrower braincases found in Akidolestes and Hadrocodium.

Is that yet another antorbital fenestra? Earlier we looked at several occurrences in which the antorbital fenestra occurred, not just in archosaurs and fenestrasaurs. Ornithorhynchus certainly has another one, and Akiodolestes may have one too, but crushing makes it difficult to determine.

Figure 6. Ornithorhynchus adult and juvenile skulls with bones identified. Note the posterior postorbitals that appear to be laminated to the expanded cranium and the septomaxilla that help form the rostrum.

Figure 6. Ornithorhynchus adult and juvenile skulls with bones identified. Note the posterior postorbitals that appear to be laminated to the expanded cranium and the septomaxilla that help form the rostrum. The antorbital fenestra appears lateral to the rostrum. The postorbital rams of the jugal is a primitive trait. The juvenile has teeth. Images courtesy of Digimorph.org and used with permission. The great expansion of the braincase may be convergent with therians.

Note
the mandible of Akidolestes and Ornithorhynchus is NOT the classic basal mammal dentary with a very large coronoid process. The postorbital ramus of the jugal in the adult Ornithorhynchus is not a trait retained by other basal mammals, but is found in Chiniquodon. Chiniquodon apparently has a large sesamoid near the knee joint (Fig. 5) that may be homologous with the parafibular process found in Akidolestes and Ornithorhynchus (Fig. 3). The ilium in Ornithorhynchus is relatively short IFig. 6).

The pelvis of Ornithorhynchus with elements colorized. The ilium does not have the classic extended appearance common to all other basal mammals.Rather the acetabulum is within a few vertebrae of the anterior tip.

The pelvis of Ornithorhynchus with elements colorized. The ilium does not have the classic extended appearance common to all other basal mammals.Rather the acetabulum is within a few vertebrae of the anterior tip.

A great deal of phylogenetic distance
separates Akidolestes from related taxa in the large reptile tree. Unfortunately candidate and putative sister taxa that I have found are largely represented by mandibles and teeth (but see below). Whether their addition to the matrix will resolve the nesting of the monotremes remains an issue for future consideration. I’m just catching up to the data one taxon at a time in this node of the cladogram.

Figure 7. Ventral view of the skull and dentary of two monotremes, Ornithorhynchus and Tachyglossus, with elements colorized. Note the ventral placement of the the ear ossicles (former posterior jaw elements.

Figure 7. Ventral view of the skull and dentary of two monotremes, Ornithorhynchus and Tachyglossus, with elements colorized. Note the ventral placement of the the ear ossicles (former posterior jaw elements.

Akidolestes was covered
in a dense ball of fur (Fig. 1) and it had a full set of teeth, but it did not have the broad swimming tail of Ornithorhynchus. Nevertheless, it was found with a fish fossil. Akidolestes was preserved in a lake environment. Other Yixian formation mammals include eutriconodontans, multituberculates, symmetrodonts, metatherians and eutherians, so Akidolestes was a late survivor of an earlier (Late Triassic radiation). Megazostrodon and Hadrocodium fossils are found in Early Jurassic strata.

Figure 8. Ornithorhynchus and Akidolestes to scale.

Figure 8. Ornithorhynchus and Akidolestes to scale.

I cannot comment
on the tooth traits that diagnose Akidolestes, only on the other traits (see above), all of which nest it as an early representative of the monotreme radiation. Since Ornithorhynchus juveniles have teeth that transform as adults, I wonder if there was some phylogenetic transformation in the teeth of Ornithorhynchus over the last 100-200 million years. The phylogenetic branches that separate Akidolestes from Ornithorhynchus might go back to the Triassic, or just to the Early Cretaceous.

Li and Luo note: “The shoulder girdle and forelimb are similar to those of zhangheotheriids. However, Akidolestes differs from zhangheotheriids but is similar to monotremes in many features in the posterior part of the skeleton.”

According to Wikipedia, Zhangheotherium is a genus of symmetrodont …mammals. Zhangheotherium has many primitive characteristics. Among them is a spur at the foot, seen today in the modern platypus. In addition, it walked with a reptilian sprawl, like Monotremes and many Mesozoic mammals such as Jeholodens and Repenomamus.” I see that Zhangheotherium has a smaller coronoid process, but not an elongate rostrum.

Note added a day later: 
Zhangheotherium could not be scored for many skull traits because it is preserved dorsal side down and crushed. Given those limitations, Zhangheotherium nested as a basal eutherian between Juramaia and the remaining eutherians in the large reptile tree. Like Ornithorhynchus, Zhangheotherium is reported to have had an ankle spur. There is some disturbance in the tarsus, so this needs to be confirmed. The femora could be adducted close to the body, unlike Ornithorhynchus, and like most eutherians.

Not sure how much deeper this should be examined
in the context of reptile evolution, but you can clearly see that one answer leads to several more questions… which keeps the science endlessly interesting. I have studied basal mammals now for only a few hours, so I present findings, but I’m no expert. High resolution images have been requested.

Last minute addition 
Sinoconodon rigneyi (Early Jurassic, Patterson and Olson 1961) was added to the LRT and nests between the monotremes and Megazostrodon + Hadrocorium.

References
Li and Luo 2006. A Cretaceous symmetrodont therian with some monotreme-like postcranial features. Nature 439|12 January 2006|doi:10.1038/nature04168.
Patterson B and Olson EC 1961. A triconodont mammal from the Triassic of Yunnan. In Vandebroek G (ed.), International Colloquium on the Evolution of Lower and Non Specialized Mammals. Koninklijke Vlaamse Academir voor Wetenschappen, Letteren en Schone Kunsten can Belgie 129-191.
Rich TH, Hopson JA, Musser AM, Flannery TF and Vickers-Rich P 2005. Independent origins of middle ear bones in monotremes and therians. Science 307 (5711): 910–914.

The platypus (Ornithoryhnchus anatinus) now nests in the large reptile tree

Updated August 20, 2016 with an updated cladogram with more taxa.

The oddest of all mammals,
the platypus (Ornithorhynchus, Shaw 1799; Fig. 1), is also the most primitive of all living mammals, other than its egg-laying sisters, the echindas (Tachyglossus and Zaglossus).

The only issue
raised by the large reptile tree (subset Fig. 2) is the nesting of Ornithohynchus more primitive than Megazostrodon + Hadrocodium rather than more derived, as indicated by Li and Luo 2006). That would make it the most primitive of all known mammals, given the present dataset. (That will become expanded tomorrow with greater insight).

Figure 1. New mammal family tree, a subset of the large reptile tree. Here one can trace a gradual accumulation of derived traits, something the traditional paradigm fails to do. Here the clade names in black refer to small discrete clades in the gray column at right. The red clade names refer to taxa identified by color bars

Figure 1. New mammal family tree, a subset of the large reptile tree. Here one can trace a gradual accumulation of derived traits, something the traditional paradigm fails to do. Here the clade names in black refer to small discrete clades in the gray column at right. The red clade names refer to taxa identified by color bars

Akidolestes cifellii (Li and Luo 2006) is a spalacotheroid symmetrodont, described as a “relative of modern therians (marsupials + placentals) shares several traits with Ornithorhynchus, but has premaxillary teeth and a narrow snout.” According to Li and Luo,  it was a monotreme mimic. That may be challenged tomorrow with more details to come.

References
Li and Luo 2006. A Cretaceous symmetrodont therian with some monotreme-like postcranial features. Nature 439|12 January 2006|doi:10.1038/nature04168.
Shaw G 1799. The Naturalist’s Miscellany.

 

Carrano et al. 2012: Basal Tetanurae interrelations

The classification of theropods
has been going on for a hundred years, spurred every year by the discovery of new taxa. Before computers the main division was based on size. The use of software has clarified that issue.

Several years ago,
Carrano, Benson and Sampson (2010) undertook a large study of theropod dinosaurs, focusing on the basal Tetanurae (closer to birds than to Ceratosaurus), up to and not including Coelurosauria (Compsognathus, Ornitholestes and further derived taxa including birds and kin. The authors note: “Tyrannosauridae is now universally included within Coelurosauria (Novas 1991a; Holtz 1994a), whereas ceratosaurs and coelophysoids are basal to Tetanurae.”

They also note, “The placement of many individual taxa within any of these frameworks also varies. ‘Megalosaurs’ pose an even greater and more complex problem. Many of the taxa that have at one time been referred to Megalosauridae have now been dispersed elsewhere, but a large number of putative megalosaur species remain.”

“In summary, although a great deal of progress has been achieved in recent years (measured mainly by increased consensus), several points of uncertainty remain in tetanuran phylogeny and are therefore of primary interest here. These are: (1) whether spinosauroids (= megalosauroids) and allosauroids form a clade, or are serially arranged outside Coelurosauria; (2) whether ‘megalosaurs’ form a valid clade and, if so, its membership; (3) placement of fragmentary forms of potential geographic and temporal importance; and (4) placement of relatively well known but problematical forms (e.g. Cryolophosaurus, Marshosaurus, Monolophosaurus, Neovenator and Piatnitzkysaurus).”

Their work involved firsthand examination
of hundreds of theropod specimens, but no reconstructions were made. Looking at hundreds of specimens is a very good thing, but reconstructions are the notes that let the reader know how bones were interpreted. Without them one must laboriously go through the raw numbers to check for accuracy. No one wants to do that. Reconstructions are a sort of shorthand enabling one to quickly make comparisons of hundreds of characters.

Zanno and Makovicky (2013) recovered a virtually identical theropod tree topology.

In counterpoint
The large reptile tree (subset: Fig. 1) keeps growing without changing topology. Perhaps it offers some insight into theropod relations. Some of the stability of this tree may be due to the inclusion set. Some taxa are tested together here for the first time. There are fewer theropod taxa here than in the works referenced below, but several theropod taxa are included here that are not included in the referenced works.

Figure 1. Basal theropod subset of the large reptile tree showing troodontids basal to birds and separate from dromaeosaurs.

Figure 1. Basal theropod subset of the large reptile tree showing troodontids basal to birds and separate from dromaeosaurs. See the large reptile tree for included taxa not shown here.

References
Carrano MT, Benson RBJ and Sampson SD 2012. The phylogeny of Tetanurae (Dinosauria: Theropoda). Journal of Systematic Palaeontology 10(2):211–300.
Zanno L and Makovicky PJ 2013. Neovenatorid theropods are apex predators in the Late Cretaceous of North America. Nature Communications | 4:2827 | DOI: 10.1038/ncomms3827 |www.nature.com/naturecommunications

It’s not Hovasaurus – and it’s not in a museum

A slight departure today
to the world of fossil commerce. This reptile is new to Science, so it should be presented to a museum for study, but it’s for sale online. And it was misidentified by the proprietors (who have been notified).

Figure 1. Specimen wrongly interpreted as Hovasaurus from FineFossils.com

Figure 1. Specimen wrongly identified as Hovasaurus from FineFossils.com

Cruising around the Internet
I found this specimen (Fig. 1) at FineFossils.com misidentified as Hovasaurus (Fig. 2). The differences are pretty obvious, so I won’t belabor them here. The new specimen is from the same strata and location as Hovasaurus, which is probably the reason for the mistake.

Figure 1. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes, apparently have no scapula.

Figure 2. Tangasaurus, Hovasaurus and Thadeosaurus, three marine younginiformes compared. Hovasaurus, as you can see bears little resemblance to the FineFossils.com specimen mislabeled as Hovasaurus.

From the FineFossils
website: Hovasaurus boulei was a small aquatic Diapsid reptile, of the order Eosuchia, and dates from the late Permian Period, 260m to 251m years old. This specimen was discovered in the Middle Sankamena Formation, Sankamena Valley, Madagascar. It is very rare to find such a complete specimen in perfect condition, displaying a wonderfully preserved skeleton.

These reptiles are known to have a laterally flattened tail [but this one does not have such a tail!], very much like a modern day sea snake, making them extremely agile in the water.  Stones have been found in the abdomens of these creatures [but no stones were found here], indicating that they swallowed small stones to give them ballast, preventing them from floating to the surface when they were hunting prey underwater. 

This Hovasaurus is an amazing example of this very ancient reptile, and is of museum quality [other than the upside-down skull, the specimen has no obvious errors]. We have seen other specimens, but the majority are dis-articulated or incomplete.
The only restoration to this piece is at the tip of the tail.

Size:    matrix  47cms x 15cms
Size:    reptile   46cms long

The description of this specimen
recalls the mid 1800s in the earliest days of fossil collection when every pterosaur discovered was referred to  Pterodactylus, despite readily observable differences from the holotype. This specimen (Fig. 1)  is probably more marketable with a name. The name might also imply it is common enough to be sold to private individuals, like the Green River fossil fish magnets that adorn American refrigerators.

Figure 3. The FineFossils.com specimen traced and reconstructed. This previously unknown specimen nests at the base of the Diapsida, close to Eudibamus, but has an extended rostrum.

Figure 3. The FineFossils.com specimen traced and reconstructed. This previously unknown specimen nests at the base of the Diapsida, close to Eudibamus, but has an extended rostrum.

In this case, however,
the specimen is new to Science. It has not been assigned a generic name. It has not been studied yet (other than by what you’re reading here). The FineFossils specimen has a longer rostrum than other basal diapsids and hints at a broader radiation at this node. It is basal to Eudibamus, Aphelosaurus, Petrolacosaurus (Fig. 4) and Araeoscelis on one branch. It is basal to Spinoaequalis and all the marine and terrestrial Younginiforms, including birds and crocs, ichthyosaurs and plesiosaurs, on the other branch. The rostrum appears to have an antorbital fenestra (Fig. 4), but that is due to crushing and shifting of the elements.

Figure 4. Fine fossils skull wrongly attributed to Hovasaurus traced and reconstructed. This is an unnamed genus new to Science.

Figure 4. Fine fossils skull wrongly attributed to Hovasaurus traced and reconstructed. This is an unnamed genus new to Science. The apparent antorbital fenestra is an illusion produced by taphonomic shifting.

So, if anyone has deep pockets out there
you can make a purchase and a museum donation that will be much appreciated by reptile paleontologists everywhere. This is a unique specimen nesting at a key node on the family tree that I can only chat about online, since it currently has no museum number. It can’t find a permanent place on the large reptile tree without that museum number.

It would be worthy of a publication!

It’s rare. It’s unique.
And if you work it right, it might be named for you as in ‘Rogersaurus’, ‘Marysaurus’ or, better yet… Diapsidsaurus longirostrum would make a suitable name for the reasons listed above.

Figure 2. Petrolacosaurus is an earlier sister to Araeoscelis with a definite diapsid temporal configuration, but oddly the upper temporal fenestra is largely lateral in this taxon.

Figure 5. Petrolacosaurus is an earlier sister to Araeoscelis with a definite diapsid temporal configuration, but oddly the upper temporal fenestra is largely lateral in this taxon. The parietals are quite broad.

Speaking of basal diapsids
Once hailed as the most basal disapsid, Petrolacosaurus (Lane 1945, Reisz 1977) is now much more derived with several more primitive diapsid taxa preceding it on the large reptile tree, including the FineFossils.com specimen. All this hints at an earlier radiation, the kind we talked about earlier here.

References
Lane HH 1945. New Mid-Pennsylvanian Reptiles from Kansas. Transactions of the Kansas Academy of Science 47(3):381-390.
Reisz RR 1977. Petrolacosaurus, the Oldest Known Diapsid Reptile. Science, 196:1091-1093. DOI: 10.1126/science.196.4294.1091

wiki/Petrolacosaurus

News at the genesis of snakes: Tetrapodophis ‘highly suggestive’ as aquatic

A year ago
Martill et al. 2015 described the stem snake, Tetrapodophis (Fig. 1) and considered it a burrowing squamate. A new paper by Lee et al. 2016 reports that Tetrapodophis had aquatic adaptations.

Figure 2. Tiny Tetrapodophis at full scale if your monitor produces 72 dpi images (standard on many monitors).

Figure 1. Tiny Tetrapodophis at full scale if your monitor produces 72 dpi images (standard on many monitors).

This new study confirms 
what you read here, here and here when we nested Tetrapodophis with the following aquatic pre-snaketaxa: PontosaurusAdriosaurus and Aphanziocnemus in the large reptile tree (subset Fig. 2).

From the Lee et al abstract
“The exquisite transitional fossil Tetrapodophis – interpreted as a stem-snake with four small legs from the Lower Cretaceous of Brazil – has been widely considered a burrowing animal, consistent with recent studies arguing that snakes had fossorial ancestors [not so here]. We reevaluate the ecomorphology of this important taxon using a multivariate morphometric analysis and a reexamination of the limb anatomy. Our analysis shows that the body proportions are unusual and similar to both burrowing and surface-active squamates. We also show that it exhibits striking and compelling features of limb anatomy, including enlarged first metapodials and reduced tarsal/carpal ossification – that conversely are highly suggestive of aquatic habits, and are found in marine squamates. The morphology and inferred ecology of Tetrapodophis therefore does not clearly favour fossorial over aquatic origins of snakes.”
Figure 2. Scleroglossan subset of the large reptile tree. Generalists taxa duplicated in the Yi and Norell tree are shown in bright green. Burrowers shared in the Yi and Norell tree are in dark green. Legless taxa are black. Vestiges are in gray. Unknown are striped.

Figure 2. Scleroglossan subset of the large reptile tree. Generalists taxa duplicated in the Yi and Norell tree are shown in bright green. Burrowers shared in the Yi and Norell tree are in dark green. Legless taxa are black. Vestiges are in gray. Unknown are striped.

Wonder why 
prior workers have not performed a phylogenetic analysis on this taxon that includes the above named aquatic squamates and Jucaraseps?

References
Lee MSY, Palci A, Jones MEH, Caldwell MW, Holme JD & Reisz RR 2016. Aquatic adaptations in the four limbs of the snake-like reptile Tetrapodophis from the Lower Cretaceous of Brazil. Cretaceous Research (advance online publication)
doi:10.1016/j.cretres.2016.06.004 online for sale
Martill DM, Tischlinger H and Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science 349 (6246): 416-419. DOI: 10.1126/science.aaa9208

They’re out there somewhere!

Back in the ’90s, 
I built several full scale prehistoric reptile models out of wood, wire, foam, glass (eyes) and what have you. Two of them are shown here (Fig. 1).

Figure 1. Baby Camarasaurus and featherless Deinonychus models built by David Peters in the 1990s.

Figure 1. Baby Camarasaurus and featherless Deinonychus models built by David Peters in the 1990s.

At the time, 
like the the extinct Steve Czerkas and the extant Charlie McGrady, I wanted to be build dinosaurs, not just illustrate them in books. At the time, St. Louis did not have a Science Museum and that’s when (so I was told) you are supposed to get in on the ground floor. Also at the time the late sculptor Bob Cassilly was building squids, pterosaurs, sharks and rays for the St. Louis Zoo based on illustrations in my book Giants. (Bob was instrumental in bringing Sharovipteryx, Longisquama and the other Russian dinosaur exhibit to St. Louis.) Alas, that phase fizzled and the writing of papers followed. Early on you’re driven by enthusiasm and reined in by naiveté. In evolutionary terms, it worked out for that time and place.

Along with
the baby Camarasaurus and adult Deinonychus, I built a plesiosaur, Tanystropheus, fuzzy Dimorphodon, Pterodactylus and the several pterosaur skeletons seen here. The fleshed out sculptures went to the AMNH in NYC. The baby sauropod went to Martin Lockley in Colorado. The skeletons all went to Mike Triebold. Many artists want to see their art hanging in museums. Well, it happened to me, sort of, with those pterosaur skeletons. They’re out there, all over the world. The AMNH ultimately decided to display only skeletons in their renovated prehistoric displays and sold off what they had purchased.

I have no idea
where the various pieces are now or what shape they are in. But it was fun for awhile and the mailman probably told his kids about the address that had dinosaurs under the carport. Now a longer list of illustrated and animated prehistoric reptiles can be found on the Internet here.

The origin of the amniote astragalus – Piñeiro et al. 2016

A new PeerJ paper
by Piñeiro, et al. (2016) attempts to shed light on the origin of the amniote astragalus by comparison to the ontogenetic development of the mesosaur tarsus. They claim that Mesosaurus is a very primitive amniote, following the thinking of traditional paleontologists.  By contrast, the  large reptile tree nests mesosaurs with highly derived thalattosaurs and ichthyosaurs, following basal pachypleurosaurs (sauropterygians) in the revived clade Enaliosauria, derived from marine younginiforms, diapsids, prodiapsids and basal archosauromorphs, all arising during the preceding Carboniferous.

In any case
The new paper seeks to answer the question, ‘Did the astragalus arise from one bone (the intermedium) or the fusion of several bones, including the tibiale, centralia and intermedium?’ Piñeiro et al. found that among embryo mesosaurs, a single four-part bone, creates the astragalus.

Two false paradigms affect the Piñeiro et al study 
1: They follow traditional beliefs that amniote skeletal structures should be present in basalmost amniotes. In reality, as we all know, amniotes are defined ONLY by the way they protect their embryos, with an amniotic sac, a structure lacking in amphibians. As we learned earlier, the basalmost amniote in the large reptile tree is Gephyrostegus bohemicus, a late-surviving member of a an earlier Viséan radiation. It has no traditional amniote traits. But a revised list of amniote traits can be seen here and in the six blogs that follow.

2: They believe that Mesosaurus is a basal amniote. The Early Permian is indeed early, but with basal reptiles already diversifying in the Viséan, some 40 million years earlier, the Permian is not early enough. Moreover a basal nesting is not supported in the large reptile tree. Along the same lines they do not understand that Diadectes, Tseajaia, Westlothiana and others nest within the Amniota (= Reptilia) in the only study that tests their relationships in a large gamut study of other tetrapods and amniotes, the large reptile tree.

Piñeiro et al. used an interesting graphic technique
of presenting the tarsal elements of several taxa as a series of interlocking hexagons. That’s fine on one level, but does not let us see the actual elements or reconstructions of the same, which is an unfortunate loss. They also mix up left and right, dorsal and ventral views of skeletal elements. It would be helpful to flip certain elements in order to present all the elements consistency for ready comparison. This should be standard operating procedure.

In the large reptile tree the intermedium remains a separate element
from the tibiae in stem (pre) amniotes like Proterogyrinus, Seymouria and the basal amniote Gephyrostegus (Fig. 1).

Figure 1. The intermedium remains separate from the tibiale in Proterogyrinus, Seymouria and Gephyrostegus.

Figure 1. The intermedium remains separate from the tibiale in Proterogyrinus, Seymouria and Gephyrostegus.

BTW, while researching Seymouria
I came across a bizarre reconstruction in Berman et al. 2000 (Fig. 1 under red circle) that did not match the bone tracings and added another row of central tarsals that no other tetrapods have.

Essentially
the Reptilia is, in reality, two clades, the Lepidosauromorpha and Archosauromorpha. Let’s take the former first. The tibiale does not fuse to the intermedium in all reptiles. So the astragalus is not present in every amniote.

Figure 2. Sample lepidosauromorph tarsi compared to Gephyrostegus. Here are Captorhinus, Emeroleter and Tjubina, a basal tritosaur lepidosaur.

Figure 2. Sample lepidosauromorph tarsi compared to Gephyrostegus. Here are Captorhinus,Orobates, Emeroleter and Tjubina, a basal tritosaur lepidosaur. Note the separate tibiae in Gephyrostegus, Orobates and Emeroleter. So an astragalus appears most of the time in this clade, not all of the time.

The appearance
and or fusion of tarsal elements varies within the Reptilia. And it varies with ontogeny within certain taxa, like mesosaurs. Older individuals often have more bones and more sharply defined bones. In the Archosauromorpha (Fig. 2), perhaps eight taxa precede the first appearance of the astragalus (fusion of tibiae and intermedium) in Casineria.

Figure 2. Comparison of archosauromorph tarsi, including Mesosaurus, the latter from Piñeiro et al

Figure 2. Comparison of archosauromorph tarsi, including Mesosaurus, the latter from Piñeiro et al Not to scale. Note the generally conservative pattern here, despite the liberal changes in relative bone sizes.

Here
(Figs. 1, 2) the astragalus (yellow/orange element) is only composed of the tibiale and intermedium in these taxa and a small perforation marks the division. The elements of the centralia may fuse together, but not with other elements in the above listed taxa. These fusion patterns occur by convergence in the two basal reptile clades.

According to Piñeiro et al
the astragalus changes greatly during the ontogeny of Mesosaurus. They interpret (“with doubts”) the embryo astragalus as the fusion of the tibiae, intermedum and two centralia. I don’t see any more than three centralia in the above illustrated taxa, and sometimes they fuse together, but not with the tibiale or intermedium (Fig. 2).

As a final note
I find it odd that workers are eager to change the names of some fused bones, like the astragalus and navicular, but are not interested in renaming other fused bones (like the postfrontal + postorbital). Instead, one bone is typically said to be present while the other is said to be absent. And that doesn’t make sense when both are present, just fused.

Let’s fix that in consensus.

References
Berman DS, Henrici AC, Sumida SS, Martens T. 2000. Redescription of Seymouria
sanjuanensis (Seymouriamorpha) from the Lower Permian of Germany based on complete mature specimens with a discussion of paleoecology of the Bromacker locality assemblage. Journal of Vertebrate Paleontology 20(2):253268
Piñeiro et al. 2016. The ontogenetic transformation of the mesosaurid tarsus: a contribution to the origin of the primitive amniotic astragalus. PeerJ 4:e2036; DOI 10.7717/peerj.2036

When Synapsids and Diapsids split

At some point
on every reptile cladogram the Synapsida emerges and somewhere else the Diapsida emerges.

In contrast to all prior cladograms,
on the large reptile tree, the traditional Diapsida is diphylletic, with lepidosaurs no longer related to archosaurs except by way of the basalmost Viséan reptiles (at the archosauromorph/ lepidisauromorph split). The reduced Diapsida (sans lepidosaurs) arises from the Prodiapsida, which splits from the Synapsida at the common base of both clades, near Protorothyris (Fig. 1), a basal archosauromorph. What happened at that split is today’s topic.

One of the basalmost synapsids
is Varanosaurus. One of the basal prodiapsids is Heleosaurus (Fig. 1). Both have a synapsid temporal morphology. Among traditional paleontologists, both are considered traditional synapsids.

Now let’s take a look
at some of the characters that split these sister taxa that otherwise share so many traits and put forth some hypotheses as to what they may mean in the grand scope of reptile evolution.

Figure 1. Taxa at the split between Synapsida and Diapsida (Prodiapsida): Varanosaurus and Heleosaurus to scale along with their common ancestor, Protorothyris.

Figure 1. Taxa at the split between Synapsida and Diapsida (Prodiapsida): Varanosaurus and Heleosaurus to scale along with their common ancestor, Protorothyris.

In many respects,
Varanosaurus was just a bigger Heleosaurus. And both were much larger than their predecessor, Protorothyris. So size was a major factor in the Early Permian. Basal synapsids were larger than prodiapsids and both were larger than their Carboniferous predecessors.

Distinct from Varanosaurus,
Heleosaurus had 19 rather minor traits in the large reptile tree. As a rule they’re not very interesting or informative (but see the next topic header):

  1. Remained < 60 cm long
  2. Slightly wider skull relative to height at orbit
  3. The nasal shape retains ‘narrows anteriorly’ description (not arrowhead)
  4. Orbit stays in anterior half of the skull
  5. Supratemporal/squamosal overhang
  6. Shorter jugal quadratojugal process
  7. Quadrate rotates to vertical
  8. Lateral temporal fenestra larger, circumtemporal bones more gracile
  9. Occiput remains close to quadrates
  10. Basipterygoid lateral processes prominent
  11. Mandible tip straight
  12. Mandible fenestra remains absent
  13. Olecranon process not present (Heleosaurus clade only)
  14. Clavicles medially not broad
  15. Radius + ulna > 3x longer than wide
  16. Retained pubis angled ventrally
  17. Acetabulum opens ventrally (Heleosaurus clade only)
  18. Tibia < 2x ilium length
  19. Dorsal osteoderms present (restricted to Heleosaurus

In summary,
these Heleosaurus traits break down to four major and a few minor distinctions from Varanosaurus:

  1. Smaller size, larger orbit, shorter rostrum, relatively less bone in the skull – all attributable to neotony (retention of embryo/juvenile traits)
  2. Relatively longer hind limbs and more slender tail (shorter chevrons and transverse processes (ribs). Together these two make prodiapsids speedy, not lumbering. Ideal for avoiding larger enemies and attacking insect prey.
  3. Relatively larger orbit: possible nocturnal hunter.
  4. Longer, more gracile ribs: fast locomotion requires more efficient and rapid respiration provided by expanding ribs
  5. Minor traits: Fewer teeth, ‘solid’ palate, larger choanae: all part of the insectivore, rapid respiration bauplan.

In my opinion
the smaller size of Heleosaurus helped it retain an insect diet, rather than moving into carnivory, piscivory or herbivory, as proposed for the pelycosaurs. Heleosaurus was probably faster and more agile than its larger and smaller relatives, better adapted to hunt insects and avoid predators.

Later taxa
‘improved’ on these traits as the clade Diapsida appeared, followed quickly by a division into terrestrial younginiforms and aquatic younginiforms.

These lizardy archosauromorph diapsids competed with
outwardly similar lepidosauromorphs lepidosaur pseudo-diapsids, like Tjubina. The lepidosaur branch retained insectivory, for the most part. The archosauromorph branch did not, for the most part, with the exception that several extant mammals and birds today are insectivores.

Tiny Iberomesornis

Figure 1. Iberomesornis revisited. One of the smallest of the enantiornithes birds has a long pedal digit 4.

Figure 1. Iberomesornis revisited. One of the smallest of the enantiornithes birds has a long pedal digit 4. Note the placement of the longest toe beneath the center of balance at the shoulder glenoid in this volant biped, a configuration convergent with pterosaurs.

Iberomesornis romerali (Sanz and Bonaparte 1992, LH-22 (Las Hoyas Collection), Barremian, Early Cretaceous, 125 mya) tiny bird (8.7 cm axial column) with short wings (20 cm wingspan), pygostyle.  The long coracoids indicated powerful flapping muscles. The combination of a long p4.4 and short p3.3 makes the foot unique. This tiny taxon continued the phylogenetic size reduction that coincided with improvements in the ability to fly, as indicated by the longer coracoids and caudal fusion.

The Enanitornithes, or opposite birds, are so-named because their scapula/coracoid joint tabbed the opposite way of living birds. No living birds are enantiornithes.

References
Sanz JL and Bonaparte JF 1992. A New Order of Birds (Class Aves) from the Lower Cretaceous of Spain. In JJ Becker (ed): Papers in Avian Paleontology Honoring Pierce Brodkorb. Natural History Museum of Los Angeles County Contributions in Science 36:38-49

wiki/Iberomesornis

The skull of Lesothosaurus revisited

Lesothosaurus diagnostics (Galton 1978, Early Jurassic, 2.2m) was originally considered a basal ornithopod. Sereno (1991) suggested it may be one of the most primitive of all ornithischian dinosaurs. Butler et al. (2008) proposed that Lesothosaurus was a basal member of Neornithischia (pachycephalosaurs, ceratopsians and ornithopods), or Thyreophora (stegosaurs + ankylosaurs). The large reptile tree nests Lesothosaurus with quadrupedal Scutellosaurus and stegosaurs, not ankylosaurs. Galton, Sereno, Butler et al. published before the discoveries of more basal ornithischians like Daemonosaurus and Chilesaurus, but Wikipeidia needs another excuse for these deletions.

Just slight differences here (Fig. 1)
from traditional imagery and interpretations, chiefly those of Sereno 1991.

Figure 1. The skull of Lesothosaurus (BMNH 8501) traced and reconstructed.

Figure 1. The skull of Lesothosaurus (BMNH 8501) traced and reconstructed. Lower teeth imagined.

This new interpretation clears up
some of the problems in the large reptile tree (now 694 taxa) that were preventing high bootstrap scores at certain nodes. A bit of posterior palpebral appears above the fused postorbital/postfrontal. You also find that in Stegosaurus. The naris is close to the ventral rim, atypical for basal phytodinosaurs and basal ornithischians, but convergently appearing in certain derived ornithischians. Likewise a toothless premaxilla developed several times in the Ornithischia.

References
Butler RJ, Upchurch P, Norman, DB 2008. The phylogeny of the ornithischian dinosaurs. Journal of Systematic Palaeontology 6 (1): 1–40. doi:10.1017/S1477201907002271.
Galton PM 1978. Fabrosauridae, the basal family of ornithischian dinosaurs (Reptilia:
Ornithopoda). Paläontolgische. Zeitschrift 52:138–59.
Knoll F, Padian K and de Ricqles A 2009. Ontogenetic change and adult body size of the early ornithischian dinosaur Lesothosaurus diagnosticus: implications for basal ornithischian taxonomy”. Gondwana Research online preprint: 171. doi:10.1016/j.gr.2009.03.010.
Sereno PC 1991. Lesothosaurus, “fabrosaurids,” and the early evolution of Ornithischia. Journal of Vertebrate Paleontology 11(2):168-197

wiki/Lesothosaurus