Phylogenetic bracketing and pterosaurs – part 2

Two posts ago we looked at part 1 of this topic.

Since pterosaurs (and other tritosaurs) nest between rhynchocephalians and squamates, there are a few traits they likely shared based on phylogenetic bracketing (unless specifically excepted based on fossil evidence). Putting the rhynchocephalians aside for the moment, according to Evans (2003) squamate traits include:

(1)  a specialized quadrate articulation with a dorsal joint typically supplied by the deeply placed supratemporal, reduced squamosal, and distally expanded paroccipital process of the braincase; reduction/loss of pterygoid/quadrate overlap; loss of quadratojugal — all present in basal tritosaurs, but quadrate becomes immobile in Macrocnemus and later taxa.

(2) loss of attachment between the quadrate and epipterygoid, with the development of a specialized ventral synovial joint between the epipterygoid and pterygoid — also present up to Huehuecuetzpalli, but absent in Macrocnemus and later taxa.

(3) subdivision of the primitive metotic fissure of the braincase to give separate openings for the vagus nerve (dorsally) and the perilymphatic duct and glossopharyngeal nerve (via the lateral opening of the recesses scalae tympani ventrally). This leads to the development of a secondary tympanic window for compensatory movements and is associated with expansion of the perilymphatic system and closure of the medial wall of the otic capsule — in fossil tritosaurs these details may not be known and certainly not by me… yet.

(4) loss of ventral belly ribs (gastralia) — Basal tritosaurs, up to Homoeosaurus have gastralia. Then they don’t until Macrocnemus and all later taxa.

(5) emargination of the anterior border of the scapulocoracoid — Basal tritosaurs share this trait. Macrocnemus and tanystropheids refill the emargination. Fenestrasaurs, including pterosaurs expand the emargination resulting in a strap-like scapula and stem-like coracoid, both representing the posterior rims of these bones.

(6) hooked fifth metatarsal with double angulation — shared with tritosaurs and more complex mesotarsal joint — in tritosaurs the mesotarsal joint is simple.

(7a) a suite of soft tissue characters including greater elaboration of the vomeronasal apparatus;

(7b) a single rather than paired meniscus at the knee;

(7c) the presence of femoral and preanal organs;

(7d) fully evertible hemipenes;

(7e) and pallets on the ventral surface of the tongue tip — none of these have been noted in soft tissue fossils.

References
Evans SE 2003. At the feet of the dinosaurs: the origin, evolution and early diversification of squamate reptiles (Lepidosauria: Diapsida). Biological Reviews, Cambridge 78: 513–551.

 

The many faces of Tanystropheus

Added September 21, 2020:
Think about a bubble net, as in humpback whales, coming form the long, dead=air storage vessel that is that elongate trachea. That long neck rotating like an inverted cone to surround confused fish just above the jaws.

Tanystropheus is well known
as the sometimes giant reptile with the hyper-elongate neck (Figs. 1, 2). Several specimens are known, all by letters in the alphabet based on Wild (1973). Few specimens have skulls.

The smaller Tanystropheus specimens (Fig. 1) have multicusp posterior teeth, and some workers consider these juveniles that change their diet and teeth as they grow. Others, including yours truly, think these are two different species, if not different genera. Remember, guyz and galz, you don’t get giant species without first going through the medium and large size ranges. We learned this earlier with Pteranodon.

Wild’s (1973) reconstruction of the skull was taken as gospel for a good long time. Then Nosotti (2007) came along and rebuilt the small skull in convincing fashion. Here we’ll take a look at a skull from a small individual (Fig. 1, Exemplar a) and compare it to two skulls from the larger forms (Fig. 2, Exemplars i and q). Then you can decide if the differences are ontogenetic or phylogenetic.

Tanystropheus exemplar a.

Figure 1. Tanystropheus exemplar a.

Exemplar a has a low rostrum and large orbit. The frontals extend over the orbits like brow ridges. The nasals are not visible on any articulated skulls, and displaced samples can be placed on the skull two different ways. The ascending process of the premaxilla is also a big question mark. It could be present or absent. The pineal opening is not large in any sister taxa, so it redevelops here. The posterior skull leans down, which, by analogy with basal synapsids indicates a bit of posterior pull on the mandible, as if Exemplar a was tugging at its meals.

Figure 2. Tanystropheus with skull reconstructions based on two specimens, exemplar i and exemplar m.

Figure 2. Tanystropheus with skull reconstructions based on two specimens, exemplar i and exemplar q.

Among the giant specimens…

Exemplar i is the skull that Wild (1973) used for his ‘adult’ specimen. Like  Exemplar a, the frontals are wide, the nasals are unknown and the ascending process of the premaxilla is apparently gone. This creates quite a large confluent set of nares dorsally oriented. The posterior skull does not descend posteriorly. Only a few teeth are preserved and in dorsal view the rostrum is wide and rather flat, like a hat brim. One gets the impression that a great circle of procumbent teeth emanated from these jaws because the premaxilla appear to be quite flat in situ with no indication of any depth.

Exemplar q is lower, longer and had a reduced pterygoid and vomers. Here the nares are also very large, but divided by a slender and fragile ascending process of the premaxilla (pretty much busted up in situ). Rather than wide and flat, this rostrum is more traditionally box-like with ventrally oriented teeth. The pterygoid is greatly reduced and so are the vomers. The nasals are preserved here only as posterior rims to the large nares. The brow ridges are gone here, so Exemplar q could look up without moving its head.

The appearance of those giant nares on these tiny skulls links to that hyper-elongate neck and within, a hyper-elongate trachea that needs to be flushed of CO2 and filled with O2 every so often.

So the skulls of the big taxa are different.
It might be worthwhile to see how the post-crania also differs. There’s a PhD project waiting for someone out there, probably in Europe, where the fossils are. Or wait a few weekends and I’ll probably get around to it.

References
Bassani F 1886. Sui Fossili e sull’ età degli schisti bituminosi triasici di Besano in Lombardia. Atti della Società Italiana di Scienze Naturali 19:15–72.
Li C 2007. A juvenile Tanystropheus sp.(Protoro sauria: Tanystropheidae) from the Middle Triassic of Guizhou, China. Vertebrata PalAsiatica 45(1): 37-42.
Meyer H von 1847–55. Die saurier des Muschelkalkes mit rücksicht auf die saurier aus Buntem Sanstein und Keuper; pp. 1-167 in Zur fauna der Vorwelt, zweite Abteilung. Frankfurt.
Nosotti S 2007. Tanystropheus longobardicus (Reptilia, Protorosauria: Reinterpretations of the anatomy based on new specimens from the Middle Triassic of Besano (Lombardy, Northern Italy). Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano, Vol. XXXV – Fascicolo III, pp. 1-88
Peyer B 1931. Tanystropheus longobardicus Bass sp. Die Triasfauna der Tessiner Kalkalpen. Abhandlungen Schweizerische Paläontologie Gesellschaft 50:5-110.
Wild R 1973. Die Triasfauna der Tessiner Kalkalpen XXIII. Tanystropheus longobardicus (Bassani) (Neue Ergebnisse). – Schweizerische Paläontologische Abhandlungen 95: 1-16.

wiki/Tanystropheus

Phylogenetic bracketing and pterosaurs – part 1

Since pterosaurs (and other tritosaurs) nest between rhynchocephalians and squamates, there are a few traits they likely shared based on phylogenetic bracketing (unless specifically excepted based on fossil evidence). According to Evans (2003) these include:

(1) A derived skin structure with a specialized shedding mechanism involving distinct epidermal generations that are periodically lost and replaced, linked to
a cyclic alternation between a and b keratogenesis. — Ttritosaurs had scales. Pterosaurs also had pycnofibers, hair-like structures that first appear in Sharovipteryx. Unfortunately there is no evidence of skin shedding in any fossil lepidosaur.

(1A) The possession of a crest of projecting scales along the dorsal midline of the body and tail may also be unique to members of this group. — this reaches its acme with the tritosaur fenestrasaur, Longisquama.

(2) Paired male hemipenes housed in eversible pouches at the posterior corners of a transverse cloacal slit. These hemipenes are well developed in squamates and rudimentary in Sphenodon. — the fossil record does not include such structures.

(3) Notching of the tongue tip, possibly in relation to the development of the vomero-nasal system. — Barely notched in Iguana. I don’t see this in known rhynchochephalians or tritosaurs based on the division of the choanae into anterior and posterior fenestra, which appears in basal scleroglossans only.

(4) Separate centres of ossification in the epiphyses of the limb bones (a condition acquired independently in mammals and some birds). — This has never been noted in tritosaurs.

(5) Specialized mid-vertebral fracture planes in tail vertebrae to permit caudal autotomy facilitated by the organisation of associated soft tissue. — This has never been confirmed in any tritosaur, but then again, they are rare as fossils.

(6) A unique knee joint in which the fibula meets a lateral recess on the femur (not end to end as in many tetrapods) — This must be a very subtle trait. I see this trait in Tupinambis, Varanus and Bahndwivici, but not in very many other lepidosaurs.

(7) Specialized foot and ankle characters including a (a) hooked fifth metatarsal, (b) a specialized mesotarsal joint with a fused astragalocalcaneum and (c) an enlarged fourth distal tarsal. —  (a) The hook comes and goes. In basal rhynchocephalians, not present. It is present in Sphenodon through Mesosuchus, starts to fade with Rhynchosaurus and is gone in Hyperodapedon. Something of twisted fifth metatarsal present in most tritosaurs. Minor hook in basal squamata, becomes larger in Varanus, absent in snakes and other limbless lizards, of course. (b) In tritosaurs no ankles are fused except in drepanosaurs. (c) Also large in tritosaurs.

(8) Other soft tissue features include a sexual segment on the kidney; reduction or absence of the ciliary process in the eye; presence of a tenon (cartilaginous
disc) in the nictitating membrane and its attachment to the orbital wall. — These have never been observed in any lepidosaur fossil. But that doesn’t mean they weren’t there.

(9) In addition to these characters, all lepidosaurs show one of two kinds of tooth implantation, pleurodonty and acrodonty. — Basal tritosaurs have pleurodont teeth. Macrocnemus and later tritosaurs have thecodont teeth that happen to be much larger.

Part 2 is posted here.

References
Evans SE 2003.
At the feet of the dinosaurs: the origin, evolution and early diversification of squamate reptiles (Lepidosauria: Diapsida). Biological Reviews, Cambridge 78: 513–551.

 

Delorhynchus – getting closer to Eunotosaurus

A recent paper by Reisz et al. (2014) presented a large portion of the anterior skeleton of a specimen previously known from smaller scraps.

Figure 1. Delorhynchus compared to its closest sisters, Acleistorhinus and Eunotosaurus to scale.

Figure 1. Delorhynchus compared to its closest sisters, Acleistorhinus and Eunotosaurus to scale.

Delorhynchus cifelli (Reisz et al. 2014, Fig. 1) is an Early Permian terrestrial reptile from Oklahoma. Derived from a sister to AcleistorhinusDelorhynchus was basal to Eunotosaurus and was larger than both. The jugal had two posterior processes. The squamosal is largely unknown. No expanded ribs were found with this specimen.

Reisz et al. nested Delorhynchus with Lanthanosuchus and Acleistorhinus, but Lanthanosuchus nests with Macroleter in the large reptile tree here as we discussed earlier here.

References
Reisz RR, Macdougall MJ and Modesto S 2014. A new species of the parareptile genus Delorhynchus, based on articulated skeletal remains from Richards Spur, Lower Permian of Oklahoma. Journal of Vertebrate Paleontology 34:1033–1043.

Ikrandraco – the tip of the jaws

This is what you get when you reconstruct a pterosaur with rotating jaws (Fig. 1).

Figure 1. Ikrandraco jaw tips. Here the mandible extends slightly beyond the the rostrum, which has extremely tiny premaxillary teeth.

Figure 1. Ikrandraco jaw tips. Here the mandible extends slightly beyond the the rostrum, which has extremely tiny premaxillary teeth. Yes, that’s a tooth at the mandible tip. Very sharp.

And, going back one post, Ikrandraco does nest between the crested ornithocheirids and the uncreated istiodactylid ornithocheirid. And there’s a set of dorsal ribs beneath the tip of that plant material. That means there’s probably a scapulocoracoid under it, if anyone wants to do a little digging from the back.

 

 

 

Ikrandraco avatar – a new pterosaur & DGS reveals a few more bones

A recent paper
by Wang et al. (2014) described a wonderful new Chinese ornithocheirid, Ikrandraco avatar, with a crest below the mandible, not above the rostrum. Actually two specimens were found with slightly different preservations.

Figure 1. Click to enlarge. Ikrandraco avatar skull. As originally traced (below) with DGS (above).

Figure 1. Click to enlarge. Ikrandraco avatar skull. As originally traced (below) with DGS (above).

This wonderful new ornithocheirid (not a pteranodontoid) from the Jiufotang formation (Early Cretaceous). The authors report, “We propose that this pterosaur fed on fishes from nearby freshwater lakes by flying low over the water, capturing its prey by lowering the mandible in the water, being capable of a reduced and temporary skimming. We also propose that it had a more developed throat pouch then in other pterosaur species.”

The mandible was quite sharp. The rostrum was not. Oddly the teeth appear to have emerged from the sides of the rostrum and mandible. The alveoli were like portholes on a ship’s hull.

Figure 2. Ikrandraco in situ. Below as originally traced. Above with femur, tibia, pelvis and tail traced and manual 4.4 identified (not a rib).

Figure 2. Ikrandraco in situ. Click to enlarge. Below as originally traced. Above with femur, tibia, pelvis and tail traced and manual 4.4 identified (not a rib).

The authors were able to identify the tiny metatarsals and sacrum, but overlooked the tibia, femur, pelvis and tail, shown here (Fig. 2). And yes, that fossil is no closer to me than half a world away. I wouldn’t have looked fro the tibia and femur, but there was the metatarsus, all by itself, lined up with a crack that split the tibia. What I see of the bones are either impressions of bones that were once there, or they remained buried, just below the surface.

Figure 3. DGS restores the known elements to a more in vivo pose of Ikrandraco.

Figure 3. DGS restores the known elements to a more in vivo pose of Ikrandraco. The tiny premaxilla had tiny teeth. Rather than having a nice anhanguerid-like rake and spoonbill, this pterosaurs evolved a very narrow set of jaws. The deep angle of the quadrate absorbed shocks as the lower jaw slid through the water. 

The authors considered this an adult individual based on the fusion of the extensor tendon, but that is a phylogenetic feature shared with several ornithocheirids, large and small.

Other ornithocheirids can be seen here for comparison. Most have longer teeth. Wang et al. (2014) nested Ikrandraco with Nurhachius and Istiodactylus among pterosaurs known from more than just scraps. I haven’t done the work, but that seems reasonable, except that the orbit doesn’t have the keyhole shape. Among pterosaurs with rostral crests and large round orbits, we look to Criorhyrhynchus and Coloborhyrhynchus, which are related to istiodactylids. Unfortunately the authors nested ornithocheirds derived from a sister to sharp-snouted Pteranodon from the Late Cretaceous. That doesn’t make sense. The large pterosaur tree derives ornithocheirids from scaphognathids, sisters to cycnorhamphids and Yixianopterus is a basal ornithocheirid (not included in Wang et al. (2014).

The Ornithocheiridae.

Figure 4. The Ornithocheiridae. Click to enlarge and expand. Look for the taxon that shares the most traits with Ikrandraco. In size Ikrandraco was relatively small, about as tall as Boreopterus. 

 

 

 

References
Wang X, Rodrigues T, Jiang S, Cheng X and Kellner AWA 2014. An Early Cretaceous pterosaur with an unusual mandibular crest from China and a potential novel feeding strategy. Scientific Reports 4 : 6329, pp. 1-9. | DOI: 10.1038/srep06329

The Evolution of Frogs

While they are the most common of living amphibians, frogs are among the oddest amphibians of all time with their short torso and long hind legs. The skull has undergone great changes from the primitive state with many bones reduced (Fig. 1).

Frog skull evolution. Here, starting with the Permian Utegenia (with relatives back to the Visean) you can see stages in the evolution of the frog skull through Doleserpeton and Gerobatrachus to Rana, the bull frog.

Figure 1. Frog skull evolution. Here, starting with the Permian Utegenia (with relatives back to the Visean) you can see stages in the evolution of the frog skull through Doleserpeton and Gerobatrachus to Rana, the bull frog.

The wide flat skull of Utegenia (Earliest Permian with roots in the Visean, Laurin 1996) is our starting point. It represents a basic seymouriamorph skull with a full complement of skull bones, palatal fangs and sharp marginal teeth.

In Doleserpeton (Early Permian, Sigurdsen and Bolt 2010) the palate changes the most as the interpterygoid vacuity greatly expands. The palatal fangs are gone. The marginal teeth are tiny and continue behind the orbit. The vomer expands with a shagreen of tiny teeth. The intertemporal fuses to the postfrontal. The otic notch deepens. Note the palatines, now form part of the ventral orbit in lateral view.

In Gerobatrachus (Early Permian, Anderson et al. 2008) these trends continue as the skull widens, the orbits enlarge and the palatal bones become more gracile. Here the intertemporal bones may not have fused to the postfrontal, but the specimen has many cracks and is exposed ventrally.  Here the otic notch is not so deep and the voters are tiny.

In Rana (extant) the skull bones are reduced to struts (no more post parietal shelf) to accommodate the giant orbit and narial opening. The otic notch is angular here, not a smooth curve. It appears as if the intertemporal were still present, but sources available at present don’t delineate the cranial bones. Send a good reference if you have it and I’ll make changes as necessary.

The evolution of frogs from Utegenia, through Doleserpeton, Gerobatrachus, Triadobatrachus and Rana.

Figure 2. The evolution of frogs from Utegenia, through Doleserpeton, Gerobatrachus, Triadobatrachus and Rana. Click to enlarge. It wasn’t until Triadobatrachus in the Early Triassic that frogs developed their long hind limbs, and long ankle bones, but they still retained a short pelvis and long torso.

We skipped one
In Triadobatrachus (Fig. 2, Early Triassic, Piveteau 1936, Rage and Rocek 1989) the hind limbs and ankle bones begin to elongate and the vertebral count is reduced. Distinct from Gerobatrachus, the skull of Triadobatrachus was relatively smaller with a narrower skull roof and a larger orbit that extended into the cheek where the bones were reduced. The frontal and parietal were fused together, but note the parietals were not completely fused to each other! The intertemporal was fused to the parietal. A large postparietal shelf was absent. The lower jaw was toothless. The presacral vertebral count was reduced to 14. The tail was reduced to a nub of at least six vertebrae. The ilium included a larger anterior process, but note the thigh muscles did not extend to the anterior tip of the ilium as in reptiles and mammals. The femur, tibia and fibula were elongated as were two proximal ankle bones, the tibiale and fibulare. Unfortunately the hands and feet remain unknown. Despite the much longer hind legs, Triadobatrachus was considered an ineffective jumper.

References
Anderson JS et al. 2008.  A stem batrachian from the Early Permian of Texas
and the origin of frogs and salamanders. Nature 453:
Kuznetzov VV and Ivakhnenko MF 1981. Discosauriscids from the Upper Paleozoic in Southern Kazakhstan. Paleontological Journal 1981:101-108.
Laurin M 1996. A reappraisal of Utegenia, a Permo-Carboniferous seymouriamorph (Tetrapoda: Batrachosauria) from Kazakhstan. Journal of Vertebrate Paleontology 16(3):374-383.
Piveteau J. 1936. Une forme ancestrale des amphibiens anoures dans le Trias inférieur de Madagascar. Comptes Rendus hebdomadaires des séances de l’Académie des Sciences 202:1607–1608.
Rage J-C and Rocek Z 1989. Redescription of Triadobatrachus massinoti (Piveteau, 1936) an anuran amphibian from the early Triassic. Palaeontographica Abt. A 206(1-3):1-16. online pdf
Sigurdsen T and Bolt JR 2010. The Lower Permian amphibamid Doleserpeton (Temnospondyli: Dissorophoidea), the interrelationships of amphibamids, and the origin of modern amphibians. Journal of Vertebrate Paleontology 30(5):1360-1377.

wiki/Doleserpeton
wiki/Gerobatrachus
wiki/Utegenia
wiki/Triadobatrachus