Friday, February 19, 2010

Query: Were Mesosaurs Parareptiles or Basal Amniotes?


I'm a little bit confused. I've been reading a few different papers on parareptiles and I am getting a conflicting picture from various group's cladistic analyzes. Some place mesosaurs as parareptiles. Others have been placing them as basal amniotes. So, my appeal to the professional paleo types that read my blog at least occasionally:

What are they?

Thanx.

11 comments:

220mya said...

Pretty much all recent analyses that I know of place them as basal parareptiles. I think that is a fair placement.

Will Baird said...

Tsuji et al 2009 (History and definition of parareptilia) is what is making me scratch my head.

Neil said...

Just to echo Randy, here's what Modesto (2005) has to say:

"Although these analyses are not comprehensive treatments of mesosaurid relationships (because there are numerous postcranial characters that remain to be evaluated), the available phylogenetic evidence suggests strongly that mesosaurs are not 'stem sauropsids', but instead are nested within Reptilia. Unfortunately, the nomenclature of the eureptile sister group is problematic because of the phylogenetic lability of turtles and because of the phylogenetic reconception of the long established name Anapsida (Modesto & Anderson, 2004)."

So almost certainly well above the Synapsid/Sauropsid split.

I'm not aware of any recent trees that place them as stem-amniotes (i.e. below the split).

Tsuji et al. 2009 considers parareptiles to be 'basal amniotes' so Mesosaurs would be both Parareptiles and 'basal amniotes' but not stem-amniotes. Good example of why "basal" is not a particularly useful term, and why paleontologists should probably get out of the habit of using it.

Will Baird said...

Thank you. This helps.

That makes a lot more sense. Parareptiles are an odd bunch. Very, very diverse. Extraordinarily so.

Anonymous said...

Wait, so testudines / chelonians are now considered parareptiles? Do I have that right?



Doug M.

Will Baird said...

massive bone of contention, Doug.

Some hold they are parareptiles. Others hold that they are derived diapsids. I think that the genetic evidence leans towards the latter, but the molecular clock has turned out to be less accurate than anticipated, sooo...still up in the air.

The idea that turtles are anapsids (sorta an old name for pararetiles) isn't very new though.

Anonymous said...

Interested Statement: Is this for a possible new Xenopermian post?

Possible Answer: The currently favored idea is that mesosaurs are past the synapsid/sauropsid split, and are either very basal diapsids or else basal parareptiles. It doesn't matter to me either way, regardless of their evolutionary history they are still meatbags.

Will Baird said...

Related to the Xenopermian? Only tangentially.

Anonymous said...

! Derived diapsids? I had never heard that before! I thought conventional wisdom was still "turtles are anapsids, which split off from other amniotes well back in the Permian."

And that would be one derived damn diapsid.

OTOH, "derived diapsid" covers everything from Brachiosaurus to Buddy the Parakeet. So maybe.


Doug M.

Will Baird said...

There have been a lot of reshuffles lately.

They were argued to be anapsids. Anapsida turned out to be an inaccurate clade. Most of anapsida was rolled into parareptilia while conducting a raid on diapsida.

Recently, its been argued based on genetics and a few other bits that they are actually derived diapsids that closed their fenestra. (note: some parareptiles developed fenestrae).

We'll see. :)

nick.gardner said...

"I think that the genetic evidence leans towards the latter, but the molecular clock has turned out to be less accurate than anticipated, sooo...still up in the air."

A note, because I keep seeing this confusion appearing on this blog: molecular clocks aren't the same as molecular phylogenetics.

Further, it's a bit more complicated than just are turtles parareptiles or are they diapsids. For one thing, the diapsid camp is divided as to whether they are lepidosauromorphs (Rieppel, Mueller) or archosauromorphs (Bhullar and Bever, various molecular phylogenies). And within the parareptile camp, Lee (2001) found turtles to be related to pareiasaurids and the recent Lyson et al. (2010) paper found turtles to be basal ankyramorphs outside of the clade containing pareiasaurs, procolophonoids, and 'nycteroleters' (sensu Tsuji and Mueller 2009).