The other Slavoia and the holotype

Earlier we looked at the basal amphisbaenid, Slavoia darevskii (Fig. 3 below, Talanda 2015).

I just read about the holotype (Sulminski 1984) and at least 45 other specimens attributed to Slavoia, like this one (Fig. 1, ZPAL MgR-III/77, Campanian, Late Cretaceous). Six of the 46 skulls are associated with postcranial skeletons, like the holotype, Fig. 2, ZPAL MgR-I/8). If you think this skull looks like Macrocephalosaurus, you’re not the only one.

Slavoia specimen ZPAL MgR III/77 nests not with amphisbaenids, but with Macrocephalosaurus, a contemporary from the same horizon.

Slavoia specimen ZPAL MgR III/77, one of 46 skulls,  nests not with amphisbaenids, but with Macrocephalosaurus, a contemporary from the same horizon. Talanda reports, “The specimen has only half of the elements visible in this drawing. The skull roof and the middle part are not preserved.”

Sulminski nested Slavoia with scincomorphan lizards, but he reported, “It is interesting that described here lizard displays some characters similar to macrocephalosaurid and polyglyphanodontid  species discovered in the same localities of Mongolia. This concerns also particularly the structure of the temporal region, palatal construction and in number of teeth.”

The #77 and #8 specimens nested with macrocephalosaurs in the large reptile tree.

On the other hand,
the #112 specimen nested at the base of the amphisbaenids, as we learned earlier. So the #112 specimen needs a new generic name, or there are other issues that need be dealt with.

Dragging
the amphisbaenid #112 specimen over to the macrocephalosaur specimens adds 17 steps to the most parsimonious tree score. That’s a very low number considering that there are only 17 taxa separating the macrocephalosaurs from the amphisbaenids in the large reptile tree. So, there is a bit of convergence going on here between the macrocephalosaurids and amphisbaenids. The authors note all the skulls vary in size and shape, which they attribute to ontogeny and intraspecific variation. And, of course, none are perfectly preserved. Talanda reports, “The [#77] specimen has only a half of the elements visible in this drawing. The skull roof and the middle part are not preserved.”

Figure 2. The holotype of Slavoia (#8) compared to the lateral view skull (#77). While larger, the #77 skull is relatively shorter. These two nest together in the large reptile tree along with macrocephalosaurids.

Figure 2. The holotype of Slavoia (#8) compared to the lateral view skull (#77). While larger, the #77 skull is relatively shorter. These two nest together in the large reptile tree along with macrocephalosaurids. Note the large size of the limbs.

Does this represent a solution?
Sulimski (1984) recognized the similarity between his skinks and macrocephalosaurids. Talanda (2015) considered his specimen a basal amphisbaenid, a clade derived from skinks in the large reptile tree, but Talanda nested his amphisbaenids between Cryptolacerta and Dibamus + snakes. So there is disagreement here.

Figure 1. basal amphisbaenid Slavoia from Talanda 2015, showing in situ fossil, tracing by Talanda and colorizing added here. Several bones, like the lacrimal and prefrontal, are missing in the Talanda tracing, which evidently was not traced from this photograph.

Figure 3. The #112 specimen from Talanda 2015 which both he and I nested as a basal amphisbaenid. Note the similarity to macrocephalosaurids (above). The teeth appear to be more robust here, as they are in the palate view specimens that have more of an amphisbaenid palate. I don’t see large limbs here, but limb size varies in the amphisbaenids.

Phylogeny is sometimes simple and straightforward.
Sometimes it is not.

This case shows the importance
of using specimen-based taxa in analyses, not specific, generic or suprageneric taxa. It would not be okay to take the best traits from several Slavoia specimens because some may not be Slavoia specimens! This case also highlights a need to determine where every one of these varied Slavoia specimens do nest. And it will be okay if some are lumped while others are split.

The limbs are large in the #8 specimen, but are not visible in the #112 specimen. In amphisbaenids limbs, even in basal taxa, can be vestiges, but not vestiges in the very derived Bipes.

We all have a lot to learn here. It’s not all set in stone.

References
Sulimski A 1984. A new Cretaceous scincomorph lizard from Mongolia. Palaeontologia Polonica, 46, 143–155.
Talanda M 2015.
 Cretaceous roots of the amphisbaenian lizards. Zoologica Scripta. doi:10.1111/zsc.12138

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