Nesbitt (2011) and his Characters – part 1

Following remarks from fellow paleontologists asking for my study to include more Nesbitt (2011) characters in the large reptile study, I thought we should dive right into them, taking a few days to digest them all — a bite at a time — then use the ones that make sense to use (will be readily visible across a long list of taxa).

Nesbitt Characters for Archosauriforms
Sterling Nesbitt (SN) reported, “My analysis finds pterosaurs well nested within archosauriforms as the sister taxon to Dinosauromorpha; this is a result obtained by all recent numerical analyses.”
Note: none of these included lepidosaurs including fenestrasaurs.

SN: “Therefore, I present a list of characters that unambiguously support archosauriform clades and indicate whether the character is present in (basal) pterosaurs:”

1) Absence of a parietal foramen (63-1). Basal pterosaurs lack a parietal foramen.
Note: As in certain lepidosaurs, including all fenestrasaurs.

(2) Jugal-quadratojugal contact present (70-1). Present in basal pterosaurs.
Note: As in certain lepidosaurs, including all fenestrasaurs.

(3) Ectopterygoid forms all of the lateral edge of the lateral pterygoid flange (88-1). Not known in basal pterosaurs.
Note: Actually this trait is known. This condition does not occur in any pterosaur. The ectopterygoid is fused to the palatine to form the ectopalatine and the pterygoid does not have a lateral flange.

(4) Ossified laterosphenoid present (92-1). Present in pterosaurs (Bennett, 1996).
Note: Bennett (1991, 2001) identified an indistinct portion of the entirely fused braincase of Pteranodon as the laterosphenoid. Cosesaurus has that same structure. Due to lack of sutures, this trait in pterosaurs has not been definitively determined. And if so, then it has to be applied also to Cosesaurus. 

(5) Antorbital fenestra present (136-1). Present in basal pterosaurs.
Note: As in all fenestrasaurs.

(6) Lateral mandibular fenestra present (138-1). Present in Dimorphodon and a specimen refered to Eudimorphodon (BPS 1994 I 51).
Note: These are bogus identifications both due to a slipped surangular. No other pterosaurs have this trait.

(7) Presence of tooth serrations (168-1). Present in Dimorphodon.
Note: This was claimed for only the third premaxillary tooth in one specimen, but was not visible in the evidential photograph. No other pterosaur but the two specimens of Austriadatylus have serrations, but several have multiple cusps, like those found in higher fenestrasaurs. Here is why we should score characters at the genus level. Choosing Austriadactylus as our “pterosaur” would yield one score. Any other pterosaur would yield the other. 

So far, not a strong case, and a case that makes sense to Nesbitt (2011) only by keeping a blind eye to competing candidates among the fenestrasaurs.

Tomorrow: Erythrosuchus + Archosauria

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.

Advertisements

9 thoughts on “Nesbitt (2011) and his Characters – part 1

  1. Which lepidosaurs have a laterosphenoid? As far as I know, that’s considered unique to archosauriforms.

    For serrated teeth, while I agree that the supposed Dimorphodon serrations are not visible in the photo, you didn’t mention Preondactylus or Austriadactylus, which both have obvious serrations.

    • Let’s be cautious about what we are qualifying as “serrations.” It is possible that the microserrations of the keel of various archosaurs (sebecosuchian or otherwise) and of Dimosauromorpha are not homologous with the “serrated” teeth of “eudimorphodont” grade pterosaurs. I would use “cusps” in this case until you can settle whether the marginal carinae produce actual cusps in pterosaurs, rather than other pressures causing the teeth to behave in similar ways. “Serrations,” as in that of theropod dinosaurs, also appear in non-reptiles, including mammals, and it would be uncouth to presume homology when analogy won’t do.

      • None of this matters, since phylogenetic analyses determine homology. Just because sharks’ or Smilodon’s serrations aren’t homologous with archosaurs’ doesn’t mean we shouldn’t still code them as serrated in an analysis. If a theropod had a tooth like Preondactylus, I would code it as serrated in my analysis, just as I do Troodon. To do otherwise is to put the cart before the horse.

      • Understand that what I am referring to aren’t even “marginal serrations” in the sense mentioned already: They are pectinate arrays of vertically-oriented, rather than radial or perpendicular, large growths of the crown. In mammals, the development of the cusps of a crown follow a pattern quite differently from what we know of “archosaur” serrations, as also seen in some lamniform sharks and the sabres of some large cats. Even in taxa with both cusps and serrations in the same jaw, the development is distinct (again, as in sharks and cats).

        And as you say, Mickey: One uses the analysis to provide evidence for homology: One does not assume it a priori by coding homology into the analysis — especially if distribution and development contradict the assumption they are homologous. Instead, segregate the two patterns.

      • I agree with both of you. Use the character, but know that it has wide convergence. Other traits, like the migration of the centralia to become the preaxial carpal elements and the presence of a sternal complex and hyper-enlarged fourth finger are more restricted and thus have more clout.

  2. To Rob’s question. It may not be necessary to do this with regards to pterosaurs as the scorings note above. However, when we get down to dinosaurs, which was Mickey’s point, then yes if the characters are visible or not on available taxa. I have only gotten so far, so far. There are a week’s worth of Nesbitt character blogs before we get to dinos.

    To Mickey’s question: My earlier errors have just been corrected. Thank you for noting them. I had not thought about Austriadactylus for some time so I needed that reminder. And the laterosphenoid thing made me scratch my head. I must have been distracted while writing it as several other answers simply required a copy/paste. Thanks for being a good editor.

  3. It couldn’t hurt to put these characters in though. And excluding characters based on what you think they will do is not the same as testing. Without testing you can’t know for sure if this will reinforce your tree or not.
    Anyway, looking forward to seeing the rest of the week’s stuff.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s