Here’s a project ripe for a PhD dissertation: Youngina and kin

Summary for those in a hurry:
Specimens nesting at the base of the marine and terrestrial younginiforms need a good review, as in a doctoral dissertation. Many of the specimens below have not been described and the collection has not been tested in a phylogenetic analysis, except here in the LRT. And let’s not forget headless Galesphyris (Fig. 4), the last common ancestor of this monophyletic clade of (at present) wastebasket “young-” younginids (Youngina, Youngolepis and Youngoides) needs to be part of the picture. The Late Carboniferous diapsid, Spinoaequalis (Fig. 2), is the outgroup taxon in the LRT.

A new ‘Youngina’ specimen came to my attention
(Fig. 1) published in Sues 2019. Unfortunately no museum number was provided. Pending acquisition of that number, the new specimen was added to the large reptile tree (LRT, 1694+ taxa) just to see where the new one would nest among the many Youngina, Youngoides and Youngolepis specimens (Figs. 2, 3) already in the LRT. Scale bars indicate it’s a big one.

Figure 1. Unidentified specimen attributed by Sues 2019 to Youngina capensis. Here it nests to scale with the much smaller BPI 375 specimen basal to protosaurs, like the AMNH 9520 specimen assigned to Prolacerta.

Relatively few workers
have published on the Youngina, Younginoides and Youngolepis specimens. That is unexpected considering the key position in the LRT of these largely ignored taxa at the bases of several major clades.

Figure 2. Terrestrial Yonginiformes + Galesphyrus representing the marine clade, all to scale except the toned area containing protorosaurs, which have their own scale.

One traditional Youngina specimen, 
short-legged BPI 3859, does not nest with the terrestrial taxa in the LRT, despite the many similarities.

Figure 3. The odd one out, the BPI 3859 specimen assigned to Youngina does not nest with the others, but with marine taxa.

However,
if headless Galesphyris turns out to be a junior synonym of Youngina, then the genus would be monophyletic across tested taxa. Let’s leave open that possibility. Otherwise, let’s rename them all appropriately.

Figure 4. If Galesphyrus was Youngina, the genus would be monophyletic.

At nine cm in length, the skull of the new specimen
is the largest skull assigned to the genus Youngina. Like the smaller BPI 375 specimen, it nests basal to protorosaurs in the LRT. Other specimens nest basal to Archosauriformes. As noted above, the BPI 3859 specimen nests basal to Claudiosaurus in the LRT along with other marine younginiformes, including plesiosaurs, mesosaurs and ichthyosaurs.

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References
Broom R 1914. A new thecodont reptile. Proceedings of the Zoological Society of London, 1914:1072-1077.
Broom R and Robinson JT 1948. Some new fossil reptiles from the Karroo beds of South Africa: Proceedings of the Zoological Society of London, series B, v. 118, p. 392-407.
Gardner NM, Holliday CM and O’Keefe FR 2010. The braincase of Youngina capensis (Reptilia, Diapsida): New insights from high-resolution CT scanning of the holotype. Paleonotologica Electronica 13(3).
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Olson EC 1936. Notes on the skull of Youngina capensis Broom. Journal of Geology, 44 (4): 523-533.
Olson EC and Broom R 1937. New genera and species of tetrapods from the Karroo Beds of South Africa. Journal of Paleontology 11(7):613-619.
Smith, RMH and Evans SE 1996. New material of Youngina: evidence of juvenile aggregation in Permian diapsid reptiles. Palaeontology, 39 (2):289–303.
Sues HD 2019. The Rise of Reptiles: 320 Million Years of Evolution.
Johns Hopkins University Press, Baltimore. xiii + 385 p.; ill.; index.
ISBN: 9781421428673 (hc); 9781421428680 (eb).

wiki/Youngina

Teyujagua paradoxa: still no paradox in the LRT

Back in 2016 Pinheiro et al.
introduced readers to a small Early Triassic proterosuchid without much of an antorbital fenestra, Teyujagua paradoxa (Fig. 1). Back then a smaller large reptile tree (LRT, subset Fig. 2) nested Teyujagua as one of several smaller descendants of Proterosuchus without an antorbital fenestra. Based on taxon exclusion Pinheiro et al. 2016 mistakenly described Teyujagua as, “transitional in morphology between archosauriforms and more primitive reptiles…as the sister taxon to Archosauriformes.” Evidently they were looking for greater glories than Teyujagua actually represented.

Figure 1. Teyujagua compared to sister taxa, including Youngoides, Proterosuchus and Chasmatosaurus. Teyujagua is a phylogenetic miniature in which the antorbital fenestra became a vestige.

 

This year (2019) Pinheiro et al. returned to Teyjagua
They wrote, “The evolution of the archosauriform skull from the more plesiomorphic configuration present ancestrally in the broader clade Archosauromorpha was, until recently, elusive.”

This is a bogus statement.
The LRT found a series of terrestrial younginiforms basal to archosauriforms and protorosauria. You read about them here in 2011. All the authors had to do was google Teyjagua to find the data needed to overturn their hypothesis.

Pinheiro et al. 2019 continue, 
“This began to change with the discovery and description of Teyujagua paradoxa, an early archosauromorph from the Lower Triassic Sanga do Cabral Formation of Brazil. In addition to providing new details of the anatomy of T. paradoxa, our study also reveals an early development of skull pneumaticity prior to the emergence of the antorbital fenestra.”

This is an backwards statement.
The LRT found Teyujagua was losing an antorbital fenestra, not gaining one. Adding taxa would have solved this problem for Pinheiro et al. 2019, as suggested three years ago.

Pinheiro et al. 2019 continue,
‘The data presented here provide new insights into character evolution during the origin of the archosauriform skull.”

The actual origin of the archosauriform skull
according to the LRT (Fig. 2). occurs in a list of excluded taxa ending with Youngoides romeri FMNH UC1528. As before Teyujagua remains a sister to Chasmatosaurus alexandri NMQR 1484 and is therefore a dead end taxon, basal to nothing.

Figure 2. Cladogram of basal archosauriforms. Note the putative basalmost archosauriform, Teyujagua (Pinheiro et al 2016) nests deep within the proterosuchids. The 6047 specimen that Ewer referred to Euparkeria nests as the basalmost euarchosauriform now.

This should be embarrassing to the authors
when an amateur without a science degree of any firsthand access to  the specimen can tell the PhDs they didn’t included enough taxa to understand what they were dealing with. Sadly, this is not the first time, and it won’t be the last. The LRT is a powerful tool, free for all to use.

Figure 3. Youngoides romeri FMNH UC1528 demonstrates an early appearance of the antorbital fenestra in the Archosauriformes. This specimen is the outgroup to Proterosuchus, the traditional basal member of the Archosauriformes.

Figure 4. Click to enlarge. Updated image of various proterosuchids and their kin. When you see them all together it is easier to appreciated the similarities and slight differences that are gradual accumulations of derived taxa. Teyujagua is a deadened taxon, less glorious than Pinheiro et al. 2016 and 2019 wish it was.

A paper on Youngoides romeri and the origin of the Archosauriformes
can be read online here at ResearchGate.org. It was rejected by the referees.

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References
Pinheiro FL, França MAG, Lacerda MB, Butler RJ and Schultz CL 2016. An exceptional fossil skull from South America and the origins of the archosauriform radiation. Nature Scientific Reports 6:22817 DOI: 10.1038/srep22817.
Pinheiro FL, De Simao-Oliveira D and Butler RJ 2019. Osteology of the archosauromorph Teyujagua paradoxa and the early evolution of the archosauriform skull.
Zoological Journal of the Linnean Society, zlz093
https://doi.org/10.1093/zoolinnean/zlz093
https://academic.oup.com/zoolinnean/advance-article-abstract/doi/10.1093/zoolinnean/zlz093/5585773

https://pterosaurheresies.wordpress.com/2016/03/13/teyujagua-not-transitional-between-archosauriforms-and-more-primitive-reptiles/

 

What?? No scapula??

In a few marine younginiforms,
Tangasaurus (Haughton 1924, Currie 1982), Hovasaurus (Piveteau 1926, Currie 1981) and Thadeosaurus (Carroll 1981, Currie 1984) the scapula cannot be found (Fig. 1). But in a young thadeosaur (if conspecific), a scapula is present (in gray). These are all currently sisters in their own clade in the large reptile tree, The lack of a scapula is not currently a scored trait in the large reptile tree.

Figure 1. Tangasaurus, Hovasaurus and some specimens of Thadeosaurus, three marine younginiformes, apparently have no scapula. Click to enlarge. The young Thadeosaurus, if that is indeed what it is (in gray box) shows what a scapula should look like.

When you first encounter these specimens
you scratch your head and search, looking for the scapulae to no avail. Then, when you realize these three sisters share this trait — it still is difficult to accept. The coracoids and sternae + interclavicle form a chest plate. What holds that pectoral girdle in place? What locks the humerus down?  It is hard to look at those naked anterior ribs. Usually something is there to cover them~ Maybe I just missed it…

It is at this node in the evolution of marine younginiforms
that they were moving from a terrestrial niche into an aquatic one. From such Late Permian taxa we get plesiosaurs, placodonts, mesosaurs, thalattosaurs and ichthyosaurs, along with the widely varied sinosaurosphargids including Atopodentatus. So the change in niche is echoed and sometimes amplified in the morphology of descendant taxa, starting with these three (Fig. 1).

References
Carroll RL 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society London B 293: 315-383
Currie PJ 1984. Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology 4(1 ): 68-84.
Currie PJ 1981. Hovasaurus boulei, an aquatic eosuchian from the Upper Permian of Madagascar. Palaeontologica Africana, 24:99-163.
Currie P 1982. The osteology and relationships of Tangasaurus mennelli Haughton. Annals of The South African Museum 86:247-265. http://biostor.org/reference/111508
Haughton SH 1924. On Reptilian Remains from the Karroo Beds of East Africa. Quarterly Journal of the Geological Society 80 (317): 1–11.
Piveteau J 1926. Paleontologie de Madagascar XIII. Amphibiens et reptiles permiens. Annls Paleont. 15: 53-180.
Reisz RR, Modesto SP and Scott DM 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B 278 (1725): 3731–3737.

wiki/Hovasaurus
wiki/Tangasaurus

 

Atopodentatus: fixing a mistake and Science marches on

Two years ago, when
Atopodentatus unicus was first described (Cheng et al. 2014) it (WIGM SPC V1107) was thought to have an odd downturned rostrum with medially facing premaxillae. That very odd (autapomorphic) interpretation was widely accepted.

Recently
Chun et al. 2016 found more fossils (IVPP V20291, IVPP V20292) that showed the crushing had introduced an illusion in the first Atopodentatus. The jaws were actually wide and flat, not deep (Fig. 2).

Figure 1. DGS tracings of the second and third specimens of Atopodentatus in several views. Note the foramina in the nasals, a likely location for the salt gland. The mandibles were both rotated axially so that the glenoid/jaw joint was found on the former medial surface.

Chun et al. 2016 reported,
“The evidence indicates a novel feeding mechanism wherein the chisel-shaped teeth were used to scrape algae off the substrate, and the plant matter that was loosened was filtered from the water column through the more posteriorly positioned tooth mesh. This is the oldest record of herbivory within marine reptiles.”

Maybe — maybe not.
To me those pmx teeth look like mud/sand rakes/sievers. And IF so, Atopodentatus was seeking burrowing organisms that could be filtered by the posterior jaws. But on the other hand, we have a living analog…

Like a modern marine iguana?
This YouTube video shows a marine iguana grazing on algae coating the underwater rocks surrounding the Galapagos. Note the narrow-snouted iguana twists its head and uses its cheek teeth on the flat part of its face to nip the plants off the rocks. The video describes those teeth as ‘razor sharp’, but they are not. They are tricuspid, acting like rakes. In a way the duplicate having three times as many single-cusp teeth.

Figure x. Marine iguana teeth are tricuspid

Unfortunately
neither Cheng et al. nor Chun et al were able to decide what sort of marine reptile Atopodentatus was. Here, in the large reptile tree, even with the new changes to the skull, Atopodentatus nests outside the Sauropterygidae, as a marine younginiform, more derived than Claudiosaurus, nesting with the turtle-like Sinosaurosphargis and the longer but still wide, Largocephalosaurus (Fig. 2). Reconstructions help.

Figure 2. Atopodentatus nests with two other pre-sauropterygian marine younginforms, Sinosaurosphargis and Largocephalosaurus. Note the narrower postorbital skull compared to these sisters.

These three were bottom feeders. 
All were from the Middle Triassic. Onlly Atopodentatus had a vertical quadrate. Only Atopdentatus had a hole in its skull, presumably for a salt gland.

Some bones missed by Chun et al. 2016 include:

1. supratemporals
2. postparietals
3. tabulars
4. quadratojugals
5. and the large lacrimals.

Figure 3. The two IVPP specimens of Atopodentatus both show the wide premaxillae ideals for scraping and raking. The pterygoids had a shagreen of tiny teeth. The maxillae also have filtering needle-like teeth. Note the presence of the lacrimal and supratemporal missed by first hand observation. Is that a salt gland location posterior to the nasals?

Was this a blunder? Or an honest mistake?
Let’s be professional about this and call it an honest mistake. And notice there is no reason to be embarrassed by such mistakes. We fix them and move on. Take these examples:

1. Elasmosaurus had the head on the wrong end originally.
2. Yi qi was thought to have an extra long wrist bone that turned out to be a displaced ulna on one side, a radius on the other side.
3. Sordes was thought to have deep chord wings and a uropatagium between the legs, not including the tail.
4. Longisquama was thought to have short hind limbs.
5. The nesting of Vancleavea with archosauriformes.
6. Or pterosaurs with dinosaurs and Scleromochlus.
7. The wrong skull and dragging tail for Brontosaurus.
8. … and all the little boo-boos that creep into everyone’s matrices (including yours truly)

Note that
one author on the original paper (Cheng L) is also an author on the new paper. Here’s how the new set of authors handled the prior mistake from the abstract, “The skull displays a pronounced hammerhead shape that was hitherto unknown.”

And from the text:
“Atopodentatus unicus was originally described as a putative sauropterygian filter feeder with a downturned rostrum, supposedly used to stir up invertebrates in soft sediment in a flamingo-like manner. Here, we describe two new specimens… that require a very different interpretation of skull morphology and provide evidence for an even more remarkable feeding strategy. The new specimens clearly demonstrate that rather than being downturned, the rostrum was developed into a “hammerhead” with pronounced lateral processes formed by the premaxillae and maxillae in the upper jaw and mirrored by the dentary in the lower jaw.”

That’s a nice way to do it. Don’t you agree?
We can all take a lesson from this.

While we’re on the subject of filter feeding marine reptiles…
Let’s not forget the oddly toothed thallattosaur, Helveticosaurus (Fig. 4, Middle Triassic). Helveticosaurus had such long cheek teeth they could not have been used for chewing or stabbing. Instead they look like baleen strainers. Helveticosaurus had long fangs anteriorly, perhaps useful for scraping rather than stabbing.

Figure 4. Helveticosaurus had cheek teeth that look like baleen strainers and long fangs anteriorly.

The proximal outgroup taxon for Atopodentatus
is the Late Permian marine younginiform, Adelosaurus, which doesn’t have any obvious marine traits. The skull is also unknown. And the phylogenetic difference between the Late Permian and Middle Triassic taxa are obvious (Fig. 5).

Figure 5. Atopodentatus compared to more primitive sister taxa, Adelosaurus and Claudiosaurus to scale.

References
Cheng L, Chen XH, Shang QH and Wu XC 2014. A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenschaften. doi:10.1007/s00114-014-1148-4.
Chun L, Rieppel O, Cheng L and Fraser NC 2016. The earliest herbivorous marine reptile and its remarkable jaw apparatus. Science Advances 06 May 2016: 2(5), e1501659
DOI: 10.1126/sciadv.1501659

 

 

 

Teyujagua: Not “transitional between archosauriforms and more primitive reptiles”

A new paper by Pinheiro et al. 2016
reports that the small skull (UNIPAMPA 653) of a new genus, Teyujagua paradoxa (Figs. 1, 2), is “transitional in morphology between archosauriforms and more primitive reptiles. This skull reveals for the first time the mosaic assembly of key features of the archosauriform skull, including the antorbital and mandibular fenestrae, serrated teeth, and closed lower temporal bar. Phylogenetic analysis recovers Teyujagua as the sister taxon to Archosauriformes…”

Well, that might be true if
you restrict the taxon list to the few (44) taxa employed by Pinheiro et al.

But when you expand the taxon list
to the size of the large reptile tree (660+ taxa) where we already have a long list of Youngina, Youngoides (Fig. 1) and Youngopsis sisters to Archosauriformes, then Teyujagua nests as a phylogenetically miniaturized sister to the NMQR 1484/C specimen attributed to Chasmatosaurus alexandri (Fig. 1). Like another phylogenetically miniaturized descendant of chasmatosaurs, Elachistosuchus huenei MB.R. 4520 and BPI 2871 (Figs. 3, 4), Teyujagua also turned its once large antorbital fenestra into a vestige (Figs. 1, 2).

Figure 1. Teyujagua compared to sister taxa, including Youngoides, Proterosuchus and Chasmatosaurus. Teyujagua is a phylogenetic miniature in which the antorbital fenestra became a vestige. Note the posterior jugal, which may or may not have supported a now missing quadratojugal anterior process.

I can see why the authors got so excited about their discovery.
The Teyjjagua skull looks like a little Chasmatosaurus skull without the antorbital fenestra. That’s because it IS one. In their own words, “This skull represents a previously unknown species that is the sister taxon to Archosauriformes and which fills a major morphological gap in understanding of early archosauriform evolution.”

Unfortunately, the authors were dealing with an antiquated cladogram
in which Youngina is basal to lizards and archosaurs… among many, many other atrocities.  They report, “Our novel cladistic analysis recovered two most parsimonious trees with 872 steps. The strict consensus of these topologies positions Teyujagua as the sister taxon of Archosauriformes, a position previously occupied by the Lower Triassic Prolacerta.”

So this is where it really pays off
to use several specimens from the Youngina grade and several specimens from the Proterosuchus grade along with 660+ opportunity taxa to nest with.

Figure 2. The rostrum of Teyujagua with the vestigial antoribital fenestra circled here. You can see how the maxilla grew over the opening. Once again, this is data that should have been announced from firsthand observation by PhD level paleontologists, not from a casual observer of photographic data.

Diagnosis (from the paper)
“Archosauromorph with the following unique character combination: confluent, dorsally positioned external nares; maxilla participating in orbital margin; antorbital fenestra absent; trapezoidal infra temporal fenestra with incomplete lower temporal bar; teeth serrated on distal margins; surangular bearing a lateral shelf; external mandibular fenestrae present and positioned beneath the orbits when the lower jaw is in occlusion (autapomorphic for Teyujagua).”

Comments on the diagnosis
The NMQR 1484/C specimen of Chasmatosaurus (Fig. 1) is pretty well preserved except for the premaxilla/narial region. Given the morphology of the Teyujagua rostrum, the NMQR specimen likely shares the trait of a dorsal naris, perhaps with a slender ascending process of the premaxila, which might be lost in both specimens. The maxilla actually does not appear to reach the orbit. The antorbital fenestra remains present as a closed over vestige. The lower temporal bar might be incomplete, but just as likely the anterior process of the quadratojugal might be taphonomically missing, as in the NMQR specimen (Fig. 1). Other proterosuchids have similar tooth serrations. The mandibular fenestra is further forward, but the posterior mandible is also deeper. The specimen is indeed distinct enough to merit a unique generic name, as is the case with several of the Chasmatosaurus/Proterosuchus specimens, which do NOT represent a growth series.

Phylogenetic miniaturization
has reduced the antorbital fenestra in BPI 2871 and Elachistosuchu, which nest at the base of the Choristodera. Both nest as descendants of larger Chasmatosaurus specimens in the large reptile tree.

Figure 3. Elachistosuchus (Janensch 1949, Sobral et al. 2015) is a sister to BPI 2871, a basal choristodere.

Not mentioned by the authors
The miniaturized skull of Teyujagua has fewer teeth than in sister or ancestors, but matching the condition in Euparkeria (Fig. 1), a related taxon only one node away at the base of a sister clade.

Figure 4. Rostral area of BPI 2871, formerly considered a younginid and here nesting at the base of the Choristodera as a descendant of Chasmatosaurus.

If you don’t remember
this earlier post (2011), Youngoides (UC1528, Fig. 5) had the genesis of an antorbital fenestra. It is the current proximal sister to the Archosauriformes in the large reptile tree.

Figure 5. GIF movie tracing the antorbital fenestra with fossa and surround bones of the FMNH UC 1528 specimen of Youngoides romeri. This is one of the earliest and most primitive appearances of the archosauriform antorbital fenestra, previously overlooked.

In summary, the authors report
“Teyujagua presents an unexpected combination of basal archosauromorph and typical archosauriform features. For example, Teyujagua resembles basal archosauromorphs in lacking an antorbital fenestra and retaining open lower temporal bars1. However, Teyujagua possesses external mandibular fenestrae and serrated teeth, features previously considered unique to Archosauriformes.”

Unfortunately, the authors appear to forget
that the antorbital fenestra can phylogenetically disappear and Chasmatosaurus demonstrates that the quadratojugal can wither phylogenetically or taphonomically disappear. It is a fragile bone.

References
Pinheiro FL, França MAG, Lacerda MB, Butler RJ and Schultz CL 2016. An exceptional fossil skull from South America and the origins of the archosauriform radiation. Nature Scientific Reports 6:22817 DOI: 10.1038/srep22817.

A new view of the first archosauriform antorbital fenestra

Earlier we looked at the FMNH UC 1528 specimen of Youngoides romeri. Today we’ll see another view of the same specimen in a GIF movie (Fig. 1). The antorbital fenestra identified here has been overlooked for several decades.

Figure 1. GIF movie tracing the antorbital fenestra with fossa and surround bones of the FMNH UC 1528 specimen of Youngoides romeri. This is one of the earliest and most primitive appearances of the archosauriform antorbital fenestra, previously overlooked.

Youngoides romeri FMNH UC1528 (Olson and Broom 1937) Late Permian, Wuchiapingian, ~255 mya was derived from a sister to Youngina AMNH 5561 and preceded Proterosuchus at the base of the Eurchosauriformes.

This nesting of Youngina and Youngoides at the base of the Protorosauria and Archosauriformes is recovered in the large reptile tree. Prior studies tended to include squamates close to this node, but the large reptile tree found squamates nesting on a completely separate branch with a last common ancestor at the origin of the Amniota/Reptilia.

References
Olson EC and Broom R 1937. New genera and species of tetrapods from the Karroo Beds of South Africa. Journal of Paleontology 11(7):613-619

wiki/Youngina

BPI 2871 – Now it is the oldest known choristodere

Updated July 5, 2015 with a lateral and occipital views of BPI 2871 and few text changes.

Earlier we looked at some of the earliest known Choristoderes recognized by traditional paleontologists. According to Wikipedia, Choristoderes are difficult to nest. “Cladistshave placed them between basal diapsids and basal archosauromorphs, but the phylogenetic position of Choristodera is still uncertain. It has also been proposed that they represent basal lepidosauromorphs. Most recently, workers have placed Choristodera within Archosauromorpha.”

Figure 1.  BPI 2871 specimen attributed to Youngina slightly modified from Gow 1975.

Gow 1975
considered the skull-only fossil, BPI 2871 (Bernard Price Institute, Fig. 1) a specimen of Youngina, despite its Late Triassic appearance (all other Youngina are Late Permian). For years his drawing (Fig. 1) was the only data I had for this specimen. Recently, and after several years of waiting, a requested image (Fig. 2) from the BPI was emailed and it clarified my understanding of this vaguely-croc-like basal archosauriform — perhaps without an antorbital fenestra — or perhaps this is one of the last taxa in the choristodere lineage to have a vestige antorbital fenestra, as it appears.

Figure 2. Dorsal, lateral and palatal views of BPI 2871 with bones colorized above. Below, reconstructed images of BPI 2871 tracings. It is more complete than illustrated by Gow 1975. Click to enlarge. Note the tiny remnant of the antorbital fenestra. The squamosal has been broken into several parts.

The new image
of BPI 2871 (Fig. 2) indicates that the skull is more complete than illustrated by Gow 1975. With this new data added to the matrix of the large reptile tree (Fig. 3) the nesting of BPI 2871 shifts it closer to the base of the Choristodera, evidently making it the oldest known member in the Late Triassic (210 mya).

Figure 3. Subset of the large reptile tree focusing on the pararchosauriformes and the Choristodera.

BPI 2871 is distinctly different
in chronology and morphology from Y. capensis, a fact overlooked or ignored by Gow who considered it another specimen of Youngina capensis. Like Bennett (1995, 1996, 2014) Gow was a lumper.

Tiny
BPI 2871 apparently lost the antorbital fenestra (requested lateral views are on the way, I am told). An absent antorbital fenestra is the reason why choristoderes have been difficult to nest in the reptile family tree. In the large reptile tree I simply popped in the traits and let the software recover the nesting. And it all makes sense. Note the resemblance of tiny BPI 2871 to the much larger Chanaresuchus, a related taxon. Like ancestral proterosuchids, BPI 2871 retains an overhanging snout and a shorter mandible. This specimen also retains postparietals.

Phylogenetic miniaturization
We keep meeting phylogenetic miniaturization at the base of novel clades and the same holds true for BPI 2871 and the Choristodera. Predecessor and successor taxa are both much larger.

Doswellia
and a rather large specimen of Proterosuchus (SAM PK-K 10603, early to Middle Triassic) also nest at the base of the Choristodera. Both retain an antorbital fenestra, rather feeble in the case of Doswellia (late Triassic).

Phylogeny
With all the new data BPI 2871 shifted one node over toward the choristoderes. This proves that inaccurate data, in the form of a simple drawing (Fig. 1), can still carry a large amount of data, that in this case, rather accurately places this taxon on the large reptile tree. More data refines that nesting.

References
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.

wiki/Youngina
wiki/Choristodera

 

 

The origin of the archosauriform antorbital fenestra

Earlier we learned that the antorbital fenestra, the hallmark of the Archosauriformes, actually had four other distinct and convergent origins in Chroniosuchians, Pararchosauriformes, Pamelaria + Jaxtasuchus and Fenestrasaurs.

Overlooked until now,
today we’ll look at the origin of the antorbital fenestra in the Archosauriformes.

Figure 1. Youngoides romeri FMNH UC1528 demonstrates an early appearance of the antorbital fenestra in the Archosauriformes. This specimen is the outgroup to Proterosuchus, the traditional basal member of the Archosauriformes.

Earlier we learned that the proximal outgroup taxon to the Archosauriformes was Youngoides romeri (FMNH UC 1528; Late Permian; Olson and Broom 1937; Figs.1, 2)

Figure 2. The origin of the antorbital fenestra in Youngoides romeri, FMNH UC1528. This trait has been overlooked until now. Yes, those are my fingers there.

The antorbital fenestra in Archosauriformes
began as a small opening in the skull below the lacrimal and above the maxilla in Youngina and Youngoides specimens. As proterosuchid descendants grew larger (Fig. 3), so did the antorbital fenestra. Euparkeriid descendants were not much larger — at first. In this clade the antorbital fenestra enlargement came at the expense of the a lateral temporal fenestra reduction as the orbit shifted posteriorly. In proterosuchids, the lateral temporal fenestra became wither taller or longer, depending on the clade.

Figure 3. The many faces of Proterosuchus to scale and in phylogenetic order, among with their closest known relatives. Note the phylogenetic miniaturization, reduction of the drooping premaxilla and loss of the antorbital fenestra after the TM 201 specimen of Chasmatosaurus. Click to enlarge.

The clade of terrestrial younginiformes,
and all of the Youngina specimens need to be reexamined as they have not been studied thoroughly as a group since Gow 1975, which predates cladistic analysis using software. Once you have an outgroup taxon for the Archosauriformes (Fig. 6), then you have a good idea where to look for the origin of the antorbital fenestra, a subject missed by Witmer 1997. The AMNH 5661 holotype of Youngina may also have had an antorbital fenestra, but the skull has several damaged areas, that one among them.

Figure 4. Youngina BPI 375. Is this a nascent antorbital fenestra? Starting small is what new traits do at their genesis.

Figure 5. Youngina capensis? BPI 375 appears to have an antorbital fenestra. Gow 1975 used dotted lines to signal he was unsure of the sutures.

The BPI 375 specimen of Youngina likewise seems to have had a small antorbital fenestra. If so, then the lack of an antorbital fenestra in most prolacertids (protorosaurs) represents a secondary loss of this trait. Gow (1975) drew the area with a dotted line (Fig 5), but the a DGS tracing of the specimen appears to show several possible fenestra between bones, the antorbital fenestra most prominent among them.

Figure 2. Subset of the large reptile tree focusing on the Protodiapsida, the Diapsida, Marine Younginiformes and Terrestrial Younginiformes, including Protorosaurs and Archosauriformes.
Click to enlarge.

In at least three lineage of archosauriformes,
the Choristodera, the Crocodylia, and derived Ornithischia, the antorbital fenestra disappeared. We’ll look at the choristodere sequence in a future blog post.

References
Gow CE 1975. The morphology and relationships of Youngina capensis Broom and Prolacerta broomi Parrington. Palaeontologia Africana, 18:89-131.
Olson EC and Broom R 1937. New genera and species of tetrapods from the Karroo Beds of South Africa. Journal of Paleontology 11(7):613-619.
Witmer LM 1997. The Evolution of the Antorbital Cavity of Archosaurs: A Study in Soft-Tissue Reconstruction in the Fossil Record with an Analysis of the Function of Pneumaticity. JVP 17(1 supp):1–76.

wiki/Youngina

The Terrestrial Younginiformes

When Alfred Romer proposed the term ‘Younginiformes‘ in 1947 as a replacement name for the taxon, Eosuchia, he had no idea this clade was diphyletic (marine and terrestrial clades). He was not yet aware of Spinoaequalis (Bickelmann, et al. 2009), which both clades share as a last common ancestor, (Fig. 2). As we learned earlier, Bickelmann, et al. (2009) also found terrestrial and marine branches for the younginiformes, but they included many unrelated taxa and did not include several pertinent taxa.

Earlier we looked at the clade of basal marine (aquatic) younginiformes. Today we’ll examine the clade of basal terrestrial younginiformes (Fig. 1). From Romer’s original list, only Youngina is included (Kenyasaurus is off the list now as we learned yesterday).

This presentation will take several blog posts
as we shed new light on a new tree topology for the base of the Archosauriformes. There is a lot to cover, many mysteries will be solved and many paradigms will be overturned, as you’ll soon see. Today: an overview (Fig. 1):

Figure 1. Terrestrial Yonginiformes + Galesphyrus representing the marine clade, all to scale except the toned area containing protorosaurs, which have their own scale.

As before
we start with the basal diapsid, Spinoaequalis (Fig.1).

Almost right from the start
two clades diverge (Fig. 3), the Protorosauria and the Archosauriformes: The both start off with little lizard-like taxa, Youngina and Prolacerta. The following taxa are in phylogenetic order within the Protorosauria as recovered by the large reptile tree.

1. The SAM K 1770 specimen(s) attributed to Youngina (the several den specimens)
2. The BPI 375 specimen attributed to Youngina
3. Prolacerta AMNH 9520
4. Prolacerta BPI/ I/475
5. Protorosaurus
6. Jaxtasuchus
7. Boreopricea
8. Azendohsaurus
9. Pamelaria

The following taxa are in phylogenetic order within the basal Archosauriformes as recovered by the large reptile tree.

1. The TM 3603 specimen attributed to Youngina
2. The RC90 specimen of Youngopsis rubidgei
3. The  TM 1490 specimen of Youngopsis kitchingi 
4. The RC91 specimen of Youngoides minor
5. Youngina capensis holotype, AMNH 5661
6. Youngoides romeri holotype, FMNH UC 1528
7. The BPI I 4016 specimen attributed to Proterosuchus
8. Euparkeria and Osmolskina.

Note that
the BPI 3859 specimen attributed to Youngina is not related to the others in the terrestrial clade, but nests within the marine clade (Fig. 2).

Figure 2. Subset of the large reptile tree focusing on the Protodiapsida, the Diapsida, Marine Younginiformes and Terrestrial Younginiformes, including Protorosaurs and Archosauriformes.
Click to enlarge.

Key to this discussion
is the basal position of various Youngina/Youngopsis/Youngoides specimens closer to Protorosaurs and Archosauriformes than traditionally considered, basal to lepidosaurs and archosaurs.

According to Wikipedia,
the bastion of traditional thinking in paleontology, “The [Eosuchia] has almost been treated as a dustbin for diapsids that are not obviously lepidosaurian or archosaurian. One consequence has been Romer’s suggestion of the alternative order Younginiformes to be applied strictly to those forms with the primitive diapsid form, in particular, a complete lowermost arch as the quadratojugal and jugal bones of the skull meet.”

Unfortunately,
the large reptile tree has put the division between lepidosaurs and archosaurs clades back to the Viséan, near the origin of the Amniota (= Reptilia). Now Younginiformes are basal to the taxa listed above (Fig. 2). This new insight arises from increasing taxon inclusion. These results require a major paradigm shift for most paleontologists.

Youngina and kin
The various specimens attributed to Youngina need to be updated. Some of the latest figures go back 40 years. Others go back 80 years. Only a few of the above figures were traced from recent photos, some taken after viewing the specimen.

A den of Youngina specimens (Smith and Evans 1996)
(SAM K7710) were considered juveniles because they were smaller than other known Youngina specimens, otherwise only known from skulls. Unfortunately, Smith and Evans did not include Spinoaequalis in their study. Here Spinoaequalis nests as an outgroup sister to the den of Youngina specimens, and it is slightly larger than the den specimens are. And the den specimens are indeed smaller than the rest of the Youngina specimens are. Thus the origin of the terrestrial younginiformes also experienced a slight amount of phylogenetic miniaturization. The den specimens are older than the rest of the Youngina specimens, and, according to the large reptile tree (Fig. 2) they are also more primitive.

Tomorrow we’ll look at the many faces of Proterosuchus. The one shown above (Fig. 1) has been considered a juvenile, but it is also the one closest in morphology to the outgroup taxa among basal Youngoides and Youngina specimens.

References
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences 46:651-661.
Smith, RMH and Evans SE 1996. New material of Youngina: evidence of juvenile aggregation in Permian diapsid reptiles. Palaeontology, 39 (2):289–303.

Kenyasaurus not a tangasaur… not a diapsid… It’s a very basal dromasaur!

Earlier we looked at marine younginiformes. Perhaps conspicuous by its absence was Kenyasaurus, which was originally considered related to tangasaurid younginiformes. Last night I found data, plugged it into the large reptile tree and was surprised at where Kenyasaurus nested.

Kenyasaurus mariakaniensis
(Harris and Carroll 1977; Early Triassic; KNM-MA1, National Museum of Kenya) is represented by a headless skeleton with only a partial forelimb and pectoral girdle (Fig. 1).

Figure 1. Kenyasaurus in situ. Click to enlarge. This rather plain specimen nests not with tangasaurids, but with dromasaurids according to the large reptile tree. Boxed area: the primitive dromasaur, Galechirus and its foot to scale for comparison. Haptodus foot for comparison, not to scale. Pink and green tarsals are absent in Kenyasaurus and dromasaurs. Note the similarity of the pes of Kenyasaurus and Haptodus, sharing the same number and proportion of pedal elements, less the two tarsals.

Originally considered
a relative of Tangasaurus and Hovasaurus, the large reptile tree nested Kenyasaurus with the arboreal herbivorous dromasaurid synapsids. If so, the purported ‘well-developed sternum’ (Fig. 1, lavender) must instead be the posterior coracoid because synapsids do not have a sternum*. Harris and Carroll (1977) noted the long tail was unlike those of the Tangasaurus and Hovasaurus and that the tarsus lacked a fifth distal tarsal, as in dromasaurs. The caudal transverse processes gradually diminished over 30 vertebrae creating a cylindrical, muscular tail similar to those found in dromasaurs, only longer.

Figure 2. Two other dromasaurs, Suminia and Galechirus. Note the similar ilium shapes.

Currie 1982
also examined Kenyasaurus. At that time Currie did not have a computer or software to test traditional nestings. And he had just been studying Tangasaurus and Hovasaurus. So he considered Kenyasaurus a tangasaurid.

Currie (1982) diagnosed Kenyasaurus on the basis of five autapomorphies:

1. low but anteroposteriorly elongate neural spines in the dorsal region
2. 56 caudal vertebrae and
3. 28 pairs of caudal ribs and transverse processes.
4. Astragalus almost triangular rather than primitive L-shape
5. Pronounced process on fifth metatarsal for insertion of peroneus brevis

How do these compare to dromasaurs?

1. Neural spines in known dromasaurs and outgroups are taller than long
2. About 45 caudal vertebrae are present in Galechirus, but they get very tiny at the tip, 52 are present in Suminia
3. 22 pairs of caudal ribs and transverse processes are present in Suminia
4. Astragalus triangular present in Suminia, square present in Galechirus
5. No pronounced process on fifth metatarsal

So, no wonder Kenyasaurus was not considered a dromasaur.

Bickelmann, Müller and Reisz 2009
did have a computer and software to test traditional nestings. They found support for two distinct families within “Younginiformes”: the aquatic Tangasauridae, and the terrestrial Younginidae. However, they found no support for the inclusion of Kenyasaurus within any of those families. Unfortunately that study also included the unrelated Lanthanolania, Palaegama, Saurosternon and Coelurosauravus (all basal lepidosaurifomes related to Triassic rib gliders) within the same clade that also included Claudiosaurus and the Younginiformes. Very odd.

Shift Kenyasaurus closer to Tangasaurus
and you’ll add 10 steps to the most parsimonious tree. Whether a sternum was present or not makes little difference.

Delete the two dromasaurs
from the large reptile tree and Kenyasaurus creates a large polytomy (loss of resolution) among basal synapsids.

Post-pectoral characters shared by Kenyasaurus and dromasaurs
to the exclusion of basal synapsids include:

1. Gastralia present and rodlike (otherwise last seen in Ophiacodon)
2. Ventral pelvis: separate plates, small medial opening
3. Pubis orientation: medial
4. Overall size: < 30 cm tall, 60 cm long

So, not a lot to work with.

Other dromasaurids
are known from the Late Permian, so Kenyasaurus would have been a late-survivor in the Early Triassic, despite its more basal nesting. That’s another black mark against Kenyasaurus being a dromasaur. Nevertheless, among the 542 taxa in the inclusion set, Kenyasaurus is most attracted to the dromasaurs with the present data set and scores.

Some final thoughts
Kenyasaurus displays no reduction of the middle phalanges of digits 3 and 4 of the manus and pes, so it resembles more primitive pelycosaur-grade synapsids in this regard. Based on this fact, the reduction of the three middle pedal phalanges may have occurred by convergence  within Therapsida, once in anomodonts and again in the main line beginning with biarmosuchids.

Basal anomodonts likely split from basal therapsids, like Stenocybus and Cutleria in the Early Permian. So Kenyasaurus was a very late (Early Triassic) remnant of that earlier radiation. So the autapomorphies that Currie listed (above) could have evolved during those tens of millions of years. And yes, I am making excuses for this taxon because it does not exactly match the ideal we might imagine. But those excuses could be true.

References
Bickelmann C, Müller J and Reisz RR 2009. The enigmatic diapsid Acerosodontosaurus piveteaui (Reptilia: Neodiapsida) from the Upper Permian of Madagascar and the paraphyly of “younginiform” reptiles. Canadian Journal of Earth Sciences 46:651-661.
Currie P 1982. The osteology and relationships of Tangasaurus mennelli Haughton. Annals of The South African Museum 86:247-265. http://biostor.org/reference/111508
Harris JM and Carroll RL 1977. Kenyasaurus, a New Eosuchian Reptile from the Early Triassic of Kenya. Journal of Paleontology 51:139–149.

* We’ll look at the mammals sternum/manubrium issue later….