Mesonyx and the other mesonychid mesonychians (mesonychians part IV)

After Andrewsarchus, the best known mesonychians are the mesonychids... and, as we saw previously, Andrewsarchus may not be a mesonychian anyway. Mesonychids are a mostly Eocene group that originated in the Paleocene; Mesonyx, from the Middle Eocene of North America, was the first member of the group to be named (Cope published the name in 1872), and it's still one of the most familiar mesonychians, by which I mean one of the kinds featured most frequently in the popular and semi-technical literature. Its limbs indicate a cursorial lifestyle [Charles Knight's Mesonyx shown below].

A number of other mesonychian taxa have conventionally been included within Mesonychidae. The two most basal taxa are Dissacus and Ankalagon (Archibald 1998, O'Leary 1999, 2001, Geisler & McKenna 2007). Yantanglestes from Paleocene Asia (originally described as a species of Dissacus) is also thought to be a basal member of the group. Dissacus was a jackal- or wolf-sized mesonychid that occurred throughout the Northern Hemisphere during the Late Paleocene (more than ten species have been named). It had slender jaws and narrow teeth, and on account of these has sometimes been suggested to be piscivorous. Relatively complete remains were described by Geisler & McKenna (2007) and confirm that the first toe was absent and that the first metatarsal was highly reduced: this is also the case in basal perissodactyls, cetaceans and artiodactyls, and it might be a synapomorphy uniting these groups.

A few dental similarities shared between Hapalodectes and Dissacus led Prothero et al. (1988) to name a new clade, Hapalodectini, which they regarded as the sister-taxon to a (mesonychid + (Andrewsarchus + cetacean)) clade (that's right, they regarded Andrewsarchus as the sister-taxon to Cetacea). This idea was contested by O'Leary (1998), however, and it's mostly agreed that, while Dissacus is a basal mesonychid, Hapalodectes is a member of another mesonychian clade that we'll be looking at later on. Ankalagon was larger than Dissacus (though the only known species, A. saurognathus, was originally described as a species of Dissacus) and is sometimes said to have been North America's first large mammalian predator. Geisler & McKenna (2007) found Ankalagon to be nested within a clade of Dissacus species, suggesting that it doesn't deserve generic separation after all. However, they also found Dissacus to be paraphyletic with respect to other mesonychids, so further study and perhaps some taxonomic revision is needed [Greg Paul's reconstruction of Ankalagon shown in adjacent image].

Harpagolestes, known from several North American and Asian species, is a notably robust-skulled mesonychid with proportionally large canines, a deep lower jaw, and relatively broad post-canine teeth that are often heavily worn [skull of H. uintensis shown here, from Szalay & Gould (1966)]. These features suggest to some authors that Harpagolestes was a carrion feeder (Szalay & Gould 1966, Archibald 1998). Harpagolestes and Mesonyx appear to be sister-taxa, and the most derived of mesonychids (O'Leary & Geisler 1999, Geisler 2001, Thewissen et al. 2007). Synoplotherium may also be part of this Harpagolestes-Mesonyx clade, and Zhou et al. (1995) found Mongolonyx and Mongolestes (both from Eocene Asia) to be part of this clade as well. Among other taxa, Pachyaena and Sinonyx appear to be successively more basal relative to the Harpagolestes + Mesonyx clade.

Pachyaena is reasonably well-known (Zhou et al. 1992, O'Leary & Rose 1995, Rose & O'Leary 1995), and also widespread, with specimens being known from the Paleocene and Eocene of eastern Asia, the Eocene and perhaps Paleocene of North America, and the Eocene of Europe. However, recent work indicates that Pachyaena is paraphyletic (Geisler & McKenna 2007), with P. ossifraga being closer to Synoplotherium, Harpagolestes and Mesonyx than to P. gigantea.

While, as noted earlier and elsewhere, Pachyaena and other mesonychids are often imagined as wolf-like, the good data we have on the osteology of this animal show that it was quite different from a canid in many respects. While the limb proportions and hoof-like phalanges indicate cursoriality, the limbs were relatively stout and show that it cannot have been a long-distance pursuit runner. Furthermore, the lumbar region wasn't as flexible as it is in carnivorans: the zygapophyses have the peculiar revolute morphology seen in modern artiodactyls (where the prezygapophyses are medially concave and prevent movement of the short, laterally convex postzygapophyses: see adjacent photos of sheep zygapophyses [and many thanks to Augusto Haro for pointing out a previous mistake made here, now corrected]). As a result, the back was relatively stiff, and Pachyaena would have been a stiff-legged runner, its gait perhaps more resembling that of a horse or antelope than that of a carnivoran. Compared to what we're used to in modern mammals, it also seems that mesonychids would have looked big-headed and also long-necked. Finally, the cheek teeth were not as sharp, or an enlarged, as those of canids and other predatory carnivorans, so mesonychids were apparently less good at slicing through tissue. Good remains of P. ossifraga show that it was a large animal of 60-70 kg [skull of Sinonyx jiashanensis from Late Paleocene China shown below, from Zhou et al. 1995].

The mesonychids mentioned here are not, of course, the only members of the group. Rather, they're the better known ones: the ones that have been included in phylogenetic studies, or the ones known from remains complete enough that allow functional or palaeobiological inferences to be made. Of course, there are a few others: Dissacusium and Jiangxia from the Asian Paleocene, Guiletes from the Asian Eocene, and Hessolestes from the North American Eocene

And there is yet more to come: the hapalodectids are next.

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)
• Mesonychians part II: Andrewsarchus was a hell of a lot weirder than all the books say

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 - -

Archibald, J. D. 1998. Archaic ungulates ("Condylarthra"). In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 292-331.

Geisler, J. H. 2001. New morphological evidence for the phylogeny of Artiodactyla, Cetacea, and Mesonychidae. American Museum Novitates 3344, 1-53.

- . & McKenna, M. C. 2007. A new species of mesonychian mammal from the lower Eocene of Mongolia and its phylogenetic relationships. Acta Palaeontologica Polonica 52, 189-212.

O'Leary, M. A. 1998. Phylogenetic and morphometric reassessment of the dental evidence for a mesonychian and cetacean clade. In Thewissen, J. G. M. (ed) The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea. Plenum Press (New York), pp. 133-161.

- . 1999. Parsimony analysis of total evidence from extinct and extant taxa and the cetacean-artiodactyl question (Mammalia, Ungulata). Cladistics 15, 315-330.

- . 2001. The phylogenetic position of cetaceans: further combined data analyses, comparisons with the stratigraphic record and a discussion of character optimization. American Zoologist 41, 487-506.

- . & Geisler, J. H. 1999. The position of Cetacea within Mammalia: phylogenetic analysis of morphological data from extinct and extant taxa. Systematic Biology 48, 455-490.

- . & Rose, K. D. 1995. Postcranial skeleton of the early Eocene mesonychid Pachyaena (Mammalia: Mesonychia). Journal of Vertebrate Paleontology 15, 401-430.

Prothero, D. R., Manning, E. M. & Fischer, M. 1988. The phylogeny of the ungulates. In Benton, M. J. (ed) The Phylogeny and Classification of the Tetrapods, Volume 2: Mammals. Clarendon Press (Oxford), pp. 201-234.

Rose, K. D. & O'Leary, M. A. 1995. The manus of Pachyaena gigantea (Mammalia: Mesonychia). Journal of Vertebrate Paleontology 15, 855-859.

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

Thewissen, J. G. M., Cooper, L. N., Clementz, M. T., Bajpai, S. & Tiwari, B. N. 2007. Whales originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450, 1190-1195.

Zhou, X. Y., Sanders, W. J. & Gingerich, P. D. 1992. Functional and behavioral implications of vertebral structure in Pachyaena ossifraga (Mammalia, Mesonychia). Contributions from the Museum of Paleontology, the University of Michigan 28, 289-319.

- ., Zhai, R. J., Gingerich, P. D. & Chen, L. Z. 1995. Skull of a new mesonychid (Mammalia, Mesonychia) from the Late Paleocene of China. Journal of Vertebrate Paleontology 15, 387-400.