SVP abstracts 11: Palacrodon returns as a drepanosauromorph?

Jenkins et al. 2020 review
“the phylogenetic placement of an enigmatic reptile from the Early Triassic Transantarctic Mountains.” This reptile has gone through some name changes, but the large reptile tree (LRT, 1751+ taxa) nested it in 2016 with similar, big-eyed, basal placodonts like Palatodonta and Pappochelys (Fig. 1). Co-authors Jenkins and Lewis (2016) nested it with rhynchocephalians, but limited their taxon list to rhynchocephalians and procolophonids. There is no indication that they included basal placodonts in 2020.

Originally
(Broom 1906) considered what little is known of Palacrodon browni (= Fremouwsaurus geludens; Early Triassic; Fig. 1) a member of the Rhynchocephalia.

Figure 1. A comparison of basal placodonts to scale (and Paraplacodus reduced to one-third shows how Fremouwsaurus (Palacrodon) is transitional between the small spike-tooth ancestors like Palatodonta and Pappochelys and the pavement toothed Paraplacodus.

From the Jenkins et al. 2020 abstract:
“The phylogenetic placement of Palacrodon has been contentious since its initial description, with workers naming it as either a rhynchocephalian, lizard, procolophonid, eosuchian, or archosauromorph.”

Taxon inclusion nests it with basal placodonts.

“The uncertainty surrounding the phylogenetic affinity of Palacrodon in large part stems from the fact that nearly all the specimens found are teeth and fragmentary portions of tooth-bearing bone. Palacrodon bears characteristic labio-lingually elongate, molariform, cuspidate teeth reminiscent of herbivorous reptiles like extinct trilophosaurs and polyglyphanodonts and modern shell-crushing lizards.”

“Because previous workers lacked any other skeletal material, Palacrodon has never been placed within a phylogeny.”

Never? The LRT placed it in 2016,

“Though its phylogenetic affinity is uncertain, Palacrodon is a cosmopolitan genus spanning most of the Triassic, with specimens found in the Early Triassic of Antarctica, Early-Middle Triassic of South Africa, and the Late Triassic of Arizona. The only specimen of Palacrodon possessing more than dentition is from the Early Triassic lower Fremouw Formation of Antarctica (specimen number BP/1/5296). That formation is the sedimentary sequence immediately preceding the Permian-Triassic mass extinction boundary in the Transantarctic Mountains and represents the only known Early Triassic paleopolar deposit with abundant tetrapod material. The Antarctic specimen of Palacrodon was described from the impression of a latex peel as a partial small skull belonging to an unknown diapsid reptile initially named Fremouwsaurus geludens, which was later synonymized with Palacrodon.”

“We CT scanned the Antarctic specimen and found that previously undescribed skeletal elements are preserved in BP/1/5296. These include limb bones, ribs, phalanges, caudal vertebrae, ankle bones, and an ilium. Of the cranial elements, portions of the right maxilla, lacrimal, prefrontal, jugal, postorbital, ectopterygoid, and dentary are preserved. Both parsimony and Bayesian analyses found Palacrodon to be a stem saurian with close affinities to drepanosauromorphs.”

See figure 2 for known drepanosaurs (all Late Triassic) and their ancestor, Jesairosaurus (Early to Middle Triassic) in the LRT.

Figure 3. Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

From the Jenkins et al. 2020 abstract:
“This finding suggests that Palacrodon is the earliest known drepanosaur, extending the temporal range of the clade by nearly 20 million years. Palacrodon is also the only known drepanosauromorph from the southern hemisphere. Further analysis of these new skeletal elements will now allow a more thorough understanding of the behavior and niche of Palacrodon and primitive drepanosuars generally.”

Excluding far fewer taxa, in the large reptile tree (LRT, 1749+ taxa) moving Palacrodon from the base of the Placodontia to the base of the Drepanosauromorpha adds 8 steps based on very few skull traits. Of course the post-crania could change things, but usually taxon exclusion changes things more.

Figure 2. The head of Palacrodon and the headless body of the Majiashanosaurus compared.

References
Broom R 1906. On a new South African Triassic rhynchocephalian. Transactions of the Philosophical Society of South Africa 16:379-380.
Gow CE 1992. An enigmatic new reptile from the Lower Triassic Fremouw Formation of Antarctica. Palaeontologia Africana 29:21-23.
Gow CE 1999. The Triassic reptile Palacrodon brown Broom, synonymy and a new specimen.
Jenkins K, Lewis P, Choiniere J and Bhullar B-A 2020. The phylogenetic placement of an enigmatic reptile from the Early Triassic Transantarctic Mountains. SVP abstracts 2020.
Jenkins KM and Lewis PJ. 2016. Triassic lepidosaur from southern Gondwana. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
Neenan JM, Li C, Rieppel O, Bernardini F, Tuniz C, Muscio G and Scheyer TG 2014. Unique method of tooth replacement in durophagous placodont marine reptiles, with new data on the dentition of Chinese taxa. Journal of Anatomy 224(5):603-613.

https://pterosaurheresies.wordpress.com/2016/10/30/is-palacrodon-a-rhynchocephalian-svp-abstract-2016/

 

Wrist supination/pronation in Megalancosaurus?

Figure 8. Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

One of the weirdest of the weird
Megalancosaurus has been studied and published previously (see refs below). A recent addition (Castiello et al. 2016) adds fused clavicles, a saddle-shaped glenoid, a tight connection between the radius and ulna that hindered pronation/suppination (but see below) and hypothetical forelimb muscles to our knowledge of this basal lepidosauriform.

Unfortunately 
the authors only go as far as labeling this taxon a drepanosaur and a drepanosauromorph without further identifying the larger and even larger clades these taxa nest within.

News

1. “unlike those of other drepanosauromorphs [the clavicles] are fused together and possess a small median process caudally directed so that the whole structure looks similar to the furcula of theropod dinosaurs, especially oviraptorids.”
2. “The scapular blade reaches the modified, expanded neural spines of the third and fourth dorsal vertebra so that the pectoral girdle formed a solid ring, which would have been very rigid.”
3. “the glenoid fossa has a saddle-shaped structure and lies on the coracoid”
4. “paired sternal plates are fused to the coracoids forming a craniocaudally elongate coracosternal complex.”
5. “the coracosternal complex was more vertically oriented than in previous reconstructions” but as figured for Drepanosaurus and Megalancosaurus (Fig. 1) at ReptileEvolution.com.
6. Rather than a separate olecranon sesamoid (Figs. 1, 2) that Megalancosaurus and all of its sisters share, the authors report on, “the elongate olecranon process of the ulna.”
7. Rather than recognizing a bone break in the ulna (Fig. 2), the authors report, “a small notch is present on the medial margin of the ulna distal to the articular surface for the humerus. This notch houses the medial corner of the proximal head of the radius, suggesting that in life, the two bones were firmly connected together at their proximal end, preventing pronation and supination of the forearm.” No other sister taxa or tetrapods have such an ulna notch. Note, the notch is not present in figure 2, but the sesamoid is pretty broken up. These bones are hollow, fragile and crushed. Be careful how you interpret them. Earlier we saw another misinterpretation of a drepanosaur forelimb.
8. When the authors present a hypothetical forelimb myology they do not present a pertinent actual forelimb myology (Fig. 3) for comparison. Such a comparison helps assure the reader that the myology for Megalancosaurus has not been invented and follows actual patterns and sizes.

Figure x. The break and the broken pieces of the Megalancosaurus ulna are reidentified here. The sesamoid is prominent and crescent-shaped as in Drepanosaurus.

Crushed hollow bones
are sometimes difficult to interpret, as we’ve seen before.

Figure 2. Elbow sesamoid in another specimen of Megalancosaurus, MPUM 8437.

The authors provided a hypothetical myology
which they phylogenetically bracketed by lepidosaurs and crocodilians (which means what??) based on prior pterosaur forelimb myology as imagined by Bennett (2003, 2008). Pterosaurs are unrelated to drepanosaurs. The Bennett pterosaur myology had problems because it located extensors and flexors anterior and posterior to the fore arm, rather than dorsal and ventral (palmar) as in Sphenodon (Fig. 3) the closest living taxon to drepanosaurs AND pterosaurs.

Figure 3 Sphenodon hand muscles. Click to enlarge. These were not referenced in the Castiello et al. study.

It would have been appropriate

1. to show that the fingers of Megalancosaurus had more phalanges (Fig. 4), as seen in sister taxa and as I see them in Megalancosaurus itself.
2. to show two versions of the manus, with spread metacarpals (as presented) and another with more closely appressed metacarpals, as in sister taxa, Hypuronector, Vallesaurus, and Drepanosaurus (Fig. 4).
3. to take a closer look at that ulna notch, knowing that such a notch mechanically weakens the cylinder, is produced by broken bone, and is not repeated in other drepanosaurs.
4. to take a closer look at that olecranon ‘process’ because sister taxa all have a large sesamoid.
5. to phylogenetically nest drepanosaurs in order to provide the most accurate myology analogy possible.

Figure 4. Click to enlarge. The sister taxa of Drepanosaurus all had an olecranon sesamoid. Drepanosaurus simply had a larger one.

The above data
has been online for the past six years. Plenty of time to consider it. No need to cite it.

Pronation/supination
Arboreal taxa in general and distant drepanosauromroph sisters (Palaegama and Jesairosaurus) are able to axially rotate the forearm by at least some degree. Like the human forearm, the radius and ulna in these taxa are separated by a long oval space that enables the radius to axially rotate on the ulna.

By contrast 
the radius and ulna of Hypuronector are appressed (Fig. 4), restricting pronation/ supination. Vallesaurus may have been similar, but taphonomic disarticulation makes it difficult to tell. The forearm was relatively shorter than the humerus. Drepanosaurus had a similar short forearm, but also had a giant elbow sesamoid that essentially extended the humerus, separated the proximal radius and ulna, as in birds, but shifted the radius to the sesamoid, deleting the parallelogram effect — AND likely reducing pronation and supination.

Unlike its sisters, but like humans,
the radius and ulna of Megalancosaurus were slender, elongate and separated by an interosseus space. I don’t see any reason to suggest that pronation and supination were restricted to 0º here, but not nearly to the extent found in humans (Homo), about 180º. The radius in Megalancosaurus still appears to articulate with the humerus and if re-inflated from its crushed state, might be a cylinder with a circular proximal articulation, enabling pronation and supination.

References
Bennett SC 2003. Morphological evolution of the pectoral girdle of pterosaurs: myology and function. In: Buffetaut E, Mazin J-M, editors. Evolution and palaeobiology of pterosaurs. Geol Soc Spec Publ. 217. London (UK): Geological Society of London. p. 191–215.
Bennett SC 2008. Morphological evolution of the forelimb of pterosaurs: myology and function. In: Buffetaut E, Hone DWE, editors. Flugsaurier: pterosaur papers in honour of Peter Wellnhofer. München: Zitteliana. B28. p. 127–141.
Calzavara M, Muscio G and Wild R 1980. Megalancosaurus preonensis n. gen. n. sp., a new reptile from the Norian of Friuli. Gortania 2: 59-64.
Castiello M, Renesto S and Bennett SC 2016. The role of the forelimb in prey capture in the Late Triassic reptile Megalancosaurus (Diapsida, Drepanosauromorpha). Historical Biology DOI: 10.1080/08912963.2015.1107552
Feduccia A and Wild R 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Natur Wissenschaften 80:564–566.
Geist NR and Feduccia A 2000. Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 4):664-675. online pdf
Renesto S 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.
Renesto S 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–179.

wiki/Megalancosaurus

Jesairosaurus and the drepanosaurs leave the Tritosauria :-(

My earlier reconstruction
of the basal lepidosauriform, Jesairosaurus (Fig. 1; contra Jalil 1997, not a protorosaur/prolacertiform) included several errors based on attempting to create a chimaera of several specimens of various sizes based on scale bars. In this case, scale bars should not have been used. Rather fore and hind parts had to be scaled to common elements, like dorsal vertebrae, as shown below (Fig. 2). I think this version more accurately reflects the in vivo specimen, despite its chimeric origins. All of the partial skeletons assigned to this genus were discovered at the same Early to Middle Triassic sandstone site and two were touching one another. A larger skull, ZAR 7, shows the variation in size from the skull to shoulders remains of the ZAR 6 specimen.

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1997). The wide and flat ribs are interesting traits for a likely arboreal reptile.

Mother of all drepanosaurs
The Drepanosauria is an odd clade of slow-moving arboreal reptiles that includes Hypuronector, Vallesaurus, Megalancosaurus and Drepanosaurus (Figs. 2, 3). Jesairosaurus was not a drepanosaur, but nested basal to this clade before the present revisions. It remains basal to the Drepanosauria now with revisions.

The revised reconstruction of Jesairosaurus 
shifts this clade away from Huehuecuetzpalli, Macrocnemus and the rest of the Tritosauria. Now Jesairosaurus and the drepanosaurs nests between Saurosternon, Palaegama and the so-called “rib” gliders, beginning with Coelurosauravus.

A short history of Jesairosaurus
Shortly after their discovery Lehman 1971 referred the several hematite encrusted specimens to the Procolophonida. Further preparation showed that they were referable to the Diapsida, according to Jalil (1990) and the, more specifically, to the Prolacertiformes (Jalil 1997) as a sister to Malerisaurus with Prolacerta as a common ancestral sister. Jalil did not include the closest sisters of Jesairosaurus as revealed by the present analysis.

With a much larger list of taxa,
the large reptile tree nests Malerisaurus between the Antarctica specimen assigned to Prolacerta (AMNH 9520) and the holotype of Prolacerta. Jesairosaurus, as mentioned above, nests with the basal lepidosauriformes. Any traits shared with protorosaurs are by convergence. Deletion of Jesairosaurus does not affect the nesting of the Drepanosauria as basal lepidosauriformes.

Figure 3. Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Arboreal
This new nesting shifts drepanosaurs closer to kuehneosaurs (Figs. 3, 4), another notably arboreal clade.

Figure 3. The new nesting for Jesairosaurus and the drepanosaurs as sisters to the Kuehneosaurs, several nodes away from Huehuecuetzpalli and the tritosaurs.

Certainly
there will someday be more taxa to fill in the current large morphological gaps in and around Jesairosaurus, but here’s what we have at present (Fig. 3) with regard to the origin of the so-called “rib” gliders (actually dermal rods, not ribs, as shown by Coelurosauravus) and the origin of the drepanosaurs.

Figure 4. Jesairosaurus to scale with sisters Palaegama and Coelurosauravus.

The shifting of a clade
like Jesairosaurus + Drepanosauria occurred due to inaccurate reconstructions used for data. Science builds on earlier errors and inaccuracies. I let the computer figure out where taxa nest in a cladogram of 606 possible nesting sites, minimizing the negative effects of bias and tradition.

It’s sad
to see the drepanosaurs leaving the Tritosauria as it contains several oddly Dr. Seuss-ian variations on the tritosaur theme.

Also note the nesting
of the basal Rhynchocephalians, Megachirella and Pleurosaurus, between the palaegamids and the tritosaurs (Fig. 4). In the course of this study, both also received updates to their skull reconstructions. The former was difficult to interpret without knowing where it nested. What appeared to be an odd sort of a squamosal in Megachirella now appears to be a pair of displaced pleurosaur-like premaxillae. For Pleurosaurus I should not have trusted a prior line drawing by another worker. Here I used DGS to create what appears to be a more accurate skull without so many apparent autapomorphies.

References
Jalil N 1990. Sur deux cranes de petits Sauria (Amniota, Diapsida) du Trias moyen d’ Algerie. Comptes Rendus de I’ Academic des Sciences, Paris 311 :73 1- 736.
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.
Lehman JP 1971. Nouveaux vertebres du Trias de la Serie de Zarzai’tine. Annales de Paleontologic (Vertebres) 57 :71-93.

 

 

A closer look at Jesairosaurus

Updated November 29, 2015 with a new reconstruction and nesting for Jesairosaurus.

Earlier we looked at Jesairosaurus (Jalil 1991) and the origin of the Drepanosauridae. Today we’ll take a closer look at Jesairosaurus.

Figure 1. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Jesairosaurus was originally considered a procolophonid, then a prolacertiform
Sure it has a long neck, but in phylogenetic analysis, it doesn’t nest with Prolacerta or Marlerisaurus, but after the basal lepidosauriform Palaegama and prior to drepanosaurs and kuehneosaurs. Note the dorsal nares and large orbit. There’s a very tall scapula here, a precursor to the tall stem in drepanosaurs.

Distinct from most lepidosauriforms,
Jesairosaurus has fairly large thecodont teeth. Gastralia appear here for the first and last time in this lineage. The ventral pelvis isn’t fused, but the thyroid fenestra is gone.

Like most lepidosauriforms,
the scapulocoracoid was not fenestrated. And there’s a nice ossified sternum there.

On a side note…
As you know, I’m always attempting to improve the data here. Several months ago I mentioned to a detractor that most prior workers reported  forelimbs present in Sharovipteryx. Only Unwin et al. (2000) thought they were buried in the matrix. My detractor claimed the opposite, that I was the only one to see forelimbs. No word yet on this issue. Still waiting.

Another detractor claimed I had seen soft tissue on a Bavarian museum fossil pterosaur. When I asked which specimen, he refused to provide the number.

In a third case I asked to see a closeup of a pterosaur mandible tip that had been published. I wondered if the sharp tip might be a tooth, as it is in other sharp mandible pterosaurs. The offer was refused with the phrase, “trust us, it’s not there.” I replied “trust” is antithetical to Science. No reply and no closeup yet.

So, is it so hard to provide a museum number? A closeup of a photograph? Or a reply to a note on forelimbs? Should we trust other scientists? Or should we test and confirm or refute? There may be cooperation among other paleontologists. Or maybe they’re all very protective of their data. Evidently I also have a very bad reputation, and that may be the reason for the lack of cooperation. These things happen when paradigms are broken.

References
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.
Unwin DM, Alifanov VR and Benton MJ 2000. Enigmatic small reptiles from the Middle-Late Triassic of Kyrgyzstan. In: Benton M.J., Shishkin M.A. & Unwin D.M. (Eds) The Age of Dinosaurs in Russia and Mongolia. Cambridge: Cambridge U. Press: 177-186.

wiki/Prolacertiformes

 

 

Drepanosaur pectoral girdle, bones identified

Along with the recent abstract on desert drepanosaurs, an earlier paper (Harris and Downs 2002) reported on a 3D drepanosaur pectoral girdle (Fig. 1).

Figure 1. 3D drepanosaur pectoral girdle. Green = overlapping clavicles, lavendar = interclavicle, pink = scapula, yellow = coracoid, blue = sternum.

Harris and Downs (2002) accurately reported on most of the elements here. But they overlooked the big one in the middle. They either overlooked the interclavicle binding all the other pectoral elements together, or else they considered it part of the scapulocoracoid. They also did not notice the right clavicle overlapped the left one.

These color images were created using a very simple version of DGS, digital graphic segregation, which also involves phylogenetic analysis and phylogenetic bracketing.

Drepanosaurs are tritosaur lepidosaurs. Earlier we looked at several other tritosaur pectoral girdles. Tritosaurs do weird things with their interclavicle/sternum.

Figure 2. Drepanosaur pectoral girdles

Here (Fig. we see in ventral view the interclavicle and overlapping clavicles of Jesairosaurus, the mother of all drepanosaurids. Cosesaurus, likewise, has these overlapping clavicles, as does Longisquama and the Pterosauria, as part of their sternal complex.

Figure 3. The broad, yet T-shaped interclavicle of Jesairosaurus, an ancestral taxon at the base of the drepanosaurids.

Hypuronector (Fig. 4 ) is also ancestral to drepanosaurids, nesting between Vallesaurus and Jesairosaurus.

Figure 4. Hypuronector pectoral girdle. Scapula = pink. Clavicle = yellow. Interclavicle = lavender. Sternum = blue. Clavicle = green. Hypuronector is also ancestral to drepanosaurs.

Vallesaurus is represented by a crushed specimen in which the interclavicle appears to be broken at or near the midline during crushing. The clavicles and sterna are likewise split medially.

Vallesaurus pectoral girdle. Here the interclavicle (in lavender/puple), if that is what it is, is neatly split in half along with the clavicles and sterna.

Dr. Seuss had it right
This clade of tritosaur lepidosaurs (drepanosaurs, tanstropheids, fenestrasaurs, pterosaurs) are indeed a bizarre bunch of the most unusual reptiles known to science.

References
Harris JD and Downs A 2002. A drepanosaurid pectoral girdle from the Ghost Ranch (Whitaker) Coelophysis quarry (Chinle Group, Rock Point Formation, Rhaetian), New Mexico. Journal of Vertebrate Paleontology 22(1):70-75.

Desert drepanosaurs – svp abstract 2013

From the abstract
Chure et al. wrote: “Drepanosaurids are enigmatic diapsids from the Late Triassic of Asia, Europe, and North America. Here we report on a new form from the Nugget Sandstone based on multiple three-dimensional, articulated skeletons with disarticulated skulls, found lying side-by-side.  The new taxon combines diagnostic features of other drepanosaurids (Dolabrosaurus, Megalancosaurus, and Drepanosaurus.) The Nugget form is most closely related to Drepanosaurus. Synapomorphies with that genus include 1) short, plate-like ulna, 2) greatly elongated radiale and ulnare replacing the ulna and forming part of the forearm, 3) hypertrophied manual ungual 2. Striking autapomorphies of the new taxon are 1) large maxillary and dentary teeth reminiscent of Trilophosaurus in being tall, much wider than long, and bearing small labiolingually arranged apical cusps, 2) manual digit 1 opposable and bearing hypertrophied ungual at least as large as that on 2, 3) large, well defined pleurocoels on posterior dorsal centra, 4) dorsal vertebrae prezygapophyses fused into a narrow, midline process at the neural spine base. Striking dimorphism is seen in the pes, with an abbreviated opposable digit I in some specimens but not others. This dimorphism is not ontogenetic but may be sexual. Drepanosaurids are generally viewed as arboreal and chameleon-like, but many specializations in the Nugget taxon (and Drepanosaurus) are similar to adaptations for burrowing and digging in extant small mammals and suggest a similar habit. In addition, these are the first drepanosaurids from an erg environment, indicating drepanosaur ecology was more diverse than previously envisioned. The age of the Early Jurassic erg deposits in western North America is poorly constrained due to lack of age diagnostic fossils or datable crystals. The Saints and Sinners Quarry is approximately 55m above the base of the eolian part of the Nugget, within interdunal sediments situated between large cross-bedded eolian packages. Elsewhere, drepanosaurids are restricted to the Late Carnian through Late Norian. These Nugget drepanosaurids suggest that either a significant part of the formation is Triassic or that the group extended into the Jurassic. In either case, the Nugget material is likely the geologically youngest record of the group.”

Figure 1. The Drepanosauria and their ancestors, Huehuecuetzpalli and Jesairosaurus. The elbow bone is really an olecranon sesamoid, not a shifted ulna.

Notes:
Earlier we established that the elbow bone was a previously overlooked sesamoid common to many drepanosaurids, not the displaced ulna. Lots of outgroups have the sesamoid and it gets larger closer to Drepanosaurus (Fig. 1). Otherwise it’s exciting to think about seeing these new desert drepanosaurs.

References
Chure D, Britt B, Engelmann G, Andrus A and Scheetz R 2013. Drepanosaurs in the desert: multipled skeletons of a new dreapanosauris from the eolian nugget sandstone (?Late Triassic – Early Jurassic), Saints and Sinners Quarry, Utah: morphology, relationships, and biostrategraphic implications.

The Antorbital Fenestra of Drepanosaurs

The drepanosaurs are a rag-tag clade of slow-moving arboreal tritosaur lizards derived from Jesairosaurus in the large reptile tree. Drepanosaurs include Hypuronector, Vallesaurus, Megalancosaurus and Drepanosaurus in order of greater derivation.

Figure 1. The skull of Vallesaurus in situ. Difficult to interpret, to be sure, but I don’t see any large maxillary plates here. Perhaps palatal elements, though.

There has been some debate as to whether or not drepanosaurs had an antorbital fenestra. If so, this would have been the fifth (or perhaps the fourth since this clade is very close to Langobardisaurus and Cosesaurus) origin of the antorbital fenestra. Unfortunately only two drepanosaurs include skulls, Vallesaurus and Megalancosaurus. Here we’ll trace and reconstruct both. It’s no easy task.

Figure 2. The skull of Vallesaurus as interpreted by Renesto (2006). He found no evidence of an antorbital fenestra. The stem-like nasal and lacrimal are both present here. The question is, what, if anything, filled the space between them? Was it folded and flattened palatal bones? Renesto did not note any palatal elements here, yet they must have been present.

Figure 3. The palatal elements of Jesairosaurus, the outgroup for the drepanosaurs. Fairly solid-looking. This contrasts with the lighter airier palates of fenestrasaurs and Megalancosaurus.

The palate
When skulls are crushed in lateral view, something has to happen to the transverse palatal elements. Typically they flip up or down, like playing cards, presenting their broadest surface to the matrix plane. It is difficult to discern the shape of the palatal elements in Vallesaurus, but if they were more solid, like the elements in Jesairosaurus, then this may account for the bone that Renesto found behind and through the rostrum (= antorbital fenestra).

In the more derived drepanosaur, Megalancosaurus (fig. 8) these solid elements were reduced to more gracile elements.

Figure 4. Interpretation of the skull (sans palatal) elements of Vallesaurus. Note the slender ascending process of the maxilla in green and the slender lacrimal stem in magenta.

Figure 5. Interpretation of the skull of Vallesaurus based on figure 4.

Vallesaurus
Talk about difficult skulls to trace (Figs. 1, 4), reconstruct and restore (Fig. 5)!! And remember, if we are looking at a light and airy skull, we’re looking through a light and airy skull to the right side elements obscured by intervening palatal elements. I did not attempt to outline the palatal elements in Vallesaurus, but considering the size of the eyes, the palate had to be nearly as wide as the height of the skull, as in Megalancosaurus.

Figure 6. Both part and counterpart of Megalancosaurus superimposed using Photoshop.

Figure 7. Interpretation of figure 6, the skull of Megalancosaurus. Struts of bone surround antorbital fenestra here. The slender ascending process of the left maxilla is convincing evidence for an antorbital fenestra in Megalancosaurus. The huge size of the naris is further evidence for skull lightening as a selective influence.

Megalancosaurus
The skull of Megalancosaurus was split in two, the part and counterpart (Figs. 6, 7). Reassembling them with software helps to realign the two crushed halves together for interpretation of the elements. What I see there is a slender ascending process of the maxilla. That can only occur if there is an antorbital fenestra. If that bone is another bone, then this will have to be reconsidered. At this point an antorbital fenestra seems likely. And I’m open to new data if anyone out there has some.

Figure 8. Megalancosaurus including the palate, the only palate ever figured for a drepanosaur.

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

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

References
Calzavara M, Muscio G and Wild R 1980. Megalancosaurus preonensis n. gen. n. sp., a new reptile from the Norian of Friuli. Gortania 2: 59-64.
Feduccia A and Wild R 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Natur Wissenschaften 80:564–566.
Geist NR and Feduccia A 2000. Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 4):664-675. online pdf
Renesto S 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.
Renesto S 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–179.
Renesto S and Binelli G 2006. ’Vallesaurus Cenensis“’ Wild, 1991, A Drepanosurid (Reptilia, Diapsida): From the Late Triassic of Northern Italy”, Rivista Italiana di Paleontologia e Stratigrafia 112: 77–94, Milano.
Wild R 1990. Ein Flugsaurierrest (Reptilia, Pterosauria) aus der Unterkreide (Hauterive) von Hannover (Niedersachsen). Neues Jahrbuch für Geologie und Paläontologie. Abhandlung 181,241–254.

wiki/Vallesaurus
wiki/Megalancosaurus

Jesairosaurus and the origin of drepanosaurs

Updated November 29, 2015 with new data and a new nesting for Jesairosaurus and the drepanosaurs. 

The authors of Wikipedia
have no idea what drepanosaurs are other than reptiles. In the large reptile tree drepanosaurs were descended from Jesairosaurus and, more primitively, the basal tritosaur lepidosauriform, Palaegama. Here we’ll start with Palaegama (Fig. 1), the father of all drepanosaurs and kuehneosaurs.

Figure 1 Drepanosaurs and their ancestor sisters, Jesairosaurus and Palaegama to scale.

Palaegama is a basal lizard-like lepidosauriform that shared few obvious traits with the highly derived drepanosaurs. It does not take a transitional taxon, like Jesairosaurus (Fig. 1) to make the case for a relationship here because deletion does nothing to tree topology.

Now we narrow our focus
to Jesairosaurus, the father of all drepanosaurs (Fig. 2). Essentially Jesairosaurus was a long torso palaegamid lepidosauriform. The skulls are readily comparable. The limbs are shorter in the derived taxon.

Figure 2. New reconstruction of the basal lepidosauriform, Jesairosaurus (Jalil 1993).

Jesairosaurus lehmani (Jalil 1997) Early to Middle Triassic ~240 mya was originally described as a prolacertiform and thus related to Prolacerta. All shared traits are by convergence here. By virtue of its nesting, Jesairosaurus is also basal to the kuiehneosaurs, like Coelurosauravus (Fig.  3).

 

Figure 3. Jesarosaurus to scale with sisters Palaegama and Coelurosauravus.

With its bigger torso and smaller limbs,
Jesairosaurus was not a speedster. This was the first step in the evolution of the slow-moving, arboreal drepanosaurs. The high scapula of drepanosaurs finds an origin in Jesairosaurus.

It is unfortunate
that the hind feet and tail are not known for Jesairosaurus, because both of these body parts underwent a great transformation in early drepanosaurs.

As in Palaegama, the pelvis of Jesairosaurus was relatively tiny and included an anterior process of the ilium that developed further in drepanosaurs. While  Palaegama employed its expanded ilium to sprint, drepanosaurs were not sprinters, but arboreal climbers and clingers, like modern day chamaeleons.

 

References
Jalil N-E 1997. A new prolacertiform diapsid from the Triassic of North Africa and the interrelationships of the Prolacertiformes. Journal of Vertebrate Paleontology 17(3), 506-525.

wiki/Prolacertiformes

A New Elbow for Megalancosaurus

Just reviewing some old posts, I noticed an autapomorphy in my tracings of the elbow of Megalancosaurus. What I thought was a suture that shortened the ulna relative to the radius was in reality a break (also noted by S. Renesto who sent me some other closeups to support his observation, see below) and the two bones were actually similar in length, as in sister taxa. Here I correct that error and recover a more crescent-shaped sesamoid, more like the giant sesamoid in Drepanosaurus, plus lots of little broken bones that would have represented the ulnar crest.

Figure 1. The break and the broken pieces of the Megalancosaurus ulna are reidentified here. The sesamoid is prominent and crescent-shaped as in Drepanosaurus. The broken portion of the ulna would have stood straight up from the matrix,  if similar to that of Drepanosaurus, which is why it broke during crushing.

More Specimens
Other specimens of this genus also demonstrate a sesamoid present at the elbow of Megalancosaurus.

Figure 2. Elbow sesamoid in another specimen of Megalancosaurus, MPUM 8437.

Figure 3. Another Megalancosaurus elbow, MPUM 6008.

These are further data supporting the hypothesis that the odd bone at the elbow of Drepanosaurus was a sesamoid, not the ulna, as Renesto (1994) proposed, nor the coracoid, as Pinna (1980) proposed.

Note, I have not seen the specimens close up, but have relied on photographs using DGS, digital graphic segregation. That’s not an excuse. That’s further support for the method.

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

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

References
Calzavara M, Muscio G and Wild R 1980. Megalancosaurus preonensis n. gen. n. sp., a new reptile from the Norian of Friuli. Gortania 2: 59-64.
Feduccia A and Wild R 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Natur Wissenschaften 80:564–566.
Geist NR and Feduccia A 2000. Gravity-defying Behaviors: Identifying Models for Protoaves. American Zoologist 4):664-675. online pdf
Pinna G 1980. Drepanosaurus unguicaudatus, nuovo genere e nuova specie di Lepidosauro del trias alpino. atti Soc. It. Sc.Nat. 121:181-192.
Pinna G 1986. On Drepanosaurus unguicaudatus, an upper Triassic lepidosaurian from the Italian Alps. Journal of Paleontology 50(5):1127-1132.
Renesto S 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of Northern Italy. Journal of Vertebrate Paleontology 14(1):38-52.
Renesto S 1994. The shoulder girdle and anterior limb of Drepanosaurus unguicaudatus(Reptilia, Neodiapsida) from the upper Triassic (Norian of Northern Italy. Zoological Journal of the Linnean Society 111(3):247-264
Renesto S 2000. Bird-like head on a chameleon body: new specimens of the enigmatic diapsid reptile Megalancosaurus from the Late Triassic of Northern Italy. Rivista Italiana di Paleontologia e Stratigrafia 106: 157–179.

wiki/Megalancosaurus
wiki/Drepanosaurus

The Tritosauria – An Overlooked Third Clade of Lizards

Traditionally there have been just two lizard clades in the Squamata. The Iguania included Iguana, Draco, Phrynosoma and other similar lizards. The Scleroglossa included Tupinambis, Chalcides, Varanus, Heloderma and all the snakes and amphisbaenids. Squamate outgroups within the Lepidosauria included members of the Rhynchocephalia (such as Sphenodon) and the basal lepidosaur, Homoeosaurus, which probably appeared in the Permian, but is only known from the Late Jurassic.

Traditional Nesting
Wikipedia reports the following about the Squamata, “Squamates are a monophyletic  group that is a sister group to the tuatara. The squamates and tuatara together are a sister group to crocodiles and birds, the extant archosaurs.” This is the traditional concept, but testing this in a larger study finds that lizards and archosaurs are not closely related. Not by a long shot.

Figure 1. Click to enlarge. The Tritosauria, a new lizard clade that was previously overlooked.

The New Heretical Tritosauria
The large study (Peters 2007) recovered a third clade of squamates just outside of the Squamata (Iguania + Scleroglossa), but inside the Lepidosauria (which includes Sphenodon and the other Rhynchocephalia). At the base of this third clade, called the Tritosauria (“third lizards”), are three very lizardy forms, none of which had fused proximal ankle bones, a trait shared by most squamates (at least those that have legs!). Lacertulus, Meyasaurus and Huehuecuetzpalli are known from crushed but articulated fossils. Lacertulus was considered a possible biped (Carroll and Thompson 1982) based on its long hind legs. It is likely that Huehuecuetzpalli (Reynoso 1998) was also a biped. All three were considered close to the base of the lepidosauria, not closely related to any living lizards.

The Tritosauria
A Clade of Misplaced and Enigmatic “Weird-Ohs”
Phylogenetically following Huehuecuetzpalli three distinct clades emerge within the Tritosauria. Some of these were formerly considered “prolacertiforms” (Peters 2000), but now we know that none are related to Prolacerta. All three subclades have some pretty weird members.

The Tanystropheidae
This clade was named by Dilkes (1998) to include “the most recent common ancestor of Macrocnemus, Tanystropheus and Langobardisaurus and all of its descendants.” Clade members include several long-necked taxa, some of which, like Dinocephalosaurus, preferred swimming to walking. Tanystropheus was the largest, attaining 4.5 meters in length.

The Jesairosauridae
This clade includes Jesairosaurus (Jalil 1991) and the drepanosaurs, from Hypuronector to Drepanosaurus.  This clade included several arboreal, hook-tailed taxa with short-toed feet that were able to grasp slender branches in their slow-motion quest for insects. All were rather small.

The Fenestrasauria
This clade was named by Peters (2000) to include “Cosesaurus, Preondactylus, their common ancestor and all of its descendants.” This clade started off with bipeds that flapped their arms, probably for display during mating rituals because some members, like Longisquama were exotically decorated with extradermal membranes and plumes. Powered gliding (as in Sharovipteryx) was followed by flapping flight in pterosaurs, the first flying vertebrates. Several pterosaurs secondarily developed a quadrupedal pace. Quetzalcoatlus was the largest tritosaur, attaining a wingspan of 10 meters.

Summary
Due to the wide gamut and large inclusion list of the present phylogenetic analysis, many former enigmas, mismatches and leftovers came together in a new clade of lepidosaurs that was previously overlooked. Together, the Tritosauria include some of the strangest and, at times largest, of all lizards. Hyper-elongated necks and hyper-elongated fingers, together with experiments in both a sedentary marine lifestyle (Dinocephalosaurus) and a homeothermic aerial lifestyle (Dimorphodon, for example) make this a truly dynamic and diverse clade. Some of these out-of-the-ordinary morphologies seem to have been kick-started by early experiments with bipedalism. While the arboreal niches of drepanosaurs and pterosaurs are relatively easy to identify, the long-necked tanystropheids may also have used bipedalism and a long neck to reach into tree boughs to snatch prey, creating their own arboreal niche.

Unfortunately, only pterosaurs and Huehuecuetzpalli survived the end of the Triassic and they did not survive the end of the Cretaceous. So tritosaurs are the only major clade of lizards that is extinct today.

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

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

References
Carroll and Thompson 1982. A bipedal lizardlike reptile fro the Karroo. Journal of Palaeontology 56:1-10.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Reynoso V-H 1998. Huehuecuetzpalli mixtecus gen. et sp. nov: a basal squamate (Reptilia) from the Early Cretaceous of Tepexi de Rodríguez, Central México. Philosophical Transactions of the Royal Society, London B 353:477-500.