The previous article was a brief, cursory introduction to the mesonychians. Time to look at things in a bit more detail…
Andrewsarchus mongoliensis is, of course, ‘the’ mesonychian for most people, and one might get the impression that it’s a typical member of the group. In fact it’s most definitely not typical, and – ironically – it’s not even part of Mesonychia in the majority of recent phylogenetic studies (O’Leary 1999, O’Leary & Geisler 1999, Gatesy & O’Leary 2001, Geisler 2001, O’Leary et al. 2003, O’Leary & Gatesy 2008)… though this might be due to the fact that it’s pretty poor known (only its skull can be coded for). Van Valen (1978) regarded Andrewsarchus as an arctocyonid (like so many Paleogene mammals, arctocyonids are of uncertain affinities – they’ve traditionally been regarded as condylarths [see previous article] – but they don’t seem closely related to mesonychians), and Gingerich (1998) implied that this is at least plausible. Anyway, it owes its fame to its large size (its skull is 83 cm long), and – importantly – to the fact that its description was published in English, and in a relatively accessible publication (the AMNH’s in-house journal American Museum Novitates; note, however, that the description in question (Osborn 1924) is annoyingly brief and mostly devoid of detailed morphological information). These facts have of course guaranteed its place in every single prehistoric animal book [adjacent image from wikipedia; by Dmitry Bogdanov].
Indeed, what can I say about Andrewsarchus that hasn’t been said before? Not much. Osborn’s paper includes the interesting observation that the holotype Andrewsarchus skull was thought for a while to be that of an entelodont and, as he noted, the two were at least superficially similar, perhaps due to similar omnivorous feeding habits (though note that the possibility of a close phylogenetic relationship between Andrewsarchus and entelodonts is real: see O’Leary & Gatesy (2008)). Szalay & Gould (1966) expanded on this, noting that the semicircular arrangement of the premaxillary teeth and premolar morphology is very similar in the two, and they even noted that the teeth of Andrewsarchus would have been identified as those of an entelodont if found on their own.
While it may well be that Andrewsarchus was an entelodont-like predator and/or omnivore (for more on entelodonts see Tet Zoo picture of the day # 23 and Giant killer pigs from hell), there are, however, indications that Andrewsarchus was freakishly weird, and doing something very special. In fact, the weirdness of its skull is routinely not depicted correctly in life restorations. Its snout is strikingly long and narrow, and ‘pinched in’ about half-way along its length. The result is that the distal end of the rostrum forms a sub-circular rosette that almost resembles that of a spinosaurid theropod. Its orbits are located way down on the sides of its skull and were widely separated by the broad base of the snout, and its entire occipital region looks narrower, and smaller, than expected in an animal of this size. The glenoid fossa is flattened compared to that of ‘proper’ mesonychians, and the associated pre- and post-glenoid structures on the zygomatic arch are small. The sagittal crest is small as well. These features all suggest that the jaws were relatively weak. And while the upper canines were said by Osborn (1924) to be enormous, they aren’t: ‘The canines are very reduced in size and in proportion to the whole dentition and to the whole skull’ (Szalay & Gould 1966, p. 154). Clearly, Andrewsarchus is just crying out for some neat functional study on cranial kinematics and function. Come on: surely someone, by now, must have thought of doing FEA or something on its skull?
More mesonychians next: Andrewsarchus and the triisodontines.
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
- Killer sperm whales
- Because it would be wrong not to mention a sperm whale named like a tyrannosaur
- Dromomerycids: discuss
Refs – –
Gatesy, J. & O’Leary, M. A. 2001. Deciphering whale origins with molecules and fossils. Trends in Ecology & Evolution 16, 562-570.
Geisler, J. H. 2001. New morphological evidence for the phylogeny of Artiodactyla, Cetacea, and Mesonychidae. American Museum Novitates 3344, 1-53.
Gingerich, P. D. 1998. Paleobiological perspectives on Mesonychia, Archaeoceti, and the origin of whales. In Thewissen, J. G. M. (ed) The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea. Plenum Press (New York), pp. 423-449.
O’Leary, M. A. 1999. Parsimony analysis of total evidence from extinct and extant taxa and the cetacean-artiodactyl question (Mammalia, Ungulata). Cladistics 15, 315-330.
– . & Gatesy, J. 2008. Impact of increased character sampling on the phylogeny of Cetartiodactyla (Mammalia): combined analysis including fossils. Cladistics 24, 397-442.
– ., Gatesy, J. & Novacek, M. J. 2003. Are the dental data really at odds with the molecular data? Morphological evidence for whale phylogeny (re)reexamined. Systematic Biology 52, 853-864.
– . & 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.
Osborn, H. F. 1924. Andrewsarchus, giant mesonychid of Mongolia. American Museum Novitates 146, 1-5.
Szalay, F. S. & Gould, S. J. 1966. Asiatic Mesonychidae (Mammalia, Condylarthra). Bulletin of the American Museum of Natural History 132, 127-174.
Van Valen, L. 1978. The beginning of the age of mammals. Evolutionary Theory 4, 45-80.