The myth of the Eureptilia

Yesterday we talked about the Parareptilia, a so-called amniote clade invalidated by novel nestings recovered by the larger numbers of taxa within the large reptile tree. Today we’ll look at its opposite “branch,” the so-called Eureptilia, which is also invalidated (shown to be diphyletic) when tested against the large reptile tree.

Muller and Reisz (2006) included the following taxa in their classification of the Eureptilia: Coelostegus, Thuringothyris and the Captorhinindae, Broffia, Paleothyris, Hylonomus, Protorothyrididae and the Diapsida.

The large reptile tree tested these relationships using many additional taxa and
did not recover the same tree topology. Most of the taxa appear together in the new Archosauromorpha (which is a good thing), but Thuringothyris and the Captorhinidae do not (which makes the clade diphyletic, which invalidates the Eureptilia).

According to Wiki, the Eureptilia is defined by the skull having greatly reduced supraoccipital, tabular, and supratemporal bones that are no longer in contact with the
postorbital. Unfortunately these traits turn out to be primitive for reptiles as they occur in
Cephalerpeton, the most basal reptile. The expansion of these bones in diadectids and other reptiles converges with various pre-reptile amphibians, hence the confusion. There is still widespread belief (not based on a comprehensive analysis) that diadectids are not reptiles. This belief has to go away with further testing, following the lead of the large reptile tree.

There is no doubt that the taxa identified by Muller and Reisz (2006) are primitive,
but the captorhinids nest on a separate branch, the new Lepidosauromorpha, while
the others nest with and around the Synapsida, some preceding that clade. Others, like the Diapsida, are actually derived from basal members of the Synapsida.

Muller and Reisz (2006) would have found the same tree topology had they
expanded their taxon list to include most of the genera used by the large
reptile tree, including the use of the reptile-like amphibian, Gephyrostegus, as
an outgroup. Here’s hoping someone will pick up this scientific challenge.

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
Muller J and Reisz RR 2006. The phylogeny of early eureptiles: Comparing
parsimony and Bayesian approaches in the investigation of a basal fossil clade.
Systematic Biology, 55(3):503-511.

The myth of the Parareptilia

The large reptile tree is not the first attempt at classifying Reptiles. It is only the most recent and the most comprehensive.

Earlier attempts invented the clade “Parareptilia,” a name coined by Olson in
1947 to refer to a group of pre-Triassic reptiles leaving no living descendants, as opposed to the Eureptilia, which included all living reptiles and their last common ancestors. This group included turtles.

Gauthier et al. (1988) attempting to understand Reptile relationships using cladistic analysis, and were among the first to do so. They divided the Amniota into Synapsida  and Sauroposida, then divided the Sauropsida into the Reptilia and Parareptilia.

Unfortunately testing in the large reptile tree (using more taxa) does not support these divisions and several new basal reptiles have been described since 1988.

No longer do synapsids split off first from the rest of the Reptilia. Now the new Archosauromorpha (chiefly insect-eaters) splits from the new Lepidosauromorpha (chiefly plant-eaters). The new Archosauromorpha includes the Synapsida, members of which evolve to become the Diapsida, which includes the Enaliosauria (Mesosauriadae + Sauropterygia + Ichthyopterygia and kin). The new Lepidosaurormorpha includes Captorhinidae, Diadectidae, Millerettidae, Lepidosauriformes and kin.

No longer are mesosaurs nested with procolophonids, but nest far from them with several other marine reptiles.

Laurin and Reisz (1995) revised the concept of the Parareptilia. In their cladogram the Synapaside split off first, followed by the Mesosauridae. The remaining taxa were considered Reptilia. Parareptilia included Millerettidae, Pareiasauria, Procolophonidae and Testudines (turtles). Their Eureptilia included Captorhininidae and Romeriida (Protorothyrididae and Diapsida).

Others (always including O. Rieppel) have moved turtles to the Sauropterygia.

When you stop including suprageneric taxa (and the dangers that follow that practice) and start including hundreds more generic taxa, new nesting patterns emerge, as demonstrated by the large reptile tree and all of its subset clades. All the groupings for Parareptilia proposed by earlier workers get split up and recombined in new patterns and clades. These new clades actually demonstrate the gradual accumulation of character traits for any and all derived taxa without introducing any “strange bedfellows.”

So the Parareptilia and its membership has been falsified in a larger, more comprehensive study. The utility of this term in paleontological work has been invalidated.

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
Gauthier J, Kluge AG and Rowe T 1988. The early evolution of the Amniota. In M. J. Benton (ed.). The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds. 103-155. Oxford: Clarendon Press.
Laurin M, Reisz RR 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113 (2): 165–223.
Olson EC 1947. The family Diadectidae and its bearing on the classification of reptiles. Fieldiana Geology 11: 1–53.

Dinocephalosaurus – a star gazer, benthic feeder

Earlier we looked at the odd macrocnemid, Dinocephalosaurus (Fig. 1). This story opens with the first description of Dinocephalosaurus (Li et al. 2004), then follows with a response by Peters, Demes and Krause (2005) that disputed the swimming and sucking abilities originally ascribed, proposing instead a benthic ambush predation mode.

Figure 1. Dinocephalosaurus. Note the very narrow cranial portion of the skull and the very wide cheeks. That, by it self, opens the orbits dorsally. Sure there’s some lateral exposure, but those eyes are looking up!

Two of the original authors, LaBarbera and Rieppel (2005) reported in their reply: “We think it unlikely that Dinocephalosaurus was a benthic ambush predator. First, we would expect that the eyes in a benthic ambush predator would be dorsally located to monitor the overlying water (as seen in living frogfish and flatfish); the eyes of Dinocephalosaurus are anteriorlaterally positioned, apparently to monitor regions to the sides and in front of the snout.”

First of all, I disagree that frogfish and flatfish have the same sort of eye orientation. But be that as it may, one look at the skull of Dinocephalosaurus makes it easy to see the narrow inter orbital region (the frontals) as compared to the much wider skull. This alone produces an orbit that looks up. Sure there’s a lateral aspect, but the dorsal aspect is plainly present to an extent not seen in related Macrocnemus and Tanystropheus specimens.

LaBarbera and Rieppel (2005) report, “Dinocephalosaurus and suggests that the relative size of the limbs indicates a “poor swimmer.” We disagree. Two pairs of 30-cm-long, flipper-shaped f ins seem more than adequate to drive a 1-m-long body. Living sea lions (Zalophus californianus) have a similar ratio of flipper to body length.”

Big limbs, really, not so much… but look at how tiny the pectoral and pelvic girdles are. That’s where the muscles anchor. That’s where the strength originates. These are not the girdles of a strong and steady swimmer.

LaBarbera and Rieppel (2005) report, “In addition, Peters’ reconstruction would have Dinocephalosaurus capture prey by sweeping its neck through the water. We find this unlikely because (i) the cervical vertebrae lack neural processes that would improve the mechanical advantage of the (necessarily small) neck muscles to drive dorsiflexion, and (ii) such motion would generate high drag forces on the neck that would tend to drag the body of the animal  along the substrate in the direction opposite to the motion of the head. (The neck would act as an oar.)”

Let’s remind ourselves that the speed of “sweeping” the neck through the water is not an issue. The neck could have arisen slowly, not moving quickly until the last moment and then just the last few inches of neck would have been involved. So, no speed, no drag, no opposite motion, which was prevented in any case by the extreme width of the extremely flattened torso (a morphology ignored originally), and wide paddles.

Figure 2. Dinocephalosaurus in resting, feeding and breathing modes. In breathing mode the throat sac would capture air that would not be inhaled until the neck was horizontal at the bottom of the shallow sea. Orbits on top of the skull support this hypothesis.

LaBabera and Rieppel (2005) report, “Peters rejects our hypothesis that Dinocephalosaurus may have employed suction feeding (driven by expansion of the cervical ribs) as a mode of prey capture on the basis that the cervical ribs are “bound to one another.” We know of no evidence to  suggest that the cervical ribs were bound to each other; indeed, the dispersal of the cervical ribs in the only available specimen would seem to indicate that tissues that surrounded the cervical ribs were quite liable to decay and thus unlikely to have been collagenous or cartilagenous.”

Well, they were bound together by their mutual length and overlapping proximity to each other (like uncooked spaghetti noodles), surrounded by skin. There’s nothing here more elaborate than anything seen in Tanystropheus and Macrocnemus, which do not have such expansion abilities. If the cervicals were able to rotate on some sort of axis, some sort of axis should be visible, but there’s nothing there. Expansion should have occurred in the cheeks or the stomach, two regions in which some small amount of expansion is already possible. The esophagus works by peristaltic motion, squeezing food toward the stomach. There little possibility for it to expand like the cheeks of a frogfish. The structure of the neck cervicals in Dinocephalosaurus, used to strengthen the extreme length of the neck, would be compromised by any lateral expansion.

If anything, let’s look for hyoids that might expand. Perhaps that’s where the confusion lies after all.

LaBabera and Rieppel (2005) report, “We have no direct evidence that Dinocephalosaurus used the cervical ribs to expand the throat, but that hypothesis is consistent with the observed morphology and we continue to search for additional tests of the hypothesis. If cervical ribs were used to power suction feeding in this animal, that function was certainly an exaptation.”

And we all appreciate this candor.

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
Li C, Rieppel O and LaBarbera MC 2004. A Triassic aquatic protorosaur with an extremely long neck. Science 305:1931.
LaBarbera M and Rieppel O 2005. Response. Science 308, p. 1113.
Peters D, Demes B and Krause DW 2005. Suction feeding in Triassic Protorosaur? Science, 308: 1112-1113.

wiki/Dinocephalosaurus

The skull of Ticinosuchus. Need confirmation, guys!

The skull of Ticinosuchus (Fig. 1) is a crushed mess. Many have looked at it. Many have shrugged their shoulders. Others have cried.

Ticinosuchus is widely considered an early (Middle Triassic) rauisuchid, smaller than most others.

Figure 1. The skull of Ticinosuchus colorized using DGS. Someone else should either duplicate or invalidate these identities and reconstruction. Let’s figure out this key taxon. Click for more data.

Some elements are easy to identify. Others defy identity.
This is a plea for someone else to identify the ALL the parts present in the skull to see if the second interpretation validates partially or completely the present interpretation.

Figure 2. Colorized elements restored to a best fit reconstruction of the skull of Ticinosuchus. Note the toothless premaxilla. This and dozens of other traits nest Ticinosuchus at the base of the Aetosauria.

The present interpretation demonstrates a close affinity with Aetosauria, a clade that had previously gone unconnected to other reptile groups.

Please submit or publish your own reconstruction and restoration of this enigma. Let’s see what you come up with.

References
Krebs B 1965. Ticinosuchus ferox nov. gen. nov. sp. Ein neuer Pseudosuchier aus der Trias des Monte San Giorgio. Schweizerische Palaontologische Abhandlungen 81:1-140.
Lautenschlager S and Desojo JB 2011. Reassessment of the Middle Triassic rauisuchian archosaurs Ticinosuchus ferox and Stagonosuchus nyassicus. Paläontologische Zeitschrift Online First DOI: 10.1007/s12542-011-0105-1

wiki/Ticinosuchus

The many and varied origins of the sterna (plural of sternum)

Basal reptiles appear do not have sterna. Neither do they have a sternum. Birds have ’em. We (mammals) have ’em. Lizards have ’em.  Crocs and turtles don’t. So what’s the story?

Figure 1. Saurosternon, the first taxon in the lepidosauromorph lineage with sterna. But don’t they look like posterior extensions to the coracoid?

I can’t find sterna within the new Lepidosauromorpha before Saurosternon (Fig. 1), a skull-less, but otherwise completed taxon with long fingers and large feet. This arboreal taxon nests at the base of the Lepidosauriformes and has twin sterna that look like posterior extensions to the coracoids (convergent with metacoracoids in therapsids and araeoscelids).

Figure 2. Homeosaurus, a sister to Dalinghosaurus

These sterna fuse to become a sternum in Sphenodon and Homoeosaurus (fig. 2 and presumably their last common ancestor, Gephyrosaurus, but it is no preserved), where they create gliding paths for the coracoids to roll upon in most living lizards. The sternum shifts anteriorly in fenestrasaurs then fuses to the interclavicle and clavicles in Longisquama + pterosaurs where this combo becomes known as the sternal complex. Other than here and in the basal lizard, Huehuecuetzpalli, there is no trace of a sternum in other tritosaurs, including drepanosaurs or tanystropheids. Lizards with legs (including the worm-like Bipes) have a sternum. Those that don’t, including snakes, lack a sternum.

Among the new Archosaurmorpha, there are no sterna until one gets to primitive mammals.  The sterna appear as segments growing from the posterior of the very much shortened interclavicle (the anteriormost sternal bone that articulates with the clavicles.)  The manubrium (anteriormost sternal bones) appear paired in Bienotheroides, but fused in all others.

Fig 3. Araeoscelis and the appearance of sterna between the metacoracoids.

The next sternum appears in Araeoscelis (Fig. 3), as a central bone or bones above the elongated interclavicle and between the metacoracoids. Altogether these bones create in immobile pectoral girdle. There is no such sternum in Galechirus, a therapsid which includes metacoracoids. Thadeosaurus has paired sterna. Again creating an immobile girdle. No enaliosaurs have a sternum. The giant coracoids do the job.

Prolacertiformes don’t have sterna. Neither do choristoderes. Neither do any of the basal archosauriformes. The sternum reappears in Archaeopteryx and sterna appear in Velociraptor. In more primitive theropods in situ gaps suggest an unossified sternum was present. In both of these birdy taxa the coracoids had transformed into immobile struts, a morphology indicative of flapping.

Alright, so, the sternum, or sternal bones, are not primitive to reptiles, but develop and disappear independently and convergently in several lineages.

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.

Giant Chinese Cretaceous Anurognathid

Figure 1. Click to enlarge. The Anurognathidae to scale. The Chinese anurognathid is the tall one in the middle.

This is a different take on an older story.

Most anurognathids are the size of sparrows. Some rise to the size of robins and jays. So such sizes are very common among flying creatures.

Most anurognathids and proto-anurognathids lived during the Late Triassic to Late Jurassic, a time of great diversity, but not great size, in pterosaur history.

There are two exceptions to these patterns.

A partial skeleton of a pterosaur from Mexico, “Dimorphodon” weintraubi, nests with anurognathids. Other anurognathids would not have stood as tall as its knees. Living during the Early to Early Middle Jurassic, D. weintraubi is also one of the oldest and most primitive anurognathids.

Figure 2. Click for more info. The IVPP embryo scaled to an adult size (based on matching the egg to the pelvic opening diameter, along with various views of the skeleton and an egg and hatchling.

Twice as tall as the Mexican pterosaur is an closely related unnamed Chinese anurognathid (Fig. 2). This Early Cretaceous pterosaur had longer metacarpals than any other anurognathid. It retained the narrow premaxilla of its dimorphodontid forebearers. It also retained a relatively long neck. At present this taxon is only known from its egg and embryo. The adult size is figured by multiplying the embryo eight times, as in other pterosaur embryo/parent combinations we know of, like Pterodaustro.

Formerly considered an ornithocheirid with a short skull, we know from other embryos that ornithocheirid embryos did not have a short skull. DGS and a phylogenetic analysis revealed the identity of this embryo.

Now the interesting thing about this giant anurognathid is the embryo is very nearly the size of all other adult anurognathids (Fig. 1). That fact should not have removed the possibility that the embryo was a possible anurognathid, yet it did. If someone else would like to include an accurate reconstruction of this specimen and add the data to a phylogenetic analysis that would go a long way to validating or invalidating the present hypothesis, that would be great.

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
Wang X-L and Zhou Z 2004. Palaeontology: pterosaur embryo from the Early Cretaceous. Nature 429: 623.

The Pterosaur Palate: More workers coming on board.

Pinheiro and Schultz (2012) recently reported on an “unusual” pterosaur palate from Brazil. (Actually it was not so unusual, IMHO, having reconstructed hundreds of them).

They claimed that the palatal elements were misidentified by Osi et al. (2010) who echoed Peters (2000). (Actually their labeling and interpretation of Pterodactylus micronyx (BSP 1936 I 50) was identical to those earlier interpretations. Their other interpretations for the most part followed suite.

Pinheiro and Schultz (2012) reported, “Only recently was a new interpretation of the pterosaur palate made, in a study that utilized the Extant Phylogenetic Bracket to identify homologous structures in the palates of pterosaurs, birds and crocodiles.” Actually, by now everyone should know that pterosaurs belong to an extinct clade of lizards, not crocs or birds. There has never been support, except in the absence of lizards, for a pterosaur-bird-croc relationship. The long fifth toe, the long fourth finger, the ossified sternum and the extreme thinness of the egg shell are traits pterosaurs share with living lepidosaurs to the exclusion of living archosaurs. If an antorbital fenestra is key, it only takes one reminder to note that this structure appears four times by convergence within the Reptilia.

But this brings up an interesting point. Why were the palatal shelves of pterosaurs ever considered palatines (See all works by Bennett and Wellnhofer) if they are maxillary in origin in crocs and birds??

So where did Pinheiro and Schultz (2012) go right?
They correctly identified the palatal shelves as belonging to the maxilla, separated only by the vomers. Most pre-2000 studies (anything by Bennett, Wellnhofer, other early workers) mistakenly labeled these palatines. They correctly identified the pterygoids. They correctly identified the palatines in most of their pterosaurs, but they did not understand that the palatines and ectopterygoids both fuse and diverge diagonally in Pteranodon (so their ectopterygoid is the ectopalatine). This becomes obvious after a study of Germanodactylus palates.

And where did Pinheiro and Schultz (2012) go wrong? 
Other than the aforementioned misconceptions, Pinheiro and Schultz labeled the posterior portion of the shelf of the new pterosaur as the palatine, ignoring their own graphics showing the palatine and ectopterygoid merge in most pterosaurs to form a single small L-shaped bone, the ectopalatine. Tiny, fragile and easily lost, the ectopalatine is missing from the new skull fragment of their study.

Their second mistake was using only highly derived taxa, like Pteranodon, Anhanguera and Tupuxuara, to identify palatal elements. Their single basal pterosaur, the Dorygnathus of Osi et al. (2012) was not relabeled, contra their earlier pronouncement. Their identification of the premaxilla/maxilla in Dorygnathus follows without criticism the mistake made by Osi et al. (2009) reviewed earlier here.

Pinhiero and Schultz (2012) blindly follow earlier analyses that link broad-snouted, toothy Anhanguera with sharp-snouted, toothless Pteranodon. On the face of it, what were they not thinking??? (Oh, yes, they were following tradition without critical thinking).

Their drawings show only the ventral view, but the tiny ectopalatines are often dorsal to the pterygoids.

Their lateral view reconstructions were not of the specimens employed, but were of generic and non-generic relatives. BSP 1936 I 50 belongs to the cycnorhamphids, not Pterodactylus.

Earlier pterosaurheresies covered nearly every aspect of pterosaur palate evolution starting here and going on for seven (occasionally interrupted) chapters. You can also Google “pterosaur palate” under the “images” choice.

In conclusion, I’m glad to see more workers are jumping on board with the maxilla shelf identification, and correctly identifying the palatine, but not sure why they think this is such a new thing.

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
Ösi A, Prondvai E, Frey E, Pohl B 2010. New interpretation of the palate of pterosaurs. The Anat Rec 293: 243–258. doi: 10.1002/ar.21053.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Pinheiro FL, Schultz CL 2012. An Unusual Pterosaur Specimen (Pterodactyloidea, ?Azhdarchoidea) from the Early Cretaceous Romualdo Formation of Brazil, and the Evolution of the Pterodactyloid Palate. PLoS ONE 7(11): e50088. doi:10.1371/journal.pone.0050088

T-rex: Babykiller? or Tank Buster?

Figure 1. Triceratops (left) and Tyrannosaurus rex (right) according to tank-buster tradition, illustrated by Charles Knight from a mural at the Field Museum of Natural History, Chicago.

The two most famous prehistoric combatants T-rex and Triceratops (Fig. 1), face off time and again in Charles Knight paintings and in the imaginations of every Montana paleontologist and grade-school kid. It’s worth a Google search of these two to see some really terrific recent iterations.

A recent challenge to this notion comes from Hone and Rauhut (2009) who conclude, “like modern predators, theropods preferentially hunted and ate juvenile animals leading to the absence of small, and especially young, dinosaurs in the fossil record.”

That’s an intriguing possibility. Doesn’t make such a dramatic painting or cartoon, and does this one behavior really reflect a lack of young dinosaurs in the fossil record?

From the perspective of an artist,
there’s a bit more to this picture that needs to be considered. What’s missing from the Knight painting (Fig. 1) are dozens more Triceratops of all ages and genders. Where’s the rest of the herd? They’re missing from the painting for two reasons, one dramatic and one practical.

The dramatic moment of the painting comes down to “man-to-man” here, “life or death” focusing the plight and fight on only these two combatants. Which will win? The backstory we don’t see is, how did this rogue bull Triceratops get separated from his herd?

The practical aspect, from an artist’s perspective, is the reduction to a one-on-one combat greatly simplifies not only the story line, but the layout and the amount of time needed to complete the painting. A working artist is successful when he can produce the most for the least and Knight produced THE iconic dinosaur painting of all time with just three characters.

The reality of the actual Late Cretaceous scene might have been more similar to a German wolfpack of U-boats surrounding an Allied flotilla, patiently waiting for the cover of night or the revelation of which herd member was lame, stuck in a muddy riverbank or protecting one too many juveniles. One downed prey Trike would have fed several pursuing tyrannosaurs.

Paul (2010) makes the point, “healed wounds on adult hadrosaurs and ceratopsids indicate that [T-rex] adults hunted similarly elephant-sized prey on a regular basis, using the tremendous head and teeth to lethally wound victims, such firepower and size was more than needed to hunt less dangerous juveniles.”

I’ll make the obvious point, reiterating Hone and Rauhut (2009), if every meal was a life-or-death scene for T-rex, very few would have survived to adulthood. For every meal that T-rex considered, it had to attack only those that offered the least risk and most reward.

Let’s also remember that juvenile T-rex did not have the firepower that robust adults had. Did juveniles pick off the smaller easier prey? Or did they follow in the footsteps of more powerful adults who provided to them already subdued prey? Or bits and pieces thereof?

Rauhut reports, “Juvenile dinosaurs are surprisingly rare – maybe because many of them have been eaten by predators.” Of course, juvenile dinosaur (including predatory dinosaur) fossils might be so rare because they grew up to become adults.

What about hadrosaurs?
It’s also instructive to Google search for hadrosaur + t-rex to see absolutely NO similar scenes involving hadrosaur fights. Clearly the dramatic moment with hadrosaurs cannot match the dramatic moment with horns and frills. If hadrosaurs were just slightly faster than giant theropods, they could have outrun them, unless the tyrannosaurs hunted cooperatively. Or were hadrosaurs just so easy to kill that the occasional attack against a Triceratops was born of desperation?

Alright, so, without our handy time machine, we’ll probably never know, but it seems the correct solution is an opportunistic combination of the two hypotheses, regulated by the age and health of the combatants. Artists will continue to portray T-rex and Triceratops as simply and economically as possible. The reality, pathos and chaos of the actual battle scene, filled with chicks, hens and bulls on both sides, changing meal to meal, day to day and year to year, will probably remain just out of reach of our most fertile imagination. Let’s not a priori mentally exclude all the possibilities before coming to conclusions.

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
Hone DWE and Rauhut OWM 2009. Feeding behaviour and bone utilization by theropod dinosaurs. Lethaia online, 3 August 2009.
Paul G 2010. The Princeton Field Guide to Dinosaurs. Princeton University Press, 320 pp.

YouTube Dinos – Majungasaurus

I haven’t been keeping up with my once a day blog lately. I’ve been in bed with a head-racking cold that even makes my hair hurt and my ribs tingle. So a quick one today.

Dinosaur videos are SO much better nowadays. Enjoy this one and its associated links here.

I am so impressed. Hats off to the creators.

Pterosaur and Therapsid Foot Convergence

Figure 1. The evolution of the therapsid manus including the reduction of the phalanges 3.2, 4.2 and 4.3.

Figure 2. Gephyrostegus in anterior view demonstrating the need for shorter medial toes in tetrapods with a sprawling gait. This insures the toes to not scrape the substrate during the recovery phase and also assures that all the toes contribute to the propulsive phase.

Figure 3. The reduction of phalanges 3.2, 4.2 and 4.3 in derived pterosaur feet.

The evolution from primitive, asymmetrical, reptile hand and foot to a more symmetrical therapsid hand and foot has been widely recognized (Fig. 1). Primitively the phalanges were more or less subequal and, as a consequence, the digits increased in length from 1 to 4. The primitive pattern is found on sprawling tetrapods, like Gephyrostegus (Fig. 2). It permits the digits to avoid hitting the substrate during the recovery stroke of the sprawling limbs, while maximizing digit length.

In basal therapsids, like Biarmosuchus (Fig. 1), the reduction of phalanges 3.2, 4.2 and 4.3 and the resulting increased symmetry in the manus and pes tells us the limbs were not so sprawling any more. The recovery stroke occurred more below the knees and elbows, rather than out to the sides. Later, about the time certain therapsids evolved into mammals, these three disc-like phalanges ultimately disappeared.

Pterosaurs undertook a similar evolution in the pes (Fig. 3), in which  phalanges 3.2, 4.2 and 4.3 were the shortest phalanges in those digits in derived forms. The pattern of reduction is both distinct and convergent in several pterosaur lineages. Each pattern produces a distinct foot shape that should aid in the identification of trackmakers (Peters 2011) when examining potential pterosaurian ichnites. In no pterosaur did the short phalanges ultimately disappear.

The interesting thing is the evolution of the pterosaurian pes did not match the evolution of the gait. Rather, just the opposite. Early pterosaurs (Dimorphodon) had a right angle femoral head and an upright stance, but they retained a more lizardy foot with digits of increasing length laterally. Derived pterosaurs (Pteranodon) had a more sprawling femur (judging by the angle of the head to the shaft) and a more symmetrical pes with short mid phalanges on digits 3 and 4. Not sure of a good explanation for this yet.

Figure 4. Click to animate. Plantigrade and quadrupedal Pterodactylus walk matched to tracks

Then there’s the subject of pterosaur clawed fingers. In MPUM 6009 the (non-wing) finger phalanges were more or less subequal. In the ornithocheirid pterosaurs, Brasileodactylus and Arthurdactylus, m3.2 was a mere disc. In these pterosaurs, manual digits 2 and 3 were subequal in length as a consequence. Not sure why yet, but these pterosaurs had such long proximal wing elements set so far out in front of their feet that they were not able to transfer weight to them while keeping their feet beneath the humeral glenoid, the center of balance in all pterosaurs. It also seems unlikely that in ornithocheirids the wing fingers could have had much contact with the substate sitting on top of the largely horizontal manus and wing finger. Such a pattern is unlike that of typical trackmakers like Pterodactylus and Ctenochasma (Fig. 4), which had shorter and more gracile proximal wing elements.

In Istiodactylus the manus phalangeal formula for digits 1 through 3 was 2-2-2. So there was some fusion and loss here, the only case of seen of it in pterosaurs.

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.

Reference
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification. Ichnos 18(2):114-141. http://dx.doi.org/10.1080/10420940.2011.573605