Crocodylomorpha and their place in the Archosauria

Among living archosaurs
there remain only birds and crocs. By definition, the last common ancestor of birds and crocs and all of its descendants are also archosaurs. By that reckoning only crocodylomorphs and dinos (including birds) are archosaurs according to the large reptile tree. That tree recovers a dino/croc split shortly after or including Gracilisuchus, but not the separate clade arising from Gracilisuchus that includes Saltopus and its cousin Scleromochlus  (Fig. 1).

Figure 2. Mariliasuchus nests at the base of a derived clade of narrow-skulled crocs.

Figure 1. Mariliasuchus nests at the base of a derived clade of narrow-skulled crocs. PVL 4597 and Lewisuchus are pro to-dinosaurs. All archosaurs are derived from a sister to Turfanosuchus +  the poposaurs and beyond that, Decuriasuchus.

Earlier crocodylomorpha cladograms
like the one in Wikipedia (refs therein, Fig. 2) do not mention dinosaurs or protodinosaurs. Instead they branch crocodyomorphans off from Postosuchus-like rauisuchians.

Figure 2. Crocodylomorpha according to Wikipedia.

Figure 2. Crocodylomorpha according to Wikipedia. The outgroup, CM 73372, was referred to Postosuchus, considered an outgroup by the authors of Wikipedia. In the large reptile tree Postosuchus is a terminal rauisuchian without known descendants.

On the same note,
earlier archosaur cladograms did not nest crocs and dinos together.
No, they put several more taxa between them (Fig. 3). The Wiki authors did not want to take sides in a 40-year-old debate, so they showed all the work by four widely cited paleontologists (Fig. 3).

Figure 3. Click to enlarge. Shown here are trees from Gauthier 1986, Sereno 1991, Benton 1999, and Nesbitt 2011, all from Wikipedia's article on archosaurs.

Figure 3. Click to enlarge. Shown here are trees from Gauthier 1986, Sereno 1991, Benton 1999, and Nesbitt 2011, all from Wikipedia’s article on archosaurs. As you can see, the inclusion set varies a bit between these authors. They all like to nest crocs with rauisuchians (both trending toward bipedal). Sometimes aetosaurs get a mention. Sometimes they don’t. Pterosaurs appear here by default in all four studies, based on traditional thinking. In each of these tree pterosaurs and parasuchians both share a common ancestor (which I hope you’ll agree is odd).

Shown above are trees from Gauthier 1986, Sereno 1991, Benton 1999, and Nesbitt 2011, all from Wikipedia’s article on archosaurs (refs therein). As you can see, the inclusion set varies a bit between these authors, as they do not all agree on what constitutes an archosaur or an archosauriform. They all like to nest crocs with rauisuchians (both trending toward bipedal). Sometimes aetosaurs get a mention. Sometimes they don’t. Pterosaurs appear here by default in all four studies, based on traditional thinking that cannot be supported by a demonstrated gradual accumulation of pterosaurian traits (but you CAN see such a gradual accumulation here. In each of these trees pterosaurs and parasuchians both share a recent common ancestor (which I hope you’ll agree is odd).

With the same issues
Naish (2001) wrote “Fossils explained 34/Crocodilians.” I only do this because some of these outmoded ideas are still floating around today (see above) and are still taken as gospel. The Naish article is online and uneditable so it also represents currently available hypotheses.

Overall, this is a good article
written and illustrated by a much younger Darren Naish (15 years ago). It introduces various members of the Crocodylomorpha in text and illustration and sets them into their family tree with a short list of derived traits.

However
when Naish sets up the Archosauria and the Crocodylomorpha early in the chapter. he reports on traditional thinking (see above), now falsified by the large reptile tree. Here are a few examples:

“However, numerous soft and hard-tissue features show that crocodilians are more closely related to birds than to lizards, snakes and turtles, and, together with pterosaurs, dinosaurs and other groups, birds and crocodiles are part of the reptilian subgroup called the Archosauria.” Pterosaurs are not in this clade as we learned here.

“Some fossilcrocodilians display the bony opening between the nostril and orbit, called the antorbital fenestra, that is unique to the Archosauria (‘ruling reptiles’).” Not unique as we learned here.

“Other unique archosaurian features, including the latero-sphenoid bone in the braincase and the fourth trochanter, a prominent muscle attachment site on the posteromedial surface of the femur, are evident in crocodilians.” Turtles and derived snakes both have a laterosphenoid, though neither is homologous to that of archosaurs.

“Crocodilians are not descendants of dinosaurs. While both groups do belong to the Archosauria, they are from fundamentally different lineages.” Not true. They both share a single common ancestor close to Gracilisuchus apart from all other fossil archosauriforms as shown here (Fig. 1).

If you want to excuse these trees
by saying ‘sure pterosaurs and parasuchians have a last common ancestor in deep time, we just haven’t found them yet,’ then you’ll have to compete with the cladogram of the large reptile tree that does not have to imagine such deep time ancestors, but provides contemporaneous sisters that actually look like sisters,

See this is why
I see the world of paleo as a little bit topsy turvy (trees have too many mismatched sisters), and wishy-washy (no one in academia wants to show what’s wrong with prior trees and champion a corrected tree.)

If you want to see
the taxa at the base of the archosauria, with one branch leading toward crocs and the other toward dinos, look here and here.

References
Bhullar B-AS and Bever GS 2009. An Archosaur-Like Laterosphenoid in Early Turtles (Reptilia: Pantestudines) Breviora Number 518 :1-11.
Naish D 2001. Fossils explained 34/Crocodilians. Geology Today 17(2):71-76.

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