THE RISE OF TRIBOSPHENIC MAMMALS DURING THE EARLY TO LATE CRETACEOUS TRANSITION IN NORTH AMERICA

We document two new tribosphenic mammals from the upper Cedar Mountain Formation (Albian–Cenomanian) of Utah. Addition of these new taxa completes the mammalian assemblage from the Mussentuchit local fauna as based on the current fossil record, and permits, for the first time, direct comparison to earlier and later mammalian faunas. One prominent characteristic of the Mussentuchit assemblage is that it includes diverse and abundant members of both tribosphenic and non-tribosphenic clades; in this sense it is broadly transitional between Late Jurassic and Late Cretaceous faunas of North America. Intriguingly, Tribosphenida are dominated by basal marsupialiforms; conspicuously missing are eutherians and deltatheroidans.

Tribosphenic mammals of earlier (Albian) North American faunas are generally small, with molars dominated by shearing function. By contrast, the younger upper Cedar Mountain Formation includes several larger tribosphenidans whose molar pattern shows more emphasis on crushing, suggesting a radiation of tribosphenic mammals into unexplored ecological roles in North America. The abundance and diversity of symmetrodonts and triconodonts in the Cedar Mountain Formation shows that these archaic mammals were not competitively replaced by tribosphenidans; their abundance and diversity might reflect local radiations, as neither symmetrodonts nor triconodonts are as well represented in Albian assemblages. On the other hand, observed differences between these faunas may reflect differential sampling of the fossil record.

Younger (Cenomanian–Maastrichtian) North American assemblages show considerable diversification of tribosphenidans, with re-appearance of Eutheria and Deltatheroida suggesting faunal immigration from Asia. Correspondingly, archaic groups rapidly dwindle in diversity and abundance, eventually going extinct by the beginning of the Campanian. Extinction of symmetrodonts and triconodonts represents loss of animalivorous taxa with molar function limited to shearing. The concomitant diversification of tribosphenic mammals, with molars capable of both grinding and shearing function, suggests passive replacement of archaic clades having shearing-only molars, by tribosphenidans with more versatile molar function (and presumably a more varied diet).