The Evolution of Gigantism in Pterosaurs: The Ancestry of Quetzalcoatlus

Earlier we followed up on a National Geographic article on how many generations it takes to create a blue whale and an elephant. Today we’ll look at the evolution of the giant pterosaur Quetzalcoatlus (Fig.1) from the Latest Cretaceous (65 mya) from a much smaller ancestor of the Late Jurassic (150 mya), a span of 85 million years.

Others consider azhdarchids, like Quetzalcoatlus, to be related to tapejarids based on the shared trait of an antorbital fenestra taller than the orbit. The large reptile tree did not recover that relationship, but found that antorbital trait to be a convergence.

The beauty of the large reptile tree and the large pterosaur tree is the ability to trace the ancestry of any listed taxon back to the basal tetrapod, Ichthyostega. Today we won’t go that far back. Rather we’ll start with the first Late Jurassic pterosaur in the lineage of Quetzalcoatlus that actually has the approximate proportions of Quetzalcoatlus. That pterosaur is tiny specimen inaccurately referred to Pterodactylus? elegans? BSPG 1911 I 31 (no. 42 in the Wellnhofer 1970 catalog, Fig. 1).

Quetzalcoatlus and its ancestor, no 42, note scale bars.

Fig. 1. Quetzalcoatlus and its ancestor, no 42, note scale bars. Also note the flat rostrum in both taxa, distinct from the pointed tapejarid rostrum derived from Germanodactylus.

Similar but different, especially in size.
We’re often taught that as organisms grow larger they also become more robust, with stronger, thicker bones to withstand the effects of their greater mass and weight. Here, not so much. Giant Quetzalcoatlus was just as gracile as the VERY much smaller, no. 42. Sure the humerus is more robust. So is the pectoral girdle. Otherwise it’s hard to find thicker bones and the neck is definitely more gracile in Quetzalcoatlus. That’s engineering!!

Tiny no 42 is dwarfed by the specimen referred to Quetzalcoatlus, which is dwarfed by the hypothetical skull of Q. northropi, based on wing elements.

Figure 2. Tiny no 42 is dwarfed by the specimen referred to Quetzalcoatlus, which is dwarfed by the hypothetical skull of Q. northropi, based on wing elements. Click to learn more.

What can we learn here?
Between no. 42, which was reduced from early Dorygnathus specimens, and Quetzalocoatlus, one of the largest known azhdarchids, the proportions are not much different overall. This is likely due to the restrictions imposed by flight. Only certain balances between power and weight can fly. Over time and millions of generations the lineage of Quetzalcoatlus gradually grew and reengineered itself to withstand the increasing stresses imposed by that growth.

Forelimb Take-Off
Dr. Mike Habib (2008) has noted the greater size of the humerus vs femur in Quetzalcoatlus and other large pterosaurs. He didn’t mention no. 42, which does not have a humerus greater in diameter than the femur. Habib considered the more robust humerus a sign that pterosaurs used a vampire bat-like forelimb launch sequence demonstrated here, rather than a bird-like hind limb launch demonstrated here. Unfortunately, its all math at present. We know of no pterosaur take-off tracks, nor any that document the implantation of the wing metacarpal into the substrate. Rather only the fragile first three digits make any impression. Perhaps the increased size of the humerus is a sign that the pectoral engine for wing flapping is much larger to drive the larger wings. After all, traditional physics has to enter into pterosaur evolution someplace!

Tomorrow we’ll look at the similar situation in the ancestry of Arthurdactylus and Anhanguera, two giant ornithocheirids.

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
Habib MB 2008. Comparative evidence for quadrupedal launch in pterosaurs. Zitteliana B28:159-166.
Hone and Benton 2006. Cope’s Rule in the Pterosauria, and differing perceptions of Cope’s Rule at different taxonomic levels. Journal of Evolutionary Biology 20(3): 1164–1170. doi: 10.1111/j.1420-9101.2006.01284.x
Kellner AWA and Langston W 1996. Cranial remains of Quetzalcoatlus (Pterosauria, Azhdarchidae) from late Cretaceous sediments of Big Bend National Park, Texas. – Journal of Vertebrate Paleontology 16: 222–231.
Lawson DA 1975. Pterosaur from the latest Cretaceous of West Texas: discovery of the largest flying creature. Science 187: 947-948.
Wellnhofer P 1970. Die Pterodactyloidea (Pterosauria) der Oberjura-Plattenkalke Süddeutschlands. Abhandlungen der Bayerischen Akademie der Wissenschaften, N.F., Munich 141: 1-133.

wiki/Quetzalcoatlus

Sacral Number and Bipedalism

Do more sacrals mean a bipedal stance?
Short answer: yes and no. Sacral vertebrae connect the backbone to the pelvis. Occasionally other vertebrae lie between the ilia without connecting to them. These are not real sacrals. Occasionally other vertebrae do not lie between the ilia, yet converge on them. These are real sacrals. Sometimes there is a relationship between ilial length and number of sacrals. Sometimes there isn’t.

Amphibians
Generally in basal tetrapods (pre-reptiles/amphibians), there is only one sacral. However, these basal tetrapod/pre-reptiles appear to have two sacrals: Batropetes, Microrater and Cacops. Let me know if this is wrong.

Reptiles – The New Lepidosauromorpha
Most reptiles have two sacrals or more, but two taxa in this study revert to one sacral: Milleretta and Eunotosaurus. Others in the Milleretta clade, like Acleistorhinus and the bolosaurids, may also have had one sacral, but they are known only from skulls.

Eunotosaurus

Figure 1. Eunotosaurus, a reptile with only one sacral vertebrae. Click to learn more.

Casea is the most basal taxon with three sacrals. Diadectes through Owenetta (sans Stephanospondylus + turtles) also have three or four sacrals. Noe bipeds here.

Saurosternon reverses this pattern back to two sacrals. It was much smaller than its predecessors and likely arboreal.

Jesairosaurus, Hypuronector and drepanosaurs have more than two sacrals. The fenestrasaurs continue this pattern. Cosesaurus has four. Sharovipteryx has nine. Longisquama has five. Pterosaurs have more than four. I have argued (Peters 2000) that the fenestrasaurs were increasingly bipedal with occasionally bipedal Rotodactylus tracks matching Cosesaurus feet. Beachcombing pterosaurs reverted to quadrupedalism, especially while feeding. There is no doubt that Sharovipteryx was a biped, although all that Google shows on this subject for the first few pages have been originated by me! No one else wants to concur? This is particularly strange and bears the mark of widespread bias or blindness. The drepanosaurs were likely arboreal lizards, some of which probably adopted a tripedal stance assisted by a prehensile tail. Jesairosaurus is a puzzle in this department.

Sharovipteryx mirabilis

Figure 2. Sharovipteryx mirabilis in various views. Not sure why this obvious biped has not been more widely acknowledged. Click to learn more.

Snakes from both branches revert to one sacral or none.

Reptiles – The New Archosauromorpha
Sphenacodont pelycosaurs have more than two sacrals as do therapsids. No bipeds here.  This has been attributed to increasing size and increasing leg length raising the body further off the substrate (if only slightly). The initiation of elongated dorsal spines may have contributed to this support between the pecs and pelvis.

The clade of Placodontia + Sauropterygia have more than two sacrals with convergence upon a very small ilium. This is the clade, mentioned above, with the converging sacrals. No bipeds here.

Pachypleurosaurus had more than two sacrals all converging on a tiny ilium.

Figure 3. Pachypleurosaurus had more than two sacrals all converging on a tiny ilium. Click to learn more.

Doswellia + Choristodera have more than two sacrals. So does the new Proterochampsa. No bipeds here. The biped Lagerpeton nests near here, but it had only two sacrals.

Ornithosuchids have more than two sacrals. So do a sprinkling of rauisuchians, including Vjushkovia, Smok and Postosuchus. I’m guessing on the number of sacrals in Vjushkovia based on a lateral view (which can be misleading) and would appreciate any better data. The other three look to be tentative bipeds.

Vjushkovia.

Figure 4. Vjushkovia. Not sure if this taxon had two or more sacrals, but the number of vertebrae between the pelvis indicate the possibility of the latter. Click to learn more.

Among archosaurs on the basal croc line, Pseudhesperosuchus has three or four sacrals. So do Scleromochlus + Saltopus. So do Terrestrisuchus + Saltoposuchus. All were likely bipeds. The number of sacrals decreased to two in derived quadrupedal crocs, but the quadrupedal Protosuchus may have retained three (covered by scutes).

Only two sacrals were reported for the theropod dinosaur Tawa, but three vertebrae were pressed between the ilia. Coelophysis had five or more sacrals highly pressed between the ilia. Herrerasaurus had only two sacrals, but they were greatly expanded as they attached to the broad ilia. Two other verts, one fore and one aft, had tiny sacrals between the ilia. These theropods were bipeds.

The phytodinosaurs (poposaurs + sauropodomorphs) all had three or four sacrals. Basal forms were bipeds. A few, like Lotosaurus and Shuvosaurus, reverted to quadrupedal locomotion. The other phytos – the Ornithischia had five or more and these likewise began as bipeds but most reverted to quadrupedalism, some of them quite early on.

Now, how do they stack up with regard to bipedalism?
As you can see from the above list, having more than two sacrals is a convergent trait among many clades, some of them completely aquatic. Some of this increase in sacral number was due to nothing more than increasing mass. Even so, an increased number of sacrals did develop among tentative and facultative bipeds at the fulcrum of the lever developing at the acetabulum, as a response to the stresses developing there when the forelimbs are raised. In later dinosaurs and pterosaurs the reversion to quadrupedal locomotion did not diminish the number of sacrals as members of both clades occasionally rose to hind limbs for feeding, fighting or takeoff and landing.

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.

Eudibamus: Is this the foot of a biped?

post-crania of Eudibamus

Figure 1. The post-crania of Eudibamus with enlargements of the manus and pes. PILs indicate sets of interacting phalanges. The hyper-asymmetry of the foot indicates a hyper-sprawling, rather than a bipedal type of configuration during normal locomotion. Pedal digit 5 is typically long in arboreal taxa. They use their tendril-like toes to wrap around uneven substrates. Note, however, the abbreviated unguals, something unexpected in an arboreal form. The femur appears to indicate the ability to be drawn closer to the midline of the body, a trait expected in a biped. The backbone appears stiff and robust, with centra butting up against one another.

Eudibamus cursoris (Berman et al. 2000) Early Permian ~290 mya, ~25 cm in length (Figs. 1, 2), was originally considered a relative of Bolosaurus, but here it nests at the base of the Diapsida, as old as the other basal diapsid, Petrolacosaurus. Araeoscelis has similar blunt teeth.

Figure 1. Click to enlarge. Eudibamus in situ (above), traced (middle) and reconstructed (below). The revised skull retains a large orbit and has a shorter rostrum.

Figure 2. Click to enlarge. Eudibamus in situ (above), traced (middle) and reconstructed (below). The revised skull retains a large orbit and has a shorter rostrum.

The forelimbs were quite a bit shorter than the hind limbs in Eudibamus. The carpals were elongated, as in Petrolacosaurus. The medial digits were greatly reduced on the manus.

The hind limbs were robust and much larger than the forelimbs. The tibia+fibula was longer than the femur. Metatarsal 1 and pedal digit 1 were pariticularly short while metatarsal 4 and digit 4 were robust and elongated.


Eudibamus was originally considered to be the first bipdal vertebrate due to the disparity in the lengths of its limbs. If so, it may have left impressions of only the big fourth toe when running, which was large enough to have acted independently of the others. This would be unusual in a tetrapod experimenting with bipedality, but maybe there were lots of them, most more typical. Typically when the limbs begin to swing beneath the knees, as in Biarmosuchus and Sharovipteryx, more toes tend to become more even in their lengths. The same does not hold for Eudibamus, in which the femur appears to have an offset head.

Gephyrostegus in anterior view

Figure 3. 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. PILs drawn through interphalangeal joints indicate which phalanges work in sets as PILs typically run perpendicular to the line of progress during a step cycle. In taxa with pedal asymmetry like this the PILs indicate a sprawling gait. Click to learn more about Gephyrostegus.

The other possibility is that Eudibamus used all of its toes to walk with an extremely wide splay. Gephyrostegus in anterior view (Fig. 3) provides the reason why the digits were increasingly longer laterally in splay-legged tetrapods. All the toes have to clear the substrate on the recovery stroke. And all the toes contribute to the propulsive phase. This is taken to the extreme in Eudibamus. Perhaps it clung to tree trunks with splayed limbs, or foraged between rock cracks instead of running bipedally.

PILs (parallel interphalangeal lines) have been demonstrated to show how digits extend and flex in unison. Your own hand has PILs. In Eudibamus pedal PILs indicate an extremely wide sprawl, not a bipedal configuration. The manus was likely similar, but the medial digits of the manus are not well preserved if at all.

In Eudibamus the ankle shows no tendency to rotate on a simple hinge the way the ankle of Cosesaurus does. No sister taxa of Eudibamus are bipeds. Even so, the tibia is longer than the femur, which usually signals a bipedal gait and a rapid one at that. The torso is short. The tail is attenuate. The sacrals are in no way derived (a subject we will tackle tomorrow). Nothing about this reptile indicates a particular flattening nor deepening of the skull or torso. The unguals are short, not hook-like like the arboreal Iguana. 

A Eudibamus sister, Milleropsis (Fig. 4), does have larger unguals, at least on the pes and considering the shape of that pelvis, the length of that attenuated tail and the limb and phalangeal proportions, this taxon might have made the better biped.

Milleropsis, a sister to Eudibamus. Note the larger unguals.

Figure 4. Milleropsis, a sister to Eudibamus. Note the larger unguals. Click to learn more.

Bipedal lizard video marker

Figure 1. Click to play video. Just how fast can quadrupedal/bipedal lizards run? This video documents 11 meters/second in a Callisaurus at the Bruce Jayne lab.

Confused? So am I.
Some aspects of Eudibamus point to a bipedal configuration. Others do not. Whenever the subject of bipedal lizards comes up, it’s always good to take a refresher course with Dr. Jayne and his treadmill wonders seen here. This video dispels most biases concerning bipedal running in reptiles.

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
Berman, DS, Reisz RR, Scott D, Henrici AC, Sumida SS and Martens T 2000. Early Permian bipedal reptile. Science 290: 969-972.

paleocritti/Eudibamus
wiki/Eudibamus

Sometimes the larger sister has the more juvenile traits.

A few posts ago we noted the interesting fact that the larger Garjainia  (Otchev 1958, Early Triassic ~240 mya, 2 m. long) had the relatively larger skull compared to its sister, Euparkeria  (Broom 1913 Early Triassic, ~240 mya, 60 cm). Generally a larger head is considered a juvenile trait. Garjainia also had a shorter tail and a shorter torso.

In the background is Garjainia, a basal erythrosuchid. Euparkeria is at its ankles, both to scale. Euparkeria is the more derived taxon. Below the tail of Euparkeria is a greatly reduced Garjainia. No fossils exist that show Garjainia to this size.

Figure 1. In the background is Garjainia, a basal erythrosuchid. Euparkeria is at its ankles, both to scale. Euparkeria is the more derived taxon. Below the tail of Euparkeria is a greatly reduced Garjainia as if a juvenile of Euparkeria. No fossils exist of Garjainia at this size.

On the other hand
Euparkeria did have the slightly larger orbit, a trait generally considered juvenile. The skull of Euparkeria also had smoother contours (no pmx/mx notch, less of a jugal descent).

Evolution Uses Premature Maturation
The greatly reduced size of Euparkeria is yet another example of a new clade arising from shrimps arising from old clades. Others have complained that size is not pertinent to phylogenetic matrices, but this example shows otherwise. Deciding where to draw “the line” will continue to be argued, no doubt.

Figure 3. Here Euparkeria nests between Garjainia, a basal erythrosuchid, and Ornithosuchus following the nestings recovered by the large reptile tree. All three share a suite of traits that do not include a long narrow rostrum and a dorsal naris, among other traits.

Figure 2. Here Euparkeria nests between Garjainia, a basal erythrosuchid, and Ornithosuchus, an ornithosuchid, following the nestings recovered by the large reptile tree. Note the relative size of the skull in Garjainia, much larger than the much smaller Euparkeria.

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
Broom R 1913. On the South-African Pseudosuchian Euparkeria and Allied Genera. Proceedings of the Zoological Society of London 83: 619–633.
Ewer RF 1965. The Anatomy of the Thecodont Reptile Euparkeria capensis Broom Philosophical Transactions of the Royal Society London B 248 379-435.
Otchev VG 1958. Novye dannye po pseudozukhiyam SSSR: Doklady Akademii Nauk SSR, 123(4):749-751.
Parrish JM 1992. Phylogeny of the Erythrosuchidae. Journal of Vertebrate Paleontology 12:93–102.

wiki/Euparkeria
wiki/Garjainia

Dog named ‘Kitty’ finds 300 myo fossil “juvenile mammal-like reptile” named “Superstar.”

A news story out of Nova Scotia reports the finding of “one of the most significant fossil finds ever in Nova Scotia”, a reptile fossil with a rib cage, partial sail and skull aged between 290 and 305 mya (Late Carboniferous, Early Permian.) Researchers considered it a juvenile, about a meter long.

“A new window into our ancient world has just opened,” said Deborah Skilliter, curator of geology for the Nova Scotia Museum. “This is just the beginning of the story as we undertake the task of determining exactly what type of sail-back reptile Superstar is, where, and how, it lived and died.” The report goes on to say, “The fossil comes from a branch of reptiles described as mammal-like as they are thought to be the ancient ancestors of modern mammal species.”

Here it is (Figs. 1, 2):

Figure 1. Fossil ribcage of new Nova Scotia fossil.

Figure 1. Fossil ribcage of new Nova Scotia discovery.

Figure 2. Fossil skull of new Nova Scotia discovery.

Figure 2. Fossil skull of new Nova Scotia discovery.

Maybe not what they think it is
I’m hoping someone will expand the list of possible candidates to include the amphibian Platyhistrix (Fig. 3) because the skull and sail shape more closely resemble it than any sphenacodont. The apparent lateral temporal fenestra is the otic notch. The low rostrum and high cranium match Cacops, a sister taxon known from more complete remains. The giant scapula and broad flat ribs are good matches. The age is the same and the size makes the new discovery a full grown adult. Previously the skull of Platyhistrix was unknown, but this find confirms the sisterhood of Cacops.

Figure 3. The sail of Platyhistrix mated to the rest of Cacops is a better match for the new Nova Scotia discovery by the dog named Kitty than any pelycosaur.

Figure 3. The sail of Platyhistrix mated to the rest of Cacops is a better match for the new Nova Scotia discovery by the dog named Kitty than any pelycosaur.

This identification was made with referral to reptileevolution.com without ever seeing the fossil firsthand and without using Photoshop. Thanks to Ben Creisler for posting this on the Dinosaur Mailing List.

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
More on the find on the Internet here.

Jocelyn Falconnet, as I read down the DML, pointed out that Superstar looks more like the amphibian Dendrerpeton.

David Marjanovic agrees with the temnospondyl interpretation.

Getting Big and Getting Small, a NEXT Page from Nat Geo

The most recent issue of Nat Geo included a one page note called, “Sizing Up.”

Nat Geo reporter Gretchen Parker sourced Allistair Evans, of Monash U, Australia, who noted “It takes a minimum of 3 million generations for a dolphin-sized aquatic mammal to increase to the size of a blue whale.” 1000x change in size, graphics impressive.

“It takes 1.6 million generations for a sheep-size land mammal to increase to the size of an elephant.” 100x change in size

“But it takes only a minimum of 0.1 million generations for an elephant-sized land mammals to decrease to the size of a sheep. 100x change in size.

“5 million generations” to go from rabbit-sized to elephant-size. 1000x

“24 million generations” to go from mouse-size to elephant-size. 100,000x

All this is interesting, but more interesting to PterosaurHeresies readers might be some similar hypotheses regarding prehistoric reptiles, particularly pterosaurs.

Pterodaustro embryo

Figure 1. Pterodaustro embryo. At one-eighth the size of a large adult, this embryo retains most of the proportions of the adult, including a long rostrum and tiny eye.

Chinsamy et al. (2008) noted that in Pterodaustro, the only pterosaur for which we have a complete growth series, half grown specimens appear to be sexually mature. At half size, the pelvis is also half size, able to pass eggs of half size producing hatchlings of half size, more or less. In three generations such a progression could lead to a one-eighth size adult, which would be the size of a hatchling of the original Pterodaustro. Now I’m not saying this is exactly how size reduction happened in pterosaurs. The three generations is just the ‘speed limit’ for getting small, something pterosaurs did over and over again, producing new clades following these many size decreases as size thereafter increased.

Some pterosaurs, like Quetzalcoatlus, became very large and very famous. Other pterosaurs became very small. They’re not famous. They don’t even rate a distinct genus, having been relegated to the trash heap with the label, “juvenile.” They are excluded from phylogenetic analysis  and unjustly so. They are important.

Of course getting big again simply depends on creating eggs later in life when the mother is slightly surpassing the 8x growth pattern having a larger pelvis to pass a larger egg. Like elephants, getting bigger probably took more time than getting smaller.

Overall size does affect morphology and evolution. Early and late maturation affects the next generation. Hormones count! Hormones also drive secondary sexual characteristics, like frills and crests. These things add up, or subtract out, over many generations.

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
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.

Maybe Asilisaurus had shorter arms

Updated September 3, 2020, 
but should have been updated much earlier. Asilisaurus and Lotosaurus are both poposaurids, taxa that nest outside Archosauria in the LRT (Fig. x).

Figure 5. Subset of the LRT focusing on the Poposauria and surrounding clades.

Figure 5. Subset of the LRT focusing on the Poposauria and surrounding clades.

Asilisaurus.

Figure 1. Asilisaurus. Above: as reconstructed by Nesbitt et al. (2010). Below: With shorter arms, more in accord with the thickness of the bones. The related Lotosaurus was a quadruped and had much more robust forelimbs. Other sister taxa had relatively short arms.

How Long Were the Arms of Asilisaurus?
Asilisaurus
(Nesbitt et al. 2010) is a basal dinosaur with incomplete forelimbs. It was originally reconstructed with long forelimbs in a quadrupedal configuration (Fig. 1). The question is: were the arms reconstructed too long? The humerus includes both ends but has a broken middle. The ulna/radius does not include the proximal ends.

Asilisaurus is the oldest known reptile in the dinosaur lineage, according to Nesbitt et al. (2010) and Wikipedia, but the large reptile tree finds Lotosaurus* is also a dinosaur that is just as old. Both lived during the Anisian period of the Middle Triassic (245-237 mya). Asilisaurus is a sister to Silesaurus in alll studies.

Considering only the Nesbitt drawings (my only data), it appears that the fore limbs may have been drawn too long in order to force a quadrupedal configuration. Why not let the bone diameters help determine their lengths? Moreover, sister taxa, like Pisanosaurus and Poposaurus, had short forelimbs. Silesaurus may have been bipedal. Lotosaurus was a sister taxon with robust forelimbs and a quadrupedal configuration. If Asilisaurus were indeed quadrupedal more robust forelimbs might be expected following these patterns. Even Pisanosaurus has more robust forelimbs.

*Nesbitt (2007) suggested Lotosaurus was a poposaurid, more closely related to Shuvosaurus, not to Xilosuchus, but in the same large clade of rauisuchians. The large reptile tree nested poposaurids within the Dinosauria, but Xilosuchus and Arizonasaurus, the other two finbacks, with the rauisuchids.


References
Nesbitt SJ, Sidor CA, Irmis RB, Angielczyk KD, Smith RMH and Tsuji LMA 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464 (7285): 95–98. doi:10.1038/nature08718PMID 20203608.

A Giant Mesosaur

Cymbospondylus is a primitive Triassic ichthyosaur of enormous length, approximately ten meters. It is also, due to its geneology a giant mesosaur. All ichthyosaurs and thalattosaurs are derived from mesosaurs. Cymbospondylus is one of the few ichthyosaurs to retain the long, sinuous body shape of mesosaurs.

Figure 1. Cymbospondylus overall in situ.

Figure 1. Cymbospondylus overall in situ. Overall, a very similar morphology to any basal mesosaur with the addition of flippers transformed from limbs.

Despite the Size Difference
Actually an order or two of magnitude larger in size, the giant ichthyosaur Cymbospondylus (Leidy 1868) kept the basic proportions of Stereosternum, but with a shorter neck and limbs transformed into flippers.

Stereosternum, a basal mesosaur

Figure 2. Stereosternum, a basal mesosaur

A comparison of skulls helps make the point.
The long premaxilla, the posteriorly shifted nares, the size of the supratemporal are obvious shared traits with Wumengosaurus acting as a transitional taxon. Details at reptileevolution.com.

Figure 2. A comparison of mesosaur skulls. Stereosternum at the base. Wumengosaurus with a very distinct upper temporal fenestra. And Cymbospondylus with upper temporal fenestra more dorsally oriented.

Figure 2. A comparison of mesosaur skulls. Stereosternum at the base. Wumengosaurus with a very distinct upper temporal fenestra. And giant Cymbospondylus (not to scale) with upper temporal fenestra more dorsally oriented, not quite visible in lateral view.

No other taxa are closer in the large reptile tree to thalattosaurs and ichthyosaurs than mesosaurs.

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
Leidy J 1868. Notice of some reptilian remains from Nevada: Proceedings of the American Philosophical Society, v. 20, p. 177-178.

Strange Bedfellows – Nesbitt (2011) – Part 9 – The Wrap-Up: Rauisuchians, Aetosaurs and Crocs

Sometimes we miss the big picture. 
Here then, for your approval and disapproval are comparisons between closest kin found by the Nesbitt (2011) tree versus those found by the large reptile tree.

The origin of the Aetosauria, Rauisuchia and Crocodylomorpha is today’s topic.
Nesbitt (2011) recovered the clade Loricata to include rauisuchians and their descendants, the crocodylomorpha (Fig. 1). Nesbitt (2011) mentioned Vjushkovia along with other erythrosuchids, but neglected to include it in his family tree. That was unfortunate. The large reptile tree found Vjushkovia a key taxon at the base of the Rauisuchia (Fig. 4), the Ticinosuchus/Aetosaur clade (Fig. 3) and the Crocodylomorpha (Fig. 5). Euparkeria and the Ornithosuchidae were recovered as outgroups for Vjushkovia in the LRT. Nesbitt (2011) nested the two pre-dinosaur plant-eaters together, Revueltosaurus with the Aetosauria, but otherwise there is little the two clades share.

Loricata according to Nesbitt (2011). Here Nesbitt recovers basal crocs arising from derived rauisuchians. The large reptile tree found both crocs and rauisuchians to be derived from a derived erythrosuchid, Vjushkovia, which was not listed by Nesbitt (2011).

Figure 1. Loricata according to Nesbitt (2011). Here Nesbitt recovers basal crocs arising from derived rauisuchians. The large reptile tree found both crocs and rauisuchians to be derived from a derived erythrosuchid, Vjushkovia, which was not listed by Nesbitt (2011). As a rule, major clades typically arise from generalized basal members, not derived taxa. Apparent exceptions, like the origin of pterosaurs from Longisquama, actually follow this pattern as pterosaurs originated from a basal longisquamid without several of the derived traits seen only on Longisquama.

The Nesbitt Tree Illustrated with Taxa
Here (Fig. 2) are many of the taxa recovered by Nesbitt (2011) in phylogenetic order. Much of this matches the large reptile tree. For instance, all of the rauisuchids nested together. However, derived rauisuchids did not give rise to crocs in the LRT. The morphological leap between Postosuchus and Hesperosuchus in the Nesbitt (2011) tree has a completely different pattern in the LRT.

Figure 2. The lineage of crocodylomorphs as recoverd by Nesbitt (2011).

Figure 2. The lineage of crocodylomorphs as recoverd by Nesbitt (2011). That’s a pretty big morphological jump between Postosuchus and Hesperosuchus (still waiting on data for CM73372, hence the place saver oval.) Even so, given the included taxa,  I can see the logic of Nesbitt’s tree, but the shared traits of these two are convergent when more taxa are added. There’s less of jump in the large reptile tree where Postosuchus is among the most derived of rauisuchians and Hesperosuchus and Dromicosuchus have many predecessor taxa in the Crocodylomorpha and basal Archosauria.

The Large Reptile Tree Illustrated with Taxa
In phylogenetic order, taxa within the large reptile tree (Figs. 3-5) appear to form more gradual transitions, have a better chronological order, and the most derived taxa in the various extinct clades actually lead to extinction. Generalized basal taxa give rise to derived forms. For instance, in the LRT a sister to Vjushkovia gave rise to the fish-eaters, Ticinosuchus and Yarasuchus + Qianosuchus and a sister to amored Ticinosuchus gave rise to heavily armored aetosaurs, both taxa with toothless premaxillae. Unfortunately Nesbitt (2011) did not reconstruct the skull of Ticinosuchus. Otherwise the aetosaur connection would have been more obvious.

Vjushkovia, Ticinosuchus and the base of the Stagonolepidae (aetosaurs)

Figure 3. Vjushkovia, Ticinosuchus and the base of the Aetosauria (Stagonolepidae). There’s still a pretty big jump here between Vjushkovia and Ticinosuchus, ameliorated by Qianosuchus (Fig. 4).

The Rauisuchia (Fig. 4) arose from a sister to the small derived erythrosuchid, Euparkeria. Vjushkovia was a descendant taxon that gave rise to several clades as is readily apparent here (details at reptileevolution.com):

Figure 4. The lineage of Rauisuchians, crocs and kin according to their skulls. Here the gradual accumulation of derived traits is easier to demonstrate.

Figure 4. The lineage of Rauisuchians, crocs and kin according to their skulls. Here the gradual accumulation of derived traits is easier to demonstrate.

Vjushkovia: basal to Aetosaurs, Crocs and Rauisuchids
The modifications that evolved in the descendants of Vjushkovia produced a lineage of decreasing size that ultimately produced tiny bipedal crocs (Figs. 4-5). There is no indication of a link between Postosuchus and Hesperosuchus (Nesbitt 2011) when you add these taxa.

Figure 1. Ten basal bipedal crocodylomorphs descending from a sister to Decuriasuchus.

Figure 1. Ten basal bipedal crocodylomorphs descending from a sister to Decuriasuchus.

Larger Studies Brings Greater Resolution
The LRT recovered different branching for the aetosaurs and crocs simply by adding more taxa and, in the case of Ticinosuchus, by more fully describing the formerly enigmatic skull. Images of these taxa demonstrate gradual transitions that are confirmed by 228 character scores leading to complete resolution.

Several notes in the dinosaur blog called these traits convergences, but convergence is defined by the initial phylogenetic distance and only a few traits are shared. Here large suites of traits were shared by sister taxa, the definition of homology.

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
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Strange Bedfellows – Nesbitt (2011) – part 8 – Basal Dinosaurs

Sometimes we miss the big picture.
Here then, for your approval and disapproval are comparisons between closest kin found by the Nesbitt (2011) tree (Fig. 1) versus those found by the large reptile tree.

The Split-Up of the Dinosauria is today’s topic.
The Dinosauria (however it originated) has been traditionally split up into Saurischia (Theropoda + Sauropodomorpha) and Ornithischia. Nesbitt (2011) also found this traditional branching with the addition of the Silesauridae as an outgroup and Marasuchus as their common ancestor.

Nesbitt 2011 tree with basal dinos and their outgroup, Lewisuchus.

Figure 1. Nesbitt 2011 tree with basal dinos and their outgroups, Lewisuchus and Marasuchus. Here the Silesauridae are just outside the Dinosauria. Ornitischia split off first leaving the Saurischia (Sauropodomorpha and Theropoda).

The Nesbitt (2011) Tree as Told by Skeletons
Since the Dinosauria was recovered as a single clade by both the Nesbitt study and the large reptile tree, the “strange bedfellows” here will be more difficult to see from small drawings such as these (Figs. 2-3). Back in 2011, I might have come up with a very similar tree. However, a wealth of basal dinosaurs have come to light since 2011, shifting the branches a wee bit.

Figure 1. The base of the Dinosauria according to Nesbitt (2011). Here Ornithischia split off first leaving Theropoda + Sauropodomorpha = Saurischia. From top to bottom, Lewisuchus, Lesothosaurus, Saturnalia and Herrerasaurus surrounded by the others to scale.

Figure 2. The base of the Dinosauria according to Nesbitt (2011). From top to bottom, Lewisuchus, Lesothosaurus, Saturnalia and Herrerasaurus surrounded by the others to scale. This is basically close to the results of the large reptile tree. Note the disparate morphologies here despite the fact that all are bipeds. A wealth of basal dinos published since 2011 has helped the large reptile tree find a slightly different set of branches.

Results of the Large Reptile Tree
In the large reptile tree Gracilisuchus and Turfanosuchus are the outgroup taxa. They nested together in the Nesbitt (2011) tree as in the large reptile tree, despite their many differences. Together they would have nested much closer to dinosaurs in the Nesbitt (2011) tree, except for his unfortunate inclusion of pterosaurs and lagerpetids. Trialestes (Fig. 3) was not tested by Nesbitt (2011) and that’s too bad, because it is a key taxon.

Figure 3. Basal dinosaurs and their outgroup, Gracilisuchus, according to the large reptile tree. Trialestes has a basal position. Herrerasaurus is a basal theropod. Pampadromaeus is a basal phytodinosaur. Pisanosaurus is a basal poposaur. Massospondylus is a basal sauropodomorph. Daemonosaurus is a basal ornithischian. The last two are know from skull only materials. Thecondontosaurus is a skull-less taxon related to Massospondylus. The skull of Heterodontosaurus is ghosted near Pisanosaurus. Daemonosaurus was co-authored by Nesbitt, but he did not include this key taxon in his 2011 Archosauria paper. He and his co-authors considered it an odd theropod. The large reptile tree produced a smoother (fewer changes) transition between taxa. Pampadromaeus apparently preserves only cervical ribs, not cervical centra. These ribs overlap shorter centra on Herrerasaurus, so Pampadromaeus may not have had such a long neck.

Theropods First
Herrerasaurus and the theropods, including Marasuchus, nested closer to Trialestes than any other dinosaurs, all of which were more derived plant-eaters. Pampadromaeus (Fig. 3) nested at the base of the Phytodinosauria. Pisanosaurus, always considered a basal plant-eater, nested at the base of the poposaurs, now nested within the Dinosauria with their redeveloped calcaneal heel. Massospondylus (Fig. 3) nested at the base of the sauropodomorpha. Daemonosaurus, another new taxon that Nesbitt considered a strange theropod, but did not include in his archosaur paper, nested instead at the base of the Ornithischia.

The Importance of Pampadromaeus
Prior to the inclusion of Pampadromaeus, Daemonosaurus nested as the transitional taxon into the phytodinosauria. Someday I hope we’ll see what its post-crania looks like. I anticipate an unusual transitional pelvis, not quite ornithischian in morphology. Panphagia might demonstrate the first stages of this. For now that short round skull and large premaxillary teeth are traits basal phytodinosauria share in common. Later forms independently developed longer longer skulls and other various shapes.

Pampadromaeus apparently preserves only cervical ribs, not cervical centra. These ribs overlap shorter centra on Herrerasaurus, so Pampadromaeus may not have had such a long neck as originally envisioned. The large reptile tree nesting of Pampadromaeus matches that of the original study (Cabiera et al., 2011).

Timing Is Everything
Unfortunately Nesbitt (2011) was published prior to the publication of taxa bridging the theropoda and the rest of the Dinosauria. Fortunately the web can be updated daily as new discoveries shift branches this way and that.

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
Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.
Sues H-D, Nesbitt SJ, Berman DS and Henrici AC 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society Bpublished online 
Cabreira SF, Schultz CL, Bittencourt JS, Soares MB, Fortier DC, Silva LR and Langer MC 2011. New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil. Naturwissenschaften (advance online publication) DOI: 10.1007/s00114-011-0858-0
Martínez RN and Alcober OA 2009. A basal sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the early evolution of Sauropodomorpha (pdf). PLoS ONE 4 (2): 1–12. doi:10.1371/journal.pone.0004397. PMC 2635939. PMID 19209223. online article

wiki/Pampadromaeus
wiki/Panphagia
wiki/Daemonosaurus