Hapalodectids, the once otter-like proto-whales (mesonychians part V)

We now move to another mesonychian group: Hapalodectidae. This is yet another of those obscure little groups that sounds really interesting, yet are never the subject of focus or discussion. Virtually all of the literature on them - and that's still only ten papers or so - mentions the idea that they might have been piscivorous, but I can't find any elaboration of this and would like to see some. Hapalodectidae was named by Szalay & Gould (1966) as a mesonychid 'subfamily' (following Ting & Li (1987) and others, I here refer to the group as Hapalodectidae instead of Hapalodectinae); it's a small group containing about five genera (or does it? Read on) from North America and eastern Asia, the best known of which is Hapalodectes. Five species of this genus have been named (H. leptognathus and H. anthracinus from North America, and H. serus, H. hetangensis and a new, as yet unnamed species, all from Asia).

Remains of Hapalodectes had first been reported in 1892 (when they were included within the mesonychid genus Dissacus), and for a long time members of the group were only known from lower jaw fragments. Ting & Li (1987), however, reported good cranial material from the Lower Eocene Lingcha Formation, and a complete skull [shown above] was reported from the Lingcha Formation by Ting et al. (2004). The hapalodectid skull is long and narrow, with a comparatively large, long and laterally expanded braincase. Based on data from the unnamed Upper Eocene Lushi Formation hapalodectid, Szalay (1969) noted indications that the hapalodectid head was proportionally larger than that of most modern mammals. Postcranial remains (partial limb bones) indicate that hapalodectids were terrestrial, but they weren't cursorial, in contrast to mesonychids (O'Leary 1998).

Hapalodectids were, mostly, small animals compared to mesonychids and triisodontids, though the largest member of the group, Hapalorestes, did overlap in size with the members of these other groups. Gunnell & Gingerich (1996) estimated their body masses to range from 1 to 8 kg, putting them within the 'medium-sized' category for mammals (in contrast, mesonychids are estimated to range from 10 to 250 kg). One of their characteristic features is that the lower third molar is the longest tooth in the tooth row. Another peculiarity is the presence of broad, deep 'embrasure pits' on the palate: these would have received the crowns of the lower jaw teeth when the jaws were closed, and they're peppered with small vascular foramina (Szalay 1969: adjacent reconstruction of palate shown in adjacent image). Similar pits are seen on the palates of stem-whales, and these features, combined with those comments about possible piscivory, have inspired some to imagine hapalodectids as close to whale ancestry: this possibility was depicted in a phylogram-style diagram produced by Szalay (1969). If you watched the TV series The Velvet Claw (or read the accompanying book), you might recall a scene where an otter-like Hapalodectes slips into a river and evolves directly into a humpback whale [a still from that scene is shown at the top of the article: for more on The Velvet Claw see Homage to The Velvet Claw (part I) and Homage to The Velvet Claw (part II)]. It doesn't seem that hapalodectids did look otter-like, but just what they did look like remains uncertain. Has anyone ever seen (or produced) a life restoration? [lower jaw of Hapalodectes leptognathus below, from Szalay (1969)].

How are hapalodectids related to other mesonychians? They share a number of detailed dental characters with mesonychids, but differ from them in that their lower cheek teeth are more compressed and blade-like. Szalay (1969) thought that they might have descended from mesonychids like Dissacus. Inspired by efforts to determine cetacean affinities, a larger number of recent studies have included a broad selection of mesonychians, cetaceans, artiodactyls and other placentals (see previous articles in this series!). These have mostly recovered Hapalodectes as the most basal member of a mesonychian clade, the topology of which is poorly resolved.

A number of other mesonychians have been included within Hapalodectidae by some authors, including Lohoodon, Metahapalodectes and Plagiocristodon (McKenna & Bell 1997). However, Zhou & Gingerich (1991) argued that these taxa are mesonychids, not hapalodectids, as they lack 'a re-entrant groove on the anterior surface of the lower molars' (p, 219), another characteristic hapalodectid feature.

And - are there more to come? That would be telling.

For previous articles on Paleogene mammals see...

• Homage to The Velvet Claw (part I)
• Giant killer pigs from hell
• Snow White and the six perissodactyls
• Thunder beasts in pictures
• Thunder beasts of New York
• Because we all love Paleogene 'ungulates'
• What did a dinoceratan do?
• Because Andrewsarchus is not the world's only mesonychian (mesonychians part I)

And for other stuff on neat and obscure fossil mammals see...

• Ten things you didn't know about sloths
• Five things you didn't know about armadillos
• Dude, where's my astrapothere?
• Snorki the giant's friends and relatives
• What was that skull? (glyptodonts)
• Invasion of the marsupial weasels, dogs, cats and bears... or is it?
• Long-snouted marsupial martens and false thylacines
• Marsupial 'bears' and marsupial sabre-tooths
• Scaphokogia!
• Killer sperm whales
• Because it would be wrong not to mention a sperm whale named like a tyrannosaur
• Dromomerycids: discuss

Refs - -

Gunnell, G. F. & Gingerich, P. D. 1996. New hapalodectid Hapalorestes lowei (Mammalia, Mesonychia) from the early middle Eocene of northwestern Wyoming. Contributions from the Museum of Paleontology, the University of Michigan 29, 413-418.

McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press (New York).

O'Leary, M. A. 1998. Morphology of the humerus of Hapalodectes (Mammalia, Mesonychia). American Museum Novitates 3242, 1-6.

Szalay, F. S. 1969. The Hapalodectine and a phylogeny of the Mesonychidae (Mammalia, Condylarthra). American Museum Novitates 2361, 1-26.

- . & Gould, S. J. 1966. Asiatic Mesonychidae (Mammalia, Condylarthra). Bulletin of the American Museum of Natural History 132, 127-174.

Ting, S. & Li, C. 1987. The skull of Hapalodectes (?Acreodi, Mammalia), with notes on some Chinese Paleocene mesonychids. Vertebrata PalAsiatica 25, 161-186.

- ., Wang, Y., Schiebout, J. A., Koch, P. L., Clyde, W. C., Bowen, G. J. & Wang, Y. 2004. New Early Eocene mammalian fossils from the Hengyang Basin, Hunan China [sic]. Bulletin of the Carnegie Museum of Natural History 36, 291-301.

Zhou, X. & Gingerich, P. D. 1991. New species of Hapalodectes (Mammalia, Mesonychia) from the Early Wasatchian, Early Eocene, of northwestern Wyoming. Contributions from the Museum of Paleontology, the University of Michigan 28, 215-220.