Taxonomic and phylogenetic review of Necrosuchus ionensis (Alligatoroidea: Caimaninae) and the early evolution and radiation of caimanines

Abstract

Alligatoroidea is the most species-rich crocodylomorph clade of the Cenozoic of South America, with nearly all species belonging to the Caimaninae clade. However, the earliest records of Caimaninae in South America, which are from the Palaeocene, are based mostly on incomplete specimens, which increases the importance of detailed taxonomic and phylogenetic studies on these taxa. This paper offers a taxonomic and phylogenetic review of Necrosuchus ionensis, a caimanine species from the Salamanca Formation of the Palaeocene of Argentina. Necrosuchus ionensis is considered a valid species, albeit with a different diagnosis from that proposed by previous authors. The phylogenetic analysis shows, for the first time, that N. ionensis belongs to the derived Caimaninae clade Jacarea. However, a better understanding of the Jacarea clade is needed, and alternative placements for N. ionensis might be considered. Nevertheless, the placement of N. ionensis as a derived caimanine raises interesting perspectives on the early evolution and radiation of caimanines, which are thoroughly discussed in this paper together with other results obtained in this study, such as the recovery of the North American caimanines Bottosaurus and Tsoabichi as a clade.

Redescription and phylogenetic affinities of the caimanine Eocaiman cavernensis (Crocodylia, Alligatoroidea) from the Eocene of Argentina

“…In terms of taxon sampling, we included Protocaiman peligrensis and excluded UCMP 39978 from the matrix of Cidade et al . (2020a). The scorings of Protocaiman peligrensis , a putative caimanine from the Paleocene of Argentina, follow the morphological description in Bona et al .…”

A new caimanine alligatorid from the Middle Eocene of Southwest Texas and implications for spatial and temporal shifts in Paleogene crocodyliform diversity

“…Some recent analyses place Late Cretaceous and earliest Paleocene globidontans from North America, such as Bottosaurus Agassiz 1849, Stangerochampsa Wu et al 1996 and Brachychampsa Gilmore 1911, at the root of Caimaninae, but whether this reflects accurate phylogenetic relationships or a broader phylogenetic distribution of characters currently thought to diagnose Caimaninae among globidontans is unclear ( Cossette & Brochu, 2018 ; Bona et al, 2018 ; Cidade, Fortier & Hsiou, 2020 ). The earliest unambiguous caimanines are Necrosuchus ionensis Simpson, 1937 ( Brochu, 2011 ; Cidade, Fortier & Hsiou, 2020 ), Eocaiman palaeocenicus Bona, 2007 , and Protocaiman peligrensis Bona et al, 2018 . Notocaiman stromeri Rusconi 1937 is slightly younger.…”

“…Some recent analyses place Late Cretaceous and earliest Paleocene globidontans from North America, such as Bottosaurus Agassiz 1849, Stangerochampsa Wu et al 1996 and Brachychampsa Gilmore 1911, at the root of Caimaninae, but whether this reflects accurate phylogenetic relationships or a broader phylogenetic distribution of characters currently thought to diagnose Caimaninae among globidontans is unclear (Cossette & Brochu, 2018;Bona et al, 2018;Cidade, Fortier & Hsiou, 2020). The earliest unambiguous caimanines are Necrosuchus ionensis Simpson, 1937(Brochu, 2011Cidade, Fortier & Hsiou, 2020) In North America, the caimanine Tsoabichi greenriverensis is known from the Wasatchian of Wyoming (Brochu, 2010). Orthogenysuchus olseni Mook, 1924, also from the Wasatchian of Wyoming, was recovered as a caimanine related to Mourasuchus in some analyses (Brochu, 1999), but further work with the type specimen calls this interpretation into question (Salas-Gismondi et al, 2015, andC.…”

“…The only living members of Alligatoroidea are two species in Alligatorinae ( Alligator mississippiensis Daudin 1809 from southeastern North America and A. sinensis Fauvel 1879 from China) and six currently recognized Neotropical species (depending on the division of certain species complexes; Brochu, 1999 ; Godshalk, 2014 ; Escobedo-Galván et al, 2015 ; Uetz, Freed & Hošek, 2018 ; Muniz et al, 2018 ; Borges et al, 2018 ; Bittencourt et al, 2019 ; Roberto et al, 2020 ) within Caimaninae ( Paleosuchus Gray, 1862 , Melanosuchus Gray, 1862 , and Caiman Spix, 1825 ). The earliest known members of both Alligatorinae and Caimaninae occur in Paleocene deposits in North and South America, respectively ( Bona, 2007 ; Brochu, 2011 ; Pinheiro et al, 2013 ; Cossette & Brochu, 2018 ; Bona et al, 2018 ; Cidade, Fortier & Hsiou, 2020 ), but the known fossil record suggests a reversal of diversity fortune during the Cenozoic—disappearance of stem alligatorines was a primary driver in the observed drop in crocodyliform diversity after the middle Eocene, and there appears to be a sharp increase in caimanine diversity in the Neogene.…”

“…The known fossil record of Caimaninae is improving ( Bona, Riff & Gasparini, 2013 ; Cidade, Fortier & Hsiou, 2020 ), but our understanding of early caimanine history is limited. The earliest known South American caimanines are of Paleocene and Eocene age, and they are incompletely preserved ( Simpson, 1933 , 1937 ; Bona, 2007 ; Brochu, 2011 ; Pinheiro et al, 2013 ; Bona et al, 2018 ).…”