New cladotherian mammal from southern Chile and the evolution of mesungulatid meridiolestidans at the dusk of the Mesozoic era

Abstract

In the last decades, several discoveries have uncovered the complexity of mammalian evolution during the Mesozoic Era, including important Gondwanan lineages: the australosphenidans, gondwanatherians, and meridiolestidans (Dryolestoidea). Most often, their presence and diversity is documented by isolated teeth and jaws. Here, we describe a new meridiolestidan mammal, Orretherium tzen gen. et sp. nov., from the Late Cretaceous of southern Chile, based on a partial jaw with five cheek teeth in locis and an isolated upper premolar. Phylogenetic analysis places Orretherium as the earliest divergence within Mesungulatidae, before other forms such as the Late Cretaceous Mesungulatum and Coloniatherium, and the early Paleocene Peligrotherium. The in loco tooth sequence (last two premolars and three molars) is the first recovered for a Cretaceous taxon in this family and suggests that reconstructed tooth sequences for other Mesozoic mesungulatids may include more than one species. Tooth eruption and replacement show that molar eruption in mesungulatids is heterochronically delayed with regard to basal dryolestoids, with therian-like simultaneous eruption of the last premolar and last molar. Meridiolestidans seem endemic to Patagonia, but given their diversity and abundance, and the similarity of vertebrate faunas in other regions of Gondwana, they may yet be discovered in other continents.

An African Radiation of ‘Dryolestoidea’ (Donodontidae, Cladotheria) and its Significance for Mammalian Evolution

“…The triconodont dentition of this amphilestid with three mesiodistally aligned cusps, is probably the plesiomorphic condition for the clade including 'Dryolestoidea' and Theria (Mills 1971;Kielan-Jaworowska and Dashzeveg 1998;Rougier et al 2007;Averianov and Lopatin 2011). It was preferred to Probainognathus Romer, 1970, which was used as outgroup in the analyses of , Chimento et al (2012), O'Meara andThompson (2014), and Martinelli et al (2021), because we consider Probainognathus to be too distant from the group of interest, which might cause problems of homology and of character applicability.…”

“…The 'prototribosphenic' molar of 'dryolestoids' marked a critical step in the evolution of the tribosphenic molar, with the appearance of talonid shelf in stem-cladotherians such as 'dryolestoids', which acted as an extending shearing surface and likely assisted in medial movement during occlusion (Grossnickle 2017;Martin et al 2020). Previous studies have demonstrated considerable phylogenetic uncertainty for 'dryolestoids', which are recovered either paraphyletic O'Meara and Thompson 2014; or monophyletic Chimento et al 2012;Martinelli et al 2021). 'Dryolestoids' are the most abundant mammals from the KM site, representing more than 34% of the discovered mammaliaform specimens (Fig.…”

“…This diverse grouping of mostly Jurassic and Cretaceous stem cladotherians includes dryolestids, meridiolestidans, 'paurodontids' and donodontids. Like in other cladotherians, the dentition of 'dryolestoids' consists of a series of reversed triangles, with the upper molars wider than the lowers (Martin 1999;Luo et al 2002;Kielan-Jaworowska et al 2004), and they constitute a radiation of pre-tribosphenic mammals (Martinelli et al 2021). Their lower jaw shows a distinct angular process and sophisticated musculature, allowing a complex mastication with a major transverse component in the chewing cycle (Crompton 1971;Prothero 1981;Davis 2011;Schultz and Martin 2014).…”

“…The character matrix used in the phylogenetic analyses is mainly based on a compilation of the dental and mandibular characters extracted from the matrices of and , with subsequent modifications by , and , to which we added eight new characters. These matrices were also used in other recent works (Chimento et al 2012;O'Meara and Thompson 2014;Harper et al 2019;Martinelli et al 2021). Moreover, they include a large number of 'Dryolestoidea', and especially of Meridiolestida, which are thought to be related to the KM donodontids.…”

“…As previously stated, the taxonomic composition of 'Dryolestoidea' varies depending on the studies, and there is considerable uncertainty regarding their phylogeny. They are most often recovered as paraphyletic O'Meara and Thompson 2014;), but they are still found as monophyletic by some authors (Chimento et al 2012;Martinelli et al 2021). Moreover, even if the genus Donodon is referred to 'dryolestoids' since its discovery in 1991, it has never been included in a phylogenetic analysis.…”