Didelphodon: Difference between revisions

{{short description|Genus of extinct opossummetatherian}}

[[File:Didelphodon skull restoration.jpg|left|thumb|Restoration of the skull of ''D. vorax.'']]

Although perhaps little larger than a [[Virginia opossum]], with a maximum skull length of {{convert|12.212|cm}} and a weight of {{convert|5.2|kg}},<ref name="wilson2016"/> ''Didelphodon'' was the largest [[Cretaceous]] mammal.{{Harvnb|Rose|2006|p=78}} The teeth have specialized bladelike cusps and carnassial notches, indicating that the animal was a predator; the jaws are short and massive and bear enormous, bulbous premolar teeth which appear to have been used for crushing.<ref name="KJCCL"/> Analyses of a near-complete skull referred to ''Didelphodon'' show that it had an unusually high bite force quotient (i.e. bite force relative to body size) among Mesozoic mammals, suggesting a [[durophagy|durophagous]] diet. However, its skull lacks the vaulted forehead of hyenas and other specialized bone-eating durophagous mammals, indicating that its diet was perhaps a mixture of hard foodstuffs (e.g. snails, bones) alongside small vertebrates and carrion;<ref name="wilson2016">{{cite journal | title = A large carnivorous mammal from the Late Cretaceous and the North American origin of marsupials | journal = Nature Communications | volume = 7 | pages = 13734 | date = 2016 | first1 = G.P. | last1 = Wilson | first2 = E.G. | last2 = Ekdale | first3 = J.W. | last3 = Hoganson | first4 = J.J. | last4 = Calede | first5 = A.V.| last5 = Linden | doi = 10.1038/ncomms13734 | pmid = 27929063 | pmc = 5155139 | bibcode = 2016NatCo...713734W }}</ref> although omnivorous habits were suggested in the past, it appears that it was incapable of processing plant matter, rendering it more likely to be hypercarnivorous or durophagous.<ref>{{Cite journal|author1=Leonardo M. Carneiro |author2=Édison Vicente de Oliveira |year=2017 |title=Systematic affinities of the extinct metatherian ''Eobrasilia coutoi'' Simpson, 1947, a South American Early Eocene Stagodontidae: implications for "Eobrasiliinae" |journal=Revista Brasileira de Paleontologia |volume=20 |issue=3 |pages=355–372 |doi=10.4072/rbp.2017.3.07 |doi-access=free }}</ref> Some convergence with the [[carnassial]]s of other predatory mammal groups has also been noted.<ref>{{cite journal|first1=C.|last1= de Muizon|first2= B.|last2= Lange-Badre |title =Carnivorous dental adaptations in tribosphenic mammals and phylogenetic reconstruction |date=1997 |doi =10.1111/j.1502-3931.1997.tb00481.x| journal = Lethaia | volume = 30 | issue = 4 | pages=353–366}}</ref>

''Didelphodon'' is a [[stagodontidae|stagodontid]] marsupial related to ''[[Eodelphis]]'' and ''[[Pariadens]]''. The genus appears to descend from the Campanian ''[[Eodelphis]]'', and in particular appears to be related to ''Eodelphis cutleri''. ''Pariadens'' appears to be more primitive than either ''Eodelphis'' or ''Didelphodon'', and is probably sister to their group. [[Didelphimorphia]] is an [[order (biology)|order]] that was named in 1872 by Gill. Previously, in 1821, Gray named the superfamily [[Didelphoidea]] to house the families [[Alphadontidae]], [[Pediomyidae]], [[Peradectidae]], and [[Stagodontidae]], which unites ''Didelphodon'' with many other genera.<ref name="KJCCL"/>[[File:The Childrens Museum of Indianapolis - Didelphodon mandible.jpg|thumb|Cast of the first ''Didelphodon'' mandible (the holotype of the genus), assigned to ''D. vorax'', to be discovered still containing teeth, now located in the permanent collection of [[The Children’sChildren's Museum of Indianapolis]]|left]] In 2006, a study found that the stagodontids only contained two taxa, ''Didelphodon'' and ''Eodelphis''. The previously-included ''Pariadens'' was excluded from the group because its type species, ''P. kirklandi'', lacks any of the clade's characteristics; it was reassigned to [[Marsupial]]ia ''incertae sedis''. Another species, "P." ''mckennai'' lacks marsupial features, and is probably a [[theria]]n. Another historical stagodontid, ''[[Boreodon]]'', is a ''[[nomen dubium]]''. Finally, the purported stagodontid ''[[Delphodon]]'' is probably a synonym of ''[[Pediomys]]'' or ''[[Alphadon]]''.<ref name="FN2006"/>[[File:Didelphodon.jpg|thumb|Premolar or molar of ''Didelphodon''. Like most [[Mammal|mammals]], and unlike their contemporaries, the [[Dinosaur|dinosaursdinosaur]]s, ''Didelphodon'' had very advanced dentition.|left]]

Although it has been argued on the basis of the shape of referred [[tarsal bones]] that ''Didelphodon'' and other stagodontids were semiaquatic due to having flexible feet, these traits may in fact be evidence of increased rigidity in the foot.<ref name="FN2006">{{cite journal|last1=Fox|first1=R.C.|last2=Naylor|first2=B.G.|year=2006|title=Stagodontid marsupials from the Late Cretaceous of Canada and their systematic and functional implications|url=https://www.app.pan.pl/archive/published/app51/app51-013.pdf|journal=[[Acta Palaeontologica Polonica]]|volume=51|issue=6|pages=13–36}}</ref><ref name="KJCCL">{{cite book|last1=Kielan-Jaworowska|first1=Z.|last2=Cifelli|first2=R.L.|last3=Cifelli|first3=R|last4=Luo|first4=Z.X.|year=2004|title=Mammals from the Age of Dinosaurs: Origins, Evolution and Structure|location=New York|publisher=Columbia University Press|url=https://books.google.com/books?id=WgO3KyvMXUAC|pages=441–462|isbn=9780231509275}}</ref> Nevertheless, a recently-found and as-of-yet undescribed specimen, located just {{convert|40|m|ft|abbr=on}} away from a ''[[Triceratops]]'' in a riverbed, suggests that ''Didelphodon'' may have possessed an [[otter]]-like body with a [[tasmanian devil]]-like skull. A study that is being prepared by [[Kraig Derstler]], [[Greg G. Wilson|Greg Wilson]], [[Robert Bakker]], [[Ray Vodden]] and [[Mike Triebold]] will describe this new specimen, housed in the Rocky Mountain Dinosaur Resource Center.<ref name="RMDRC"/> A study on Mesozoic mammal locomotion demonstrates that ''Didelphodon'' groups with semi-aquatic species.<ref>Meng Chen, Gregory Philip Wilson, A multivariate approach to infer locomotor modes in Mesozoic mammals, Article in Paleobiology 41(02) · February 2015 {{DOIdoi|10.1017/pab.2014.14}}</ref>

The evolution of ''Didelphodon'' and other large stagodontids (as well as large [[deltatheroideandeltatheroidan]]s like ''[[Nanocuris]]'') occurs after the local extinction of [[eutriconodont]] mammals, suggesting passive or direct ecological replacement.<ref>G. W. Rougier, B. M. Davis, and M. J. Novacek. 2015. A deltatheroidan mammal from the Upper Cretaceous Baynshiree Formation, eastern Mongolia. Cretaceous Research 52:167-177</ref> Given that all insectivorous and carnivorous mammal groups suffered heavy losses during the mid-Cretaceous, it seems likely these metatherians simply occupied niches left after the extinction of most eutriconodonts.<ref>David M. Grossnickle, P. David Polly, Mammal disparity decreases during the Cretaceous angiosperm radiation, Published 2 October 2013.{{DOIdoi|10.1098/rspb.2013.2110}}</ref>