Mimoperadectes

Beck, Robin M. D., Voss, Robert S. & Jansa, Sharon A., 2022, Craniodental Morphology And Phylogeny Of Marsupials, Bulletin of the American Museum of Natural History 2022 (457), pp. 1-353 : 313-316

† Mimoperadectes

SPECIES SCORED: † Mimoperadectes labrus (type species), † M. houdei.

GEOLOGICAL PROVENANCE OF SCORED SPECIMENS: UM Locality SC-69, lower part of Willwood Formation, Park County, Wyoming († Mimoperadectes labrus); UM Locality SC-133, lower part of Willwood Formation, Park County, Clark Fork Basin, Wyoming († M. houdei).

AGE OF SCORED SPECIMENS: UM Locality SC-69 is within Zone Wa-0 of the Wasatchian North American Land Mammal Age (Gingerich, 2001: fig. 11), whereas UM Locality SC-133 is within Zone Wa-2 (Bowen and Bloch, 2002: fig. 2, table 2 ); the maximum bound of Wa-0 is approximately 55.8 Mya, and the minimum bound of Wa-2 is approximately 55.1 Mya (Chew and Oheim, 2013: fig. 2).

ASSIGNED AGE RANGE: 55.800 –55.100 Mya.

REMARKS: Bown and Rose (1979) described † Mimoperadectes labrus based on incomplete, predominantly dental, specimens from the earliest Eocene of the Willwood Formation, and referred it to the family Didelphidae . Horovitz et al. (2009) described a broken but nearly complete cranium of † Mimoperadectes (USNM 482355) from a slightly younger site in the Willwood Formation, referred it to a new species, † M. houdei, and referred † Mimoperadectes to the family † Peradectidae . In the same year, Beard and Dawson (2009) described a third species, † M. sowasheenis, from the early Wasatchian Red Hot Local Fauna, Uppermost Tuscahoma Formation, Lauderdale County, Mississippi. Subsequently, Rose et al. (2012) described additional specimens (including an isolated astragalus) of the type species, † M. labrus, from the Wa-0 Sand Creek Divide Fauna in the Willwood Formation, and argued that the validity of † M. houdei as a distinct species should be reassessed (Rose et al., 2012: 22). Of this material, only the specimens described by Bown and Rose (1979) and Horovitz et al. (2009) have been used for scoring purposes here.

Horovitz et al. (2009) presented a morphological phylogenetic analysis that included a terminal combining craniodental character scores for † Mimoperadectes labrus and † M. houdei with dental character scores from † Peradectes spp. and three postcranial character scores from “a Peradectes -like species from the Middle Eocene of Messel, Germany, which may belong to this same genus or alternatively may be closely related to it” (Horovitz et al., 2009: supporting information). In their analysis, the “ Mimoperadectes - Peradectes ” terminal was recovered as sister to a clade comprising the living didelphids Monodelphis and Didelphis , suggesting that it was a crown-clade marsupial and a member of the order Didelphimorphia . The morphological analysis of Wilson et al. (2016) also included a “ Mimoperadectes -Peradectes ” terminal, which again fell inside Marsupialia , as sister to † Herpetotherium , with this clade sister to Didelphidae + Dasyuridae , and Dromiciops outside this. However, the relationships among extant marsupials found by Wilson et al. (2016) are in clear conflict with molecular, total-evidence, and most morphological analyses (e.g., Phillips et al., 2001; Amrine-Madsen et al., 2003b; Horovitz and Sánchez-Villagra, 2003; Asher et al., 2004; Nilsson et al., 2004, 2010; Phillips et al., 2006; Sánchez-Villagra et al., 2007; Beck, 2008, 2012; Beck et al., 2008, 2014, 2016; Horovitz et al., 2008, 2009; Meredith et al., 2008, 2009c, 2011; Mitchell et al., 2014; Gallus et al., 2015a; May-Collado et al., 2015; Lorente et al., 2016; Carneiro and Oliveira, 2017b; Maga and Beck, 2017; Carneiro et al., 2018; Duchêne et al., 2018; Abello and Candela, 2019; Carneiro, 2019; Zimicz and Goin, 2020; Álvarez-Carretero et al., 2021) in failing to support monophyly of Australidelphia.

An implicit assumption of the analyses of Horovitz et al. (2009) and Wilson et al. (2016) is that the taxa they used to score their “ Mimoperadectes -Peradectes ” terminal (i.e., † Mimoperadectes labrus, † M. houdei, † Peradectes spp., and the “ Peradectes -like species” from Messel) form a clade to the exclusion of their other terminals. However, evidence in support of this assumption is not particularly strong. The phylogenetic analyses of Williamson et al. (2012; 2014) recovered † Mimoperadectes labrus and † M. houdei as sister taxa in a clade that also included † Peradectes chesteri and † P. protinnominatus (as well as † Armintodelphys blacki and † A. dawsoni), which supports a close relationship between † Mimoperadectes and these particular † Peradectes species. However, other † Peradectes species included by Williamson et al. (2012, 2014)— namely † P. minor (the type species), † P. californicus, † P. coproxeches (= † Peradectes, sp. nov., in Williamson et al., 2012; see Williamson and Taylor, 2011), † P. elegans , and † P. gulottai —were not resolved as members of this clade, but instead formed part of a polytomy (within which † P. elegans and † P. californicus were sister taxa) that also included the † Mimoperadectes + † Armintodelphys + † P. chesteri + † P. protinnominatus clade discussed above, † Maastrichtidelphys, † Pucadelphys , † Szalinia, and †Pediomyidae. Thus, the analyses of Williamson et al. (2012; 2014) do not unambiguously support monophyly of † Peradectes nor monophyly of † Mimoperadectes + † Peradectes. Williamson et al. (2012; 2014) referred to the taxa forming this polytomy, but excluding †Pediomyidae, as “ Peradectidae sensu lato,” and Williamson et al. (2012: 629) specifically argued that future studies would place at least some of these taxa within a monophyletic † Peradectidae sensu stricto, which they defined as “the most inclusive clade containing Peradectes elegans , but not Herpetotherium fugax, Pediomys elegans or Didelphis virginiana ” (Williamson et al., 2012: 629).

Carneiro and Oliveira (2017b) and Carneiro (2018) recovered a clade comprising † Mimoperadectes , † Peradectes, and † Thylacodon (the last genus having been regarded as synonymous with † Peradectes by some authors; see Williamson et al., 2012), although they did not specify exactly which species they used to score these terminals. In two other papers, Carneiro et al. (2018) and Carneiro (2019) found a clade that they called † Peradectidae comprising † Mimoperadectes ; † Peradectes; † Thylacodon; the North American taxa † Golderdelphys, † Nanodelphys, and † Didelphidectes; † Siamoperadectes from Thailand; and an unnamed taxon from the Itaboraí Local Fauna of Brazil. Rangel et al. (2019), meanwhile, also found a † Mimoperadectes + † Peradectes clade (they did not include † Thylacodon or the other putative peradectids used by Carneiro et al. [2018] and Carneiro [2019]), but again it is unclear exactly which species were used for scoring purposes.

The “ Peradectes -like species” from Messel, meanwhile, has never had its relationship to definitive † Peradectes species formally tested, largely because the known specimens of the Messel taxon (although preserved as largely complete, articulated skeletons with some soft-tissue preservation) are heavily crushed, and details of the dentition that could be compared with known specimens of † Peradectes and similar taxa (most of which are known from dental material only) are not easily visible (Koenigswald and Storch, 1988; Kurz, 2001, 2005, 2007; Storch, 2001; Kurz and Habersetzer, 2004). Indeed, Horovitz et al. (2009: supplementary information) were able to score only three characters from the Messel specimens, all of which were from the postcranial skeleton. Kurz and Habersetzer (2004) used “continuous online recalibrated radiography” (CORR) to view the occlusal surfaces of the postcanine teeth of the Messel specimens, noting that the centrocrista of the molars is only slightly curved and that the paracone and metacone are similar in size (Kurz and Habersetzer, 2004: 18), features that have traditionally been used to distinguish peradectids from herpetotheriids, with the latter characterized has having a distinctly V-shaped centrocrista and a metacone that is markedly larger than the paracone (e.g., Crochet, 1979, 1980; Krishtalka and Stucky, 1983; Korth, 1994, 2008; Johanson, 1996b; Case et al., 2005; Hooker et al., 2008). However, the phylogenetic analysis of Williamson et al. (2012: 634) implies that a V-shaped centrocrista has evolved at least five times within Metatheria, so the presence of a relatively straight centrocrista (which is likely plesiomorphic for both Metatheria and Marsupialiformes) in the Messel specimens is not compelling evidence that they represent † Peradectes. Indeed, Morlo et al. (2004: footnote 79) remarked that “As the taxonomy is unclear, Storch (2001) and Kurz (unpublished data) regard the specimens as “ Peradectes,” in the sense of a primitive didelphimorphian from the Palaeocene of Europe”—we note here that “didelphimorphian” is presumably used by Morlo et al. (2004) to mean a dentally generalized marsupialiform, rather than a close relative of Didelphidae .

Williamson et al. (2012; 2014) did not include representatives of Didelphidae or any other extant marsupial family in the published versions of their analyses, so they did not test the relationship of † Mimoperadectes , † Peradectes, and other “peradectids” to Didelphidae , nor to crown-clade Marsupialia . Williamson et al. (2012: 631–632) reported that they had included extant didelphid Didelphis virginiana as an “experimental exercise,” but found that its inclusion led to a considerable loss of resolution in the resultant phylogeny. Beck (2012) used a modified version of Horovitz et al.’s (2009) matrix, but modified their “ Mimoperadectes -Peradectes ” terminal (which he renamed † Peradectidae ) by deleting the three postcranial character scores taken from the “ Peradectes -like species” from Messel and revising some character scores. In particular, Beck (2012: electronic supplementary material) scored his † Peradectidae terminal as lacking an alisphenoid tympanic wing, based on personal observations by R.S.V. of the holotype and only known specimen of † Mimoperadectes houdei (USNM 482355; see also Horovitz et al., 2009: fig. S3). Muizon et al. (2018: 402) maintained that a “small tympanic process is…likely to have been present in Mimoperadectes ,” but did not provide any supporting evidence. In fact, † M. houdei appears to entirely lack an ossified hypotympanic sinus floor (see char. 55). Beck’s (2012) maximum parsimony and Bayesian analyses of his revised morphological matrix using a molecular scaffold placed † Peradectidae outside Marsupialia (Beck, 2012: fig. 6), and subsequent morphological and total-evidence analyses that have used versions of his matrix have either found † Mimoperadectes /† Peradectidae to fall outside Marsupialia (Beck et al., 2014, 2016; Lorente et al., 2016; Maga and Beck, 2017; Abello and Candela, 2019; Zimicz and Goin, 2020) or have found this relationship to be unresolved (Beck, 2017b). Numerous recent studies that have used rather different morphological datasets from Beck’s (2012) have also found † Mimoperadectes or † Peradectidae outside Marsupialia (Forasiepi, 2009; Engelman and Croft, 2014; Forasiepi et al., 2014a; Suarez et al., 2015; Carneiro and Oliveira, 2017b; Carneiro, 2018; Carneiro et al., 2018; Muizon et al., 2018; Carneiro, 2019; Rangel et al., 2019; Engelman et al., 2020; Ladevèze et al., 2020; Muizon and Ladevèze, 2020).

In summary, the composition of † Peradectidae remains unsettled, but most phylogenetic analyses that have included † Mimoperadectes or a composite peradectid terminal that combines character scores from † Mimoperadectes and † Peradectes have found that this taxon falls outside Marsupialia (contra Horovitz et al., 2009; Wilson et al., 2016), The current study represents a further test of the relationship of † Mimoperadectes to the marsupial crown clade.