Ever since the LRT nested multiberculates within Glires,
we’ve been looking for non-multituberculate members of Glires (rats, rabbits, tree shrews, etc.) from the Jurassic to support that novel hypothesis. Here’s one.
Martin and Rauhut 2005
redescribed the mandible and teeth belonging to Asfaltomylos (Rauhut et al. 2002; Fig. 1) famous for being the first Jurassic mammal from South America and for apparently lacking a canine and incisors.
Is this an egg-laying monotreme (clade: Prototheria)? That’s what both Rauhut et al. 2002 and Martin and Rauhut 2005 thought based on tooth shape and a post-dentary groove in the medial dentary. They also excluded taxa listed below (and shown in figure 1). Such bias is a too common fault in traditional paleontology, as long time readers are well aware.
Based primarily on tooth morphology,
Rauhut et al. 2002 considered Asfaltomylos a member of the Australosphenida, a clade of southern Jurassic mammals that is said to convergently evolve tribosphenic molars with northern mammals and probably gave rise to monotremes. Their taxon restricted cladogram nested Asfaltomylos between Shuotherium (Fig. 2) and several untested taxa leading to several platypus-like taxa (including genus: Ornithorhynchus; Fig. 3.)
Question for you, dear readers:
Do the mandibles of Asfaltomylos (Fig. 1) and Shuotherium (Fig. 2) resemble one another? They should, given their proximity in the Rauhut et al. and Martin and Rauhut cladograms. If you think they don’t look similar, perhaps we need to expand the taxon list.
As a test, let’s add all the mammals in the LRT.
When we do, and based on very few mandible characters, Asfaltomylos foregoes the Prototheria and nests with derived members of Glires, derived from moonrats, the only members of Glires that sometimes do not have large gnawing incisors (yet another reversal).
Only the posterior molar
in the hedgehog, Erinaceus (Fig. 1), looks like the two molars in Asfaltomylos, separated in time by 166 million years. The premolar is nearly identical.
(Fig. 4) have an appropriately primitive appearance, and are different from other members of Glires in being chiefly carnivorous.
Rougier et al. 2007
considered Henosferus another member of the clade ‘Australosphenida’. With its complete dental formula on a low profile mandible, Henosferous (Fig. x) nests with other basalmost therians, like Morganucodon (Fig. 3) in the LRT, not close to Asfaltomylos. So members of the invalidated clade ‘Australosphenida’ are polyphyletic in the LRT.
Phylogenetic miniaturization and neotony
answer the problems posed by the low number of molars and the retention of the postdentary trough in Asfaltomylos. As you may recall, mammals recapitulate their phylogeny during ontogeny and Asfaltomylos matured at an earlier stage of development due to its small size.
Tooth morphology is something else to be ware of in phylogenetic analyses.
As an example, whale teeth devolved from multi-cusped in a square in their four-limbed terrestrial ancestors, to multi-cusped in a row in archaeocetes with flukes, to simple cones and toothlessness in derived odontocetes.
The problem is,
the high coronoid process and retroarticular (angular) process of Asfaltomylos are not found in Ornithorhynchus (Fig. 3) nor in other Prototheres in the large reptile tree (LRT, 1631+ taxa, Fig. 2). Prototheria are notable for their long rostra, lots of teeth and low coronoid process, traits that don’t match the Asfaltomylos mandible. The medial surface of Asfaltomylos does include a dentary trough in which tiny posterior jaws bones would soon evolve to become ear bones… except that happens by convergence in highly derived arboreal mammals, like multituberculates, that experience that reversal in the auditory region, to the chagrin of Jurassic mammal workers worldwide.
Here’s a thought:
Take a look at that tall, narrow, posterior premolar in Asfaltomylos. That’s what turns into a similar posterior premolar in moonrats and hedgehogs. That’s what turns into a large cutting premolar in Carpolestes and multituberculates.
Once again, the LRT shows why it is so important
to test all enigma taxa against a wide gamut of taxa, like the LRT. The LRT minimizes bias in the choice of the inclusion set of taxa. The number of characters for the mandible in the LRT comes down to less than dozen. Tooth cusp characters are largely omitted. So character count is, once again, shown to be not nearly as important, contra the opinions of workers who ask for more characters to no advantage.
Martin T and Rauhut OWM 2005. Mandible and dentition of Asfaltomylos patagonicus (Australosphenida, Mammalia) and the evolution of tribosphenic teeth. Journal of Vertebrate Paleontology 25(2):414–425.
Rauhut OWM, Martin T Ortiz-Jaureguizar E and Puerta P 2002. A Jurassic mammal from South America. Nature 416:165–168.
Rougier, GW, Martinelli AG, Forasiepi AM and Novacek M J 2007. New Jurassic mammals from Patagonia, Argentina : a reappraisal of australosphenidan morphology and interrelationships. American Museum novitates, no. 3566. online here.