Thursday, November 19, 2009

Prep work: update II and a note on sirenian periotics

This has been a long hiatus! I’ve been really busy doing some more prep work on the Puerto Rican Dioplotherium and the Yucatán skull (another new species of dugongine). On top of that I’ve been preparing a couple of manuscripts describing some sirenian remains from PR, which I hope to submit sometime next year.

The subject of this post is to show you more of the Puerto Rican Dioplotherium, which was featured on the previous post. I have done additional prep work on the left squamosal, which was detached from the skull.

The composite picture above shows the skull as it was back in 2006 (top picture) and an outline of the enlarged area below. All that was visible of the sqamosal were the zygomatic arch, post-tympanic process and the mastoid part of the periotic (bottom picture). It was exciting knowing that part of the ear bones were preserved, even if it was only the periotic. Fortunately I got more than I bargained for.

In this figure we see the squamosal, now free of matrix, in lateral (A) and medial (B) views. Notice that the tympanic bone was preserved as well as the periotic. And there is more!

Additional removal of matrix revealed the three auditory ossicles, in articulation! The picture below shows a posteroventral view into the middle ear (anterior is to the right, medial towards the top of the picture). This is really neat as these bones are easily lost in most fossils (they are, apparently, missing on the right side of the skull).

A little on sirenian periotics

The periotic can be divided into three parts tegmen tympani, pars mastoidea and pars petrosa. The latter can be subdivided into pars canalicularis and pars cochlearis (see picture above) (Robineau, 1969). In the pars cochlearis, the structure labeled perilymphatic foramen, is uniquely found in (most) sirenians, (most) proboscideans and Arsinoitherium (hinting at their tethytherian affinity?). The homologous structure in other mammals consists of two openings known as the fenestra cochleae (rotunda) and aqueductus cochleae (Court, 1994).

The occurrence of a perilymphatic foramen in some tethytheres (I’m not sure what is the condition in desmostylians) seems to indicate that it might be a unique derived character of the group. Nonetheless, when we look at the fossil record, primitive proboscideans (Phosphatherium escuilliei) and sirenians (Prorastomus sirenoides) do have fenestra cochleae and aqueductus cochleae (Gheerbrant et al., 2005; Court, 1990; Savage et al., 1994). Meaning that this condition is homoplasic in tethytheres (Court, 1994, Gheerbrant et al. 2005). Whether resulting from multiple origins or multiple reversals, I still think it is an interesting characteristic that is found in at least some tethytheres.

Previous post about sirenians:

Prep work: update

Sirenian diversity in the past

De la tierra al agua (English version here)

Domningia and other Indian sirenians

What's wrong with the hands of Steller's sea cow

Court, N. 1990. Perotic anatomy of Arsinoitherium (Mammalia, Embrithopoda) and its phylogenetic implications. Journal of Vertebrate Paleontology 10(2):170-182.

Court, N. 1994. The periotic of Moeritherium (Mammalia, Proboscidea): homology or homoplasy in the ear region of Tethytheria McKenna, 1975? Zoological Journal of the Linnean Society 112:13-28.

Gheerbrant, E., J. Sudre, P. Tassy, M. Amaghzaz, B. Bouya and M. Iarochène. 2005. Nouvelles données sur Phosphatherium escuilliei (Mammalia, Proboscidea) de ‘Éocène inférieur du Maroc, apports à la phylogénie des Proboscidea et des ongulés lophodontes. Geodiversitas 27(2):239-333.

Robineau, D. 1969. Morphologie externe du complexe osseux temporal chez les sireniens. Mémoires du Muséum National d’Histoire Naturelle, Série A, Zoologie 60(1)-1-32.

Savage, R. J. G., D. P. Domning and J. G. M. Thewissen. 1994. Fossil Sirenia of the west Atlantic and Caribbean region. V. The most primitive known sirenian, Prorastomus sirenoides Owen, 1855. Journal of Vertebrate Paleontology 14(3):427-449.