Weekly Spotlight (Mini Version): Pezosiren

4 09 2010

Good tidings and well-wishes! 

(NOTE: Due to my rough academic schedule, I’ve been forced to rely upon quotations and links far more heavily in this week’s spotlight than usual.) 

Although Ambulocetus, Rodhocetus, and their cetacean brethren may get all the media attention when it comes to exemplifying the evolution of aquatic amniotes, the sirenian fossil record has produced an equally fascinating parallel transition equipped with a number of amazing beasts of its own. Perhaps none, however, can exemplify the key components of this incredible journey from land to sea more effectively than Pezosiren portelli from the early Eocene of Jamaica. 

Pezosiren skeletal reconstruction

Pezosiren skull closeup.

Pezosiren is, to date, represented by a nearly complete skeleton (see above) unearthed during the nineties approximately 15 km south of Montego Bay alongside a number of fish, crocodylian, and rhinocerotoid specimens. The animal’s overall body shape has been informally compared to that of the famed Moeritherium. According to paenungulate experts Emmanuel Gheerbrant, Daryl P. Domning, and Pascal Tassy, 

“The basal sirenian morphotype, displayed by Pezosiren, comprises a long trunk (20 thoracic and 4 lumbar vertebrae) supported on relatively short legs… Four sacral vertebrae are present, with a firm sacroiliac articulation capable in most cases of supporting the body’s weight on dry land. However, in Pezosiren, these sacrals are no longer anklyosed in most cases, pointing to an incipient increase of flexibility of the sacral region for swimming by spinal undulation– a convergence with early whales that were comparable in their stage of evolution to Rodhocetus.  The tail was long and strong compared to that of most terrestrial ungulates, but the caudal vertebrae still lacked the enlarged transverse processes for the attachment of powerful tail muscles seen later in sirenians and cetaceans.”  

A) Partial right mandible, medial view, showing location of small mandibular foramen (white arrow). B) Dorsal view of sacrum. C.) Right innominate, ventromedial view. D.) Right and left innominates, ventral view. E.) Subadult left femur, anterior view. F.) Left tibia, anterior view. G.) Left tibia, lateral view. H.) Intermediate phalanx, dorsal view. Scale Bar = 5 cm. Courtesy of nature.com

The authors maintain that Pezosiren‘s tail was likely utilized to maximize the effects of spinal dorsoventral undulation. Theoretically, this locomotory style would have resulted in a strong propulsive force being delivered to the tail and hindlimbs. Additionally, it’s of vital importance to note that Pezosiren, like modern hippos, had osteosclerotic (“very dense”) appendicular limb elements, enabling the sirenian to almost effortlessly remain submerged for extended periods of time. The significance of this fact has been described at length by fellow paleo-blogger Brian Switek (whose upcoming book I simply cannot wait to get my greedy little paws on!) and ultimately boils down to the observation that Pezosiren would have been capable of a much wider range of motion underwater than upon any other form of terrestrial realm.