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. 





5 responses

6 09 2010

If i ever got my museum to an advanced stage, one of the themes of the marine mammal hall would be evolution. But instead of the usual “walking whales” i would have this guy as the example of terrestrial animals returning to the sea (seriously, the Black Hills Institute sells a cast of this animal that I’d use).

As long as it’s not just for padding, so what if you use quotations and links? as long as they help paint the picture. And i am having the reverse problem as you. I have nothing to kill the time, but it takes a while to get up my posts because i’m a slow typer (which isn’t helping my current situation: i have all these ideas for posts, but trouble getting them up).

6 09 2010

This museum sounds more epic every time I hear about it!

As for my earlier comments regarding quotations and links, while I fully agree with your sentiments on this topic, I’m relatively hesitant to use them extensively for fear of allowing TTT to degrade into a “paleo excerpt machine” devoid of original written material. Still, these venues most assuredly have their place in scientific blogging.

6 09 2010

well, yeah, i have so many ideas that if i ever had the means to simply make it the way i envision it now, it would likely be the size of the LA Museum. But i always word my ideas in such a way as to indicate that it will never happen (hell, i’d thank my lucky stars if i could establish something on the level of the Marmarth Research Institute)…

9 09 2010
Zach Miller

Excellent writeup! The evolutionary history of sirenians is fantastically well-documented but never gets much press. Pezosiren was a real shock to my system when it first came out. You rarely find such a great-looking “first” form of anything. Archaeopteryx is a good exception, but the fully terrestrial whales (Icthyolestes & Pakicetus) aren’t terribly well-preserved.

Also, I love that illustration.

9 09 2010

It’s ironic that the rare well-preserved animal lacks the press coverage of its comparatively disarticulated counterparts this time around.

And we’re definitely in agreement about the illustration! 🙂

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