Seriously trying to nest turtles with archosaurs

Recent molecule analysis of tetrapods
by Iwabe et al. 2005 report that turtles are closer to archosaurs than to any other tested extant taxa.

Okay, given that genomic goal…
let’s figure out which archosaurs are closer to turtles.

We can do this
in the taxon-rich LRT by deleting all other candidate taxa but turtles and archosaurs (crocs + dinos). We did something like this before when we successfully nested turtles with pterosaurs using taxon deletion. Hone and Benton (2007-2009) also did something similar when they deleted the closest sisters to pterosaurs, the fenestrasaurs of Peters (2000) to connive a way to nest pterosaurs uncertainly with basal archosauriformes, like Erythrosuchus. And they did not check their work for possible problems.

You just can’t learn as much from taxon exclusion
and you can be led down unproductive paths. So, it’s much better to expand your taxon list if there is any doubt as to where your taxon will nest… as you’ll soon see.

In experiment #1
only turtles and archosaurs (crocs + dinos) were included. Ichthyostega and Pederpes were chosen as outgroup taxa. Eight MPTs were recovered from the 192 non-deleted taxa. Unfortunately, turtles nested between the basal tetrapods and the archosaurs, in other words, outside the archosaurs.

So, now let’s expand the taxon list to include all archosauriformes.

In experiment #2
only turtles and archosauriformes (includes the basalmost archosauriform, Youngoides UC FMNH 1528 and all descendants. 2 MPTs were recovered (Niolamia + Meiolania currently unresolved) from the 266 non-deleted taxa. Unfortunately, turtles again nested between the basal tetrapods and the archosauriformes, outside the archosauriformes. Interestingly, the basalmost archosauriform clades were reversed, so they included the small choristoderes at the base, not the proterosuchids, which still nested with erythrosuchids, UC FMNH 1528 and Euparkeria.

So, let’s expand the taxon list to include the new archosauromorphs.

In experiment #3
only turtles and the new archosauromorphs (including enaliosaurs) were included. Turtle-like sauropterygians from within the Enaliosauria have been promoted as turtle sisters. I stopped the search after 220 million rearrangements were tried. 6 MPTs were recovered from 635 non-deleted taxa. Interestingly, turtles nested with the Anomodontia, the herbivorous basal therapsids, now shifting their nesting to between Titanophoneus and Procynosuchus — NOT with basal sauropterygians or shelled placodonts!

Now, after all that frustration,
let’s expand the taxon list to include all tested tetrapods. 

In experiment  #4
giving turtles every possible opportunity with the current list of 889 total taxa, turtles arise in two clades from two clades of small pareiasaurs, Sclerosaurus for flat-soft-shelled turtles and Elginia for  dome-hard-shelled turtles. Both of these hypotheses of relationships are heretical and unprecedented, but continue to be validated  in the LRT. And when you look at the pertinent transitional taxa (Fig. 1), the long overlooked transition from pareiasaurs to turtles is almost painfully obvious.

Figure 1. The palate and lateral skulls of Elginia, a small toothed pareiasaur, and Niolamia, a large, toothless turtle to scale. These taxa nest a the origin of turtles.

Figure 1. The palate and lateral skulls of Elginia, a small toothed pareiasaur, and Niolamia, a large, toothless turtle to scale. These taxa nest at the origin of hard-shelled turtles. No other taxa are closer. Soft-shelled turtles have their own origin. From this perspective, using these taxa, the origin of turtles is pretty clear. Here it is clear that the supratemporal horns and tabular plate/horns are large elements that become reduced shortly before turtles were able to pull the skull beneath the shell.

Meiolaniidae
is a clade of extinct turtles restricted to the Cenozoic of Patagonia and Australasia. Sterli and de la Fuente 2012 report, “Historically, meiolaniids have been described as being related to both living groups of turtles – Pleurodira and Cryptodira – or were even placed outside the crown (Joyce 2007; Sterli 2010; Sterli & de la Fuente 2011a), but only recently have other fossil turtles been allied with this group.”

Not Meiolaniidae
Peligrochelys, Chubutemys and Mongolochelys were considered Meiolaniidae by Sterli and de la Fuente 2012, but that is not a recovered topology in the LRT. All of these taxa lack horns, which disappear phylogenetically in this lineage with Proganochelys or an earlier, as yet unknown, Triassic taxon. These taxa nest between Proganochelys and sea turtles like Chelonia. Earlier authors assumed the skull horns were derived, largely because they did not include the taxon Elginia in their studies. So they got their phylogenetic order backwards. Yes, the hornless taxa currently precede Meiolaniidae in the fossil record. Elginia, which they ignore, precedes all known turtles chronologically.

Those giant horns
Here (Fig. 1) it is clear that the lateral supratemporal horns and posterodorsal tabular plate/horns are large elements that become reduced shortly before or as turtles learned to pull the skull beneath the shell. Niolamia nests closer to Elginia than smaller-horned Meiolania does in the LRT.

On another note… Guillon et al. used molecules
to lump and separate extant turtles. They found a basal dichotomy between cryptodires (vertical-neck turtles) and pleurodires (side-neck turtles), and within cryptodires a split between soft-shell turtles (Trionychia) and all remaining hard-shell cryptodires  Of course, they had no outgroup because they were testing extant taxa. Contra Guillion et al. the LRT indicates that Trionychia had a separate but parallel origin from pareiasaur ancestors without a shell and pleurodires are a clade within the cryptodires. Thus the first line of text in Guillon et al.Turtles (Testudines) form a monophyletic group with a highly distinctive body plan.” is not supported by the LRT.

References
Guillon J-M, Guéry L, Hulin V and Girondot M 2012. A large phylogeny of turtles (Testudines) using molecular data. Contributions to Zoology, 81 (3) 147-158 (2012)
Iwabe N, Hara Y, Kumazawa Y, Shibamoto K, Saito Y, Miyata T. and Katoh K 2005. Sister group relationship of turtles to the bird-crocodilian clade revealed by nuclear DNA-coded proteins, Molecular Biology and Evolution 22(4):810–813.
Sterli J and de la Fuente MS 2012. New evidence from the Palaeocene of Patagonia (Argentina) on the evolution and palaeo-biogeography of Meiolaniformes (Testudinata, new taxon name) Journal of Systematic Palaeontology 11(7):835–852.

Advertisements

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