Long before our modern mammal fauna of placentals and marsupials, the world saw a large variety of other mammal groups. I’ve talked about them quite a fewtimes already, these familiar yet strange furballs combining what one expects of a mammal with venomous spurs, undeveloped young and a variety of other features both “archaic” and modern.
For the longest time (as in, the vast majority of the Mesozoic), mammals such as eutriconodonts and symmetrodonts were a success story. They crop up in most fossil sites in Europe, Asia, North America and Africa (and, in the case of Argentoconodon and Condorodon, South America) from the early Jurassic to the mid-Cretaceous, and occupied a variety of ecological niches from miniscule insectivores to piscivores to wolverine-sized predators. Both lineages in particular are adapted towards carnivory, standing out from the more omnivorous and herbivorous contemporary mammal groups such as multituberculates, dryolestoidsand early therians.
However, around the Cenomanian the fossil record for both lineages thins out. While Africa’s mammal fossil record is sparse enough to allow room for ambiguity, they are clearly gone from Eurasia and South America, and rather suddenly as well. It was once thought that the Late Cretaceous Austrotriconodon was a late living eutriconodont, but it turned out to have been a mesungulatid dryolestoid.
In Asia, their absence was quickly met by the evolution of therian mammals specialised to carnivory, such as deltatheroideans. In particular, some researchersdo openly speculate that deltatheroideans moved into niches left vacant by large eutriconodonts. Meanwhile, in southern continents, the diversity of carnivorous notosuchians could have something to do with this, since it does conspicuously increase in the Late Cretaceous.
Eutriconodonts and symmetrodonts, however were not entirely done for. In North America, a small variety of species kept on living. These include the formidable Jugulator amplissimus and the possibly aquatic Astroconodon, both Cenomanian forms that echo the diversity that ocurred elsewhere. Both eutriconodonts and symmetrodonts are important components of these mid-Cretaceous mammal faunas, suggesting that whatever caused their extinction elsewhere took less of a toll here.
Both groups endure further on, into the Campanian. Here, however, its clear that they suffered declines, there beng only the eutriconodont Alticonodon and the symmetrodont Symmetrodontoides, in a mammalian fauna otherwise dominated by therians and multituberculates. Both occur in a variety of fossil sites in the early stages of this epoch, before disappearing altogether.
Alticonodon in particular is interesting in that it shows progressing speciation in its molars for shearing. In all eutriconodonts, a shearing mechanism is infered for their unusual molars, one of the several features that peg these animals as carnivores; mammals with carnissals may be the closest functional analogue. The degree displayed in Alticonocon, however, suggests that it had a particularly specialised diet: maybe it was a beetle specialist, where the increased sheathering efficiency allowed to masticate their exoskeletons, or perhaps it was a more dedicated scanvenger, allowing it to scrap meat from bones more efficiently.
This degree of speciation is usually seen in clades suffering from ecological displacement, in which the surviving members have been pushed into more “extreme” niches while other groups have taken the more generalist niches. While eutriconodont decline came before the expansion of therian mammals, the presence of a diverse metatherian ane eutherian as well as multituberculate fauna in the same sites as Alticonodon leaves it clear that the last eutriconodonts and symmetrodonts were either outright outcompeted by these other mammals, or only these highly specialised forms survived by this point, allowing other mammals, including our distant ancestors, to thrive.
By the late Campanian, there are no unambiguous eutriconodonts, so the clade can be confidently claimed as extinct by the KT event. However, it has been suggested that Chronoperates, a weird mammal once considered to be a surviving non-mammalian synapsid, might actually be a symmetrodont. Ocurring in the North American Paleocene, if this mammal is a symmetrodont then it would be exceptional as the very last non-therian, non-multituberculate mammal in the northern hemisphere.
