Weekly Spotlight: Tylocephalonyx

21 03 2010

Good tidings and well-wishes!

Though any assertion of my strong academic bias towards the proboscidea is doubtlessly obvious to anyone who’s so much as glanced at my humble blog, I’ll readily concede that my second-favorite animalian order is the perissodactyla, better known as the ‘odd-toed ungulates’. While this well-known yet understudied group is currently represented solely by equids (horses & kin), rhinos, and tapirs, its produced a menagerie of oddities throughout the course of the past 55 million years, including the deliciously-bizarre brontotheres and the largest land mammal known to have walked the surface of our planet. But perhaps the most eccentric (and, therefore, ‘interesting’) of these hoofed wonders hail from the chalicotheriidae family: a congregation of perissodactyls which has been described as “a bunch of horses desperately trying to be gorillas”.

A motley crew of perissodactyls

This delineation, while amusing, is ultimately guilty of simplifying the situation to a notable degree. The chalicotheriidae is divided into two subfamilies: the chalicotheriinae and the comparatively-primitive schizotheriinae. In “Mammoths, Sabertooths, And Hominids: 65 Million Years Of Mammalian Evolution In Europe”, Jordi Augsti writes:

“While the chalicotherines developed bizarre, vaguely gorilla-like proportions as an adaptation to a largely static style of feeding — probably in closed envrionments — the schizotherines, such as Ancylotherium… retained more ‘conventional’ perissodactyl proportions and a more efficient quadrupedal locomotion, although in both subfamilies, the feet were clawed instead of hoofed, and the forelimbs were likely used to manipulate branches and to bring the animals’ forequarters up toward the lower foliage of trees.”

Anisodon grande, a chalicotherine.

Ancylotherium sp., a schizotherine (as seen in BBC's 'Walking With Prehistoric Beasts').

At the onset, the schizotherines may not appear to be of any particular interest when placed alongside their rather unorthodox counterparts. However, the members of this subfamily sported a unique system of claw retraction on their forelimbs in which these claws were dorsally pulled from the ground during locomotion to prevent wear. (Schizotherines can be even more readily identified by their relatively long necks and limbs: it’s therefore been suggested that the browsing capabilities of these beasts may have been heightened by occasional bipedalism).

Furthermore, unlike the chalicotherines, the schizotherines were able to reach North America during the Miocene epoch, with Moropus and Tylocephalonyx presumably evolving on the continent during this time.

Moropus left manus reconstruction. Unlike most perissodactyls in which the middle toe is the largest, the inner toe earns this distinction in chalicotheres.

While it may seem logical to assume that Tylocephalonyx and Moropus were very close relatives on the basis of their chronologically and geographically contemporaneous status, this idea may not be as practical as one might think. In this descriptive article, Margery C. Coombs argues:

Moropus has the most similar morphology to Tylocephalonyx, but the major similarities are primitive. Tylocephalonyx and Ancylotherium share certain derived characters, such as the reduction of lower incisors… however, Tylocephalonyx lacks important derived characters of the manus and dentition of Ancylotherium…  The distribution of Tylocephalonyx in the [early to mid-Miocene] of North America is for the most part north of contemporary distribution of Moropus [which is most commonly found in Colorado, Nebraska, and Wyoming]. Floral evidence suggests that Tylocephalonyx was associated with a moist, temperate, dedicious hardwood/conifer forest which covered northwestern North America, whereas Moropus at that time was associated with drier savanna or forest-woodland border situations.”

Moropus skeleton.

As usual, I’ve saved what’s almost unquestionably the most interesting aspect of this week’s featured critter’s anatomy for last: the skull, which (as evidenced by the following photograph) contains a rather large dorsal expansion that’s chiefly composed of the frontal and parietal bones and was, theoretically, used for the purposes of intraspecific head-butting and/or general rough-housing.

The hat tip goes to my regular reader Doug for this one!

Only additional specimens and research will yield a clearer answer to how precisely this weird mammal utilized its bulbous skull dome.

May the fossil record continue to enchant us all!

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2 responses

21 03 2010
accpaleo

“Though I’ve regrettably been unable to locate any images of Tylocephalonyx’s bizarre cranium…”- Then i got a present for you!

Tylocephalonyx is one of the critters i want in my museum if i ever build it. Moropus is cool ,but he is in quite a few museums and I’m all about the lesser known guys. And that Anisodon grande. Was that once Calicotherium grande? Cause i swear I’ve heard “grande” attached to a chalicothere, but I’ve never heard of Anisodon before.

21 03 2010
tanystropheus

Thanks for the awesome image, Doug!

I believe that A. grande was once C. grande, but I’m not absolutely certain…

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