Sorry for the delay.
Several months ago I created a phylogenetic tree that was wrong (in the nesting of a particular clade). It happens all the time with the pros, and for exactly the same reason: for lack of a sufficient gamut of sister taxa. I spent the last several days in study to rectify the problem.
It all began when, out of curiosity, I added Utaherpeton, Tuditanus and Anthracodromeus to the large reptile family tree. I expected these three to nest traditionally outside the Reptilia (= Amniota), but instead they nested in a clade with Westlothiana just barely within the base of the Reptilia.
A Little Background on Microsaurs
Carroll, Bybee and Tidwell (1991) considered tiny Utaherpeton to be the oldest microsaur (and therefore an amphibian). Tuditanus had been long considered a microsaur and an amphibian. However, Müller and Reisz (2006) considered Anthracodromeus to be close to Cephalerpeton and Protorothyris, both reptiles.
Carroll, Bybee and Tidwell (1991) reported, “…no synapomorphies are recognized that support a specific sister-group relationship between microsaurs and any other group of Paleozoic tetrapods.” (More from this paper as a postscript, below)
Telling Microsaurs and Reptile Apart Can Be Difficult
Some microsaurs are so reptile-like that it can be difficult to tell them apart from reptiles. Microsaurs all lose the intertemporal bone (typically by fusion with one or another of the skull roof bones) as do reptiles. Microsaurs can also have an astragalus (fused tibiale and intermedium) when they don’t have a poorly ossified tarsus. Some have canines. Many lose intercentra. Some share the trait of a poorly ossified pectoral girdle. Some have a T-shaped interclavicle. The number of sacrals is typically one in microsaurs, as in all other pre-reptiles. But Micraroter has three and both Cacops and Doleserpeton have two. So how do you separate the microsaurs from the reptiles?
The solution (as always): add more taxa (in this case, more microsaurs and amphibians) to the matrix.
The new tree nested these three taxa and several other microsaurs, amphibians and nectrideans outside the Reptilia and I learned something about microsaurs and nectrideans along the way. I’ve added several of these taxa to the reptileevolution.com website and will be covering them in future blogs — but if you’re in a hurry, start here.
The Occiput Will Guide You When Little Else Will
Microsaurs can often be distinguished from reptiles by the structure of the occipital condyle. Microsaurs have two condyles, side by side. Reptiles have one, like a trailer hitch. Unfortunately, this area is not always so well preserved or exposed. Reptiles typically retain all five manual digits. Microsaurs and amphibians lose the lateral finger. Other than these, there appear to be no hard and fast rules, except to test their nesting with a large phylogenetic analysis.
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.
Carroll RL and Baird D 1972. Carboniferous Stem-Reptiles of the Family Romeriidae: Bulletin of the Museum of Comparative Zoology, Harvard, v. 143, n. 5, p. 321-364.
Carroll RL, Bybee P and Tidwell WD 1991. The oldest microsaur (Amphibia). Journal of Paleontology 65: 314-322.
Carroll RL and Baird D 1968. The Carboniferous amphibian Tuditanus (Eosauravus) and the distinction between microsaurs and reptiles. American Museum novitates 2337: 1-50.
Cope ED 1871. Observations on the extinct batrachian fauna of the Carboniferous of Linton, Ohio. Proceedings of the American Philosophical Society, 12: 177.
Cope ED 1875. Supplement to the Extinct Batrachia and Reptilia of North America I. Catalogue of the Air Breathing Vertebrata from the Coal Measures of Linton, Ohio. Transactions of the American Philosophical Society, New Series 15(2):261-278.
Müller J and Reisz RR 2006. The Phylogeny of Early Eureptiles: Comparing Parsimony and Bayesian Approaches in the Investigation of a Basal Fossil Clade. Systematic Biology 55(3):503–511.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proceedings of teh Royal Society, London B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Vaughn PP 1962. 1962. The Paleozoic microsaurs as close relatives of reptiles, again. Amer. Midland Nat., vol. 67, pp. 79-84.
Carroll and Baird (1968):
1. noted the supratemporal was in contact with the postfrontal and postorbital in microsaurs, but not in “reptiles.” Utaherpeton and Anthroacodromeus did not share this character, so it also represents a derived character within the clade.
2. noted the pterygoid lacked a transverse flange in microsaurs. Unfortunately the lower mandible in some fossils obscures this character, as it does in Tuditanus. Though not microsaurs, two anamniotes, Gephyrostegus and Seymouria, also had a distinct transverse flange.
3. reported the stapes had a broad footplate and little or no stem in microsaurs while in reptiles a long perforated stem is present. Unfortunately this character was not often described or is unknown in the sister taxa listed here.
4. reported the occipital condyle in microsaurs was broad, strap-shaped and concave — essential a double condyle, opposite the single convex knob of reptiles. This is a good character, but is often obscured in several transitional taxa.
5. reported the atlas was a single ossification, opposite the condition in traditional reptiles.
6. reported the axis and third vertebra in microsaurs were similar. Utaherpeton and Anthracodromeus, among several others, did not share this character.
7. reported the endochondral shoulder girdle was incompletely ossified in microsaurs but completely ossified in basal reptiles. This is not true.
8. posited the ilium had both a dorsal and posterior process in microsaurs, but actually that is only the case in a few microsaurs, such as Tuditanus. Several others had a single dorsal process.
9. noted microsaurs had fewer manual digits with losses coming from the lateral digits. This is another good guide.
10. noted some microsaurs had ossified dorsal, oval, striated scales, but similar reptiles lacked these. Casineria had such scales but Paton, Smithson and Clack (1999) described them as ventral in this crushed fossil.
11. noted the number of sacral vertebrae in microsaurus vary from a primitive single one to three in the more derived Micraroter.