A controversial paper on the origin of Tribosphenic mammals has just been published in Alcheringa:

Timothy F. Flannery, Thomas H. Rich, Patricia Vickers-Rich, E. Grace Veatch and Kristofer M. Helgen. 2022. The Gondwanan Origin of Tribosphenida (Mammalia). Alcheringa: An Australian Journal of Palaeontology. https://doi.org/10.1080/03115518.2022.2132288

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The early studies on the origins of fossil therians (a sub-class of mammals including Eutheria and Metatheria) were based solely on Northern Hemisphere fossil mammal specimens. It wasn’t until 1997 that the first Mesozoic mammal with tribosphenic teeth (Ausktribosphenos nyktos) was reported from the Southern Hemisphere that things became interesting (Rich et al 1997). Rich’s paper was quickly disputed by various northern hemisphere researchers, arguing that A.nyktos was derived from a more primitive group than the therian mammals.

Quickly following on from the discovery of A.nyktos in 1997, additional new tribosphenic mammals were discovered at the Flat Rocks site, near Inverloch in Victoria, Australia, including Bishops whitmorei (Rich et al 2001) and Kryoparvis gerriti (Rich et al 2020).

Flannery’s paper documents the discussions between the various groups of researchers on the viability of relationships between the Southern Hemisphere tribosphenic mammals and their place in the mammalian evolutionary tree and it is during this time that more Mesozoic tribosphenic mammals from different parts of Gondwana also began to appear. Ambondro mahabo was discovered from the Jurassic of Madagaskar (Flynn et al 1999), Asfaltomylos patagonicus from South America (Rauhut et al 2002), Henosferus molus (Rougier et al 2007) and an unnamed taxon from India (Prasad et al 2015).

The oldest of these six taxa of tribosphenic fossil mammals from Gondwana predate the Laurasian (Northern Hemisphere) mammals by close to 50 million years.

The paper also discusses the evolutionary change that took place in the jaw structures of the Southern Hemisphere tribosphenic mammals between the Jurassic and Early Cretaceous, as most of the fossil specimens are mandibles.

Flannery touches on the eventual extinction of the tribosphenic mammals, particularly in Australia, and hypothesizes on the possibility that this group may not have gone extinct at the end of the Cretaceous. There is tantalizing fossil evidence from the Eocene of Queensland (Godhelp et al 1992, Flannery et al 2022) in the form of a single molar, Tingamarra porterorum, that resembles features found in the Mesozoic tribosphenic mammals.

No doubt this paper will raise plenty of discussion within the palaeo-mammal research community, but what is certain is that it has thrust the Flat Rocks site, near Inverloch, onto the world stage of mammalian evolution. Not only is it the home of three tribosphenic mammal taxa, but also where the oldest and smallest monotreme, Teinolophos trusleri (Rich et al 2001) and the only evidence of multituberculate mammals in Australia, Corriebaatar marywaltersae (Rich et al 2009) were found.