Weekly Spotlight: Steropodon

24 08 2010

Good tidings and well-wishes!

Throughout its limited history, I’ve attempted to utilize “The Theatrical Tanystropheus” for the purposes of lending coverage towards bizarre prehistoric creatures who, for a variety of inexplicable reasons, have received relatively little coverage from the scientific community and the media at large. Given this favoritism towards long-extinct oddities, I figured that an ancient relative of one of the modern world’s most beloved biological eccentrics would be most fitting. With that, I give you Steropodon galmani, a Mesozoic precursor of the modern duck-billed platypus (Ornithorhynchus anatinus).

As most paleo-enthusiasts are well aware, Mesozoic mammal remains are often be maddeningly difficult to come by, as evidenced by the fact that Steropodon‘s 1985 discovery marked the first occasion in which a member of the class had ever been discovered in an Australian bed of the era’s sediment. This initial excavation was made in the mid-Albanian Girman Creek formation near the town of Lightning Ridge in New South Wales and consisted of a solitary lower jaw fragment containing three lower molars. In a taphonomic sense, the fossil is noteworthy due to the fact that the material had been long-since opalised, as seen in the following image.

Steropodon sported a compound lower jaw: a trait possessed by many ancestral mammals from this point in time including Teinolophos trusleri, another Cretaceous monotreme known from the region. The fact that the two little mammals also share the derived feature of having a notably deep dentary along with double-rooted molars (as opposed to their multi-rooted counterparts in subsequent monotremes) has led many authors to suggest that the two should be placed into their own family, the Steropodontidae which, in addition to the Ornithorhynchidae, Tachyglossidae, and extinct Kollikodontidae, makes up the Monotremata order, though some researchers have asserted that Steropodon could be more accurately viewed as an ornithorhynchid. Steropodon‘s molars are also noteworthy for their tribosphenic arrangement comparable to that found in modern insectivores and, more importantly from a cladistic standpoint, young platypuses and Obdurodon, their toothed Miocene forebear. However, this particular dental arrangement is considerably more advanced in Steropodon than in its extant duck-billed kin; a fact which has given rise to a considerable amount of debate concerning the phylogenetic affiliations of monotremes as a whole. Intriguingly, the advanced nature of these teeth in Steropodon implies to many Mesozoic paleo-mammologists that the Monotremata split from the Therian subclass of mammals (whose members give birth to live young in lieu of a shelled egg) far more recently than had been previously assumed.

With regards to the animal’s lifestyle, the dietary and ecological habits of Steropodon were likely quite analogous to those of modern amphibious platypuses, although it’s been said that despite their theoretically identical overall size, the deeply rooted teeth of this Cretaceous monotreme may have enabled it to tackle larger forms of fish and other aquatic denizens than those pursued by their present-day descendants.

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6 responses

25 08 2010
Zach Miller

Wait a second. Hold on.

*scrolls up and down the page*

Is that jaw fragment all that’s known of Steropodon? If so, I have a hard time accepting the extrapolated image of the whole beast. I have a very difficult time believing that an essentially modern platypus was swimming alongside Koolasuchus and avoiding megaraptors. I understand that monotremes are an ancient lineage, but com’on. I like how the people who did the restorations didn’t go ALL the way: they avoided the beaver tail, as if THAT is one thing that Mesozoic platypi most definately did NOT have.

*facepalm*

Also of note: tribosphenic molars probably evolved multiple times in mammals. Many of Australia’s jaw fragments were once considered placetal mammals until somebody realized that. This was back during Australia’s “Mesozoic gold rush,” when every hypsilophodont femur that came out of the ground was given a new genus and every dinosaur was thought to prove its lineage originated in Australia and was VERY ancient because of that. Also, it originated in Australia. Yay for Australia!

Made me ill at the time, particularily “Serendipoceratops” or whatever it’s called. You know, that one ulna that lacks any diagnostic features at all.

25 08 2010
tanystropheus

Zach,

This solitary jaw fragment is indeed the solre representative of this Mesozoic monotreme and, consequently, I share your concerns regarding the methodology through which the paleo-artistic community has elected to breathe life into Steropodon. As you’ve asserted, the animal almost certainly didn’t resemble a modern platypus to the degree depicted in these reconstructions. That being said, I can also sympathize with their conservative approach to the beast’s appearance: afterall, it’d doubtlessly be much worse if they took a series of wild liberties completely devoid of any resemblance of a scientific basis.

Regarding your note about tribosphenic molars, the local paleo-mammological community is far from reaching a consensus about how the presence of these teeth affects the phylogenetic affinities of the monotremata as a whole. Still, the probability of the tooth’s convergent evolution throughout the mammalia cannot be ignored, as you’ve pointed out.

25 08 2010
Brian

It might be important to remember that there are studies that indicate Ornithorhynchidae and Tachyglossidae did not diverge until late Oligocene/early Miocene times. If so, the likes of Steropodontidae, Kollikodontidae, *Monotrematum* and possibly even *Obdurodon* might be basal to this divergence. The theory is that echidnas evolved from platypus-like ancestors that took to the land.
In that case, *Steropodon* might well have looked like a platypus, but would not have had a particularly close relationship to the modern taxon.

25 08 2010
tanystropheus

Most intriguing! Do you have a link to these studies, per chance?

26 08 2010
johannes

Remember WWD’s *Steropodon*? A terrestrial predator that raided the nests of *Leaellynasaura* and looked suspiciously like a coati ? :-D

26 08 2010
Brian

The paper I alluded to is:
Phillips, MJ, Bennett, TH, and Lee, TSY. Molecules, morphology, and ecology
indicate a recent, amphibious ancestry for echidnas. PNAS Published online
before print September 23, 2009, doi: 10.1073/pnas.0904649106

An abstract and some discussion on the Dinosaur Mailing List can be read here: http://dml.cmnh.org/2009Sep/msg00465.html

I hope that helps.

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