‘Weekly’ Spoylight: Zygorhiza

11 04 2010

Good tidings and well-wishes!

It’s been quite some time since I’ve covered a cetacean, the last one being the ‘sword-nosed’ dolphin Eurhinodelphis. This week, I intend to halt this “whale famine” by focusing my attention upon a very different (but equally interesting) member of the order: Zygorhiza kochii of the American southeast.

Zygorhiza skeletal cast.

Zygorhiza hails from the well-known, albeit paraphyletic, archaeoceti suborder. More specifically, it’s a member of the (also paraphyletic) Basilosauridae family which, according to Spencer Wilkie Tinker’s “Whales Of The World”,

“The family Basiolsauridae was probably the most specialized of the Archaeoceti. Some were of rather large size, measured from about 12 to 15 meters (39 to 49 ft) in length, and had a large skull which measured about 1.5 meters (5 ft) in length. The teeth of the cheek were of an unusual serrated type which was specialized within the Archaeoceti.”

Additionally, the members of this family are united by their homogeneous lumbarization of the sacral vertebrae.

One of the best-known subfamilies of the Basilosauridae is the famed Dorudontinae a description of which provided by the aforementioned volume is as follows:

“Fossils are known from Europe, Africa, No[rth] America, and New Zealand. Size was medium (5-6 m.); hindlimbs were [generally] absent; skull was not ‘telescoped’; snout was extended; nostrils had moved half way up the skull; front teeth were rounded and conical, while side teeth were serrated; neck vertebrae were not compressed; body was not abnormally long.”

Zygorhiza reconstruction.

So what makes Zygorhiza anatomically distinctive? When this question arises, the most frequently-mentioned feature is the fact that the cingula (‘ridges surrounding teeth’) of the animal’s premolars are finely-notched (or ‘crenulated’). Additionally, unlike most Dorudontids, Zygorhiza did in fact possess rudimentary hind limbs, as shown in the following skeletal mount.

In life, Zygorhiza would have attained a total average length of approximately 6 meters, making it somewhat larger than the 5-meter Dorudon.

Zygorhiza molar

Right, then: having discussed the role of Zygorhiza in the phylogeny of whales, it’s high time for a few ‘fun facts’ about the creature.

Zygorhiza reconstruction

Firstly, Zygorhiza has been designated as a state fossil of Mississippi, an honor it shares with the much better-known (but less well-represented in the fossiliferous sense) Basilosaurus. Additionally, should be noted that, in contrast to an unfortunately-large amount of fossilized cetaceans, Zygorhiza has actually been unearthed in association with stomach contents. According to “Giant Creatures Of The Prehistoric Seas” by Judy Cutchins and Ginny Johnston,

“In the early 1980s, an amateur fossil collector in Macon, Georgia, happened to uncover the skeleton of… a Zygorhiza. Although it was smaller than Basilosaurus, the eighteen-foot-long Zygorhiza was a fearsome predator. The remains of its last meal were found in the stomach area. Not long before it died, [the] Zygorhiza ate a two-foot-long shark.”

A Zygorhiza swims alongside a much larger Basilosaurus

May the fossil record continue to enchant us all!

A trio of cetacean skulls. From left to right: Zygorhiza, Squalodon, and Phocaena (a modern porpoise).




3 responses

14 04 2010
Zach Miller

I’ve always liked the “basilosaur”-grade whales. They’re an important sidebranch, and I liked how they were portrayed in “Walking with Beasts.” At any rate, one thing I’ve always been fascinated by is how thin the archeocete skull is behind the orbits. LOTS of room for jaw muscles!

15 04 2010
Allen Hazen

Nice! As a mammalian chauvinist, I always like it when a paleoblogger features a synapsid, and Zygorhiza deserves more publicity!

Comments: Are you sure other Dorudontids didn’t have hind limbs more like Z’s than like the internal remnants in modern whales? Basilosaurus apparently did, and I had assumed that they were standard for Cetaceans of that “grade”.

In caption to last picture: porpoise is Phocaena, not Phoca. (“Phoca” is Greek for seal, and I think the name of some seal genus: my guess/phantasy is that the Greeks, in calling porpoises phocainai, were describing them as “sort of like seals but more so.”)

Going by my scanty knowledge of Greek again, “Zygorhiza” probably means something like “yoke-root,” which makes obvious sense given your image of a tooth. Teeth like this were common in archaeocetes: the older name of Basilosaurus, “Zeuglodon,” I think refers to the same feature.

15 04 2010

Zach: They’re quite an intriguing bunch, aren’t they?

Allen: Dorudon itself did in fact possess rudimentary hindlimbs, but several of its relatives lacked them (or sported a much more rudimentary pair) as I understand it.

Thanks for the Phocaena correction!

And your Greek translation’s spot on! 🙂

As for synapsids, they’re sort of my specialty…check out the ‘greatest hits’ section.

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