[Papers in Palaeontology, Vol. 7, Part 3, 2021, pp. 1205–1231] REDESCRIPTION AND PHYLOGENETIC AFFINITIES OF THE CAIMANINE EOCAIMAN CAVERNENSIS (CROCODYLIA, ALLIGATOROIDEA) FROM THE EOCENE OF ARGENTINA by PEDRO L. GODOY1 ,GIOVANNEM.CIDADE2,3 , FELIPE C. MONTEFELTRO4 ,MAXC.LANGER2 and MARK A. NORELL5 1Department of Anatomical Sciences, Stony Brook University, Stony Brook, NY USA; [email protected] 2Laboratorio de Paleontologia de Ribeir~ao Preto, FFCLRP, Universidade de S~ao Paulo, Ribeir~ao Preto, Brazil; [email protected], [email protected] 3Laboratorio de Estudos Paleobiologicos, Departamento de Biologia, Universidade Federal de S~ao Carlos, Sorocaba, Brazil 4Departamento de Biologia e Zootecnia, Universidade Estadual Paulista, FEIS, Ilha Solteira, Brazil; [email protected] 5Division of Paleontology, American Museum of Natural History, New York, NY USA; [email protected] Typescript received 24 April 2020; accepted in revised form 31 July 2020 Abstract: Caimaninae is one of the few crocodylian lin- caimanines using parsimony and Bayesian inference eages that still has living representatives. Today, most of its approaches. Our results provide evidence for a monophyletic six extant species are restricted to South and Central Amer- Eocaiman genus within Caimaninae, even though some ica. However, recent discoveries have revealed a more com- highly incomplete taxa (including the congeneric Eocaiman plex evolutionary history, with a fossil record richer than itaboraiensis) represent significant sources of phylogenetic previously thought and a possible North American origin. instability. We also found Culebrasuchus mesoamericanus as Among the oldest caimanines is Eocaiman cavernensis, from sister to all other caimanines and the North American glo- the Eocene of Patagonia, Argentina. It was described by bidontans (i.e. Brachychampsa and closer relatives) outside George G. Simpson in the 1930s, representing the first Caimaninae. A time-calibrated tree, obtained using a fos- caimanine reported for the Palaeogene. Since then, E. caver- silized birth–death model, shows a possible Campanian ori- nensis has been ubiquitous in phylogenetic studies on the gin for the group (76.97 Æ 6.7 Ma), which is older than the group, but a more detailed morphological description and age estimated using molecular data, and suggests that the revision of the taxon were lacking. Here, we present a earliest cladogenetic events of caimanines took place rapidly reassessment of E. cavernensis, based on first-hand examina- and across the K–Pg boundary. tion and micro-computed tomography of the holotype, and reinterpret different aspects of its morphology. We explore Key words: Caimaninae, morphology, CT scan, evolution, the phylogenetic affinities of E. cavernensis and other Bayesian inference, fossilized birth–death model. A T present, six extant species of Caimaninae are dis- Within Alligatoroidea, the sister group of Caimanine is tributed within the genera Caiman, Melanosuchus and Alligatorinae, which is currently represented by only two Paleosuchus. These are found mostly in South and Central extant species: A. mississippiensis of North America and America (the only exception is Caiman crocodilus, the dis- A. sinensis from China (Grigg & Kirshner 2015). Never- tribution of which extends as far north as southern Mex- theless, compared with Caimaninae, the fossil record of ico; Thorbjarnarson 1992; Brochu 1999, Grigg & Kirshner Alligatorinae has been historically regarded as much 2015). Phylogenetically, Caimaninae is defined as the richer (Brochu 2010), with a more widespread geographic group that includes C. crocodilus and all crocodylians clo- distribution and several species documented for the ser to it than to Alligator mississippiensis (Brochu 1999, Cenozoic (Brochu 1999, 2003; Whiting et al. 2016). 2003). It belongs to Alligatoroidea, one of the three main In the twenty-first century, however, new discoveries lineages of the crown-group Crocodylia, together with have revealed a higher diversity of Caimanine during the Crocodyloidea and Gavialoidea (Brochu 1999, 2003). Cenozoic, especially in South America, but also with © 2020 The Palaeontological Association doi: 10.1002/spp2.1339 1205 1206 PAPERS IN PALAEONTOLOGY, VOLUME 7 important findings from Central and North America HISTORICAL BACKGROUND AND (Aguilera et al. 2006; Bona 2007; Brochu 2010; Fortier & GEOLOGICAL SETTING Rincon 2013; Hastings et al. 2013; Pinheiro et al. 2013; Scheyer et al. 2013, 2019; Salas-Gismondi et al. 2015; The Scarritt Patagonian Expeditions and the discovery of Cidade et al. 2017; Bona et al. 2018; Souza-Filho et al. Eocaiman cavernensis 2019). These new studies and findings added complexity to both the phylogenetic and the biogeographical histories Between 1930 and 1933, two great expeditions to of caimanines. For instance, the discovery of a diverse southern Argentina, known as the Scarritt Patagonian Miocene caimanine fauna from Central America, as well Expeditions, were conducted by the American Museum as even more fossils from South America (Hastings et al. of Natural History (Madden & Scarano 2010). They 2013; Scheyer et al. 2013; Salas-Gismondi et al. 2015), were led by the eminent palaeontologist George G. revealed some species bearing a mosaic of features. Addi- Simpson and yielded several Cenozoic vertebrate fossils. tionally, the presence of North American taxa deeply Detailed records of this expedition can be found in nested within Caimaninae (Brochu 2010; Cossette & Bro- Simpson’s field notebooks (Simpson 1930a, b) and in chu 2018) has puzzled researchers about the group’s bio- his book Attending marvels: A Patagonian journal geographic history. (Simpson 1934a), in which he mentions dozens of fos- In this context, Eocaiman cavernensis (Fig. 1) has a cen- sils collected, mainly mammals. One of the most tral role in improving our understanding of caimanine important sites explored during the first expedition evolution. Described by the prominent palaeontologist (1930–1931) was the Gran Barranca (Fig. 2), in the George G. Simpson (Simpson 1933a) from Eocene rocks Chubut Province (Madden & Scarano 2010). This local- of Argentina, this was the first caimanine species reported ity provided some fossil reptiles, including the gigantic for the Palaeogene. Most early phylogenetic studies placed snake Madtsoia bai (Simpson 1933b) and the crocody- E. cavernensis as the sister taxon of all other caimanines lian E. cavernensis (Simpson 1933a). (Brochu 1999, 2010, 2011). But in the last decade, In a short descriptive paper, Simpson (1933a) classi- increased interest in the group was promoted by new fos- fied Eocaiman cavernensis as ‘a true crocodilid or alliga- sil discoveries, which led to updated phylogenetic torid’, closely related to ‘Caiman and Jacare’ (both of hypotheses for caimanines, with alternative positions for which are now assigned to the genus Caiman). This E. cavernensis (e.g. not necessarily in a sister taxon rela- was subsequently corroborated by several phylogenetic tionship with all other caimanines; Hastings et al. 2013, studies, which consistently included E. cavernensis 2016; Scheyer et al. 2013; Salas-Gismondi et al. 2015; within Caimaninae (Brochu 1999, 2010, 2011; Hastings Bona et al. 2018). Additionally, two other Eocaiman spe- et al. 2013, 2016; Scheyer et al. 2013; Pinheiro et al. cies were proposed, E. palaeocenicus and E. itaboraiensis 2013; Salas-Gismondi et al. 2015; Bona et al. 2018). (Bona 2007; Pinheiro et al. 2013), adding complexity to However, even though E. cavernensis has been continu- the biogeographical and phylogenetic histories of the ally present in numerous phylogenetic analyses since genus as well as to Caimaninae. Simpson’s work (1933a), no detailed descriptive work In this study, we provide a detailed redescription of was conducted for the specimen, which lacks a robust E. cavernensis, based on first-hand examination of the assessment of its morphology. type specimen, as well as on micro-computed tomogra- phy (lCT) data. We use the new information to provide a comprehensive morphological comparison with other Geology of the Gran Barranca caimanines and to rescore this taxon into a recent phylo- genetic data matrix. Phylogenetic analyses are performed Simpson used the term ‘Notostylops Beds’ to refer to the using both maximum parsimony and Bayesian inference, geological unit that yielded E. cavernensis, following the this latter approach applied for the first time in the con- prior designation of Florentino Ameghino (Ameghino text of Caimaninae (i.e. using only morphological charac- 1906; Simpson 1933a). In the subsequent years, different ters). We also combined the resultant topology of names were proposed for the different units of the Gran Caimaninae with information on taxon ages to obtain a Barranca (e.g. Simpson 1933c; Cifelli 1985), until Spalletti time-calibrated phylogeny of the group using the fos- & Mazzoni (1977, 1979) combined all of them into the silized birth–death (FBD) process, in a Bayesian frame- Gran Barrancan Member, which was proposed as the low- work. These results are then used for discussing est member of the Sarmiento Formation (Fig. 2). Finally, palaeoecological and palaeobiogeographical implications Re et al. (2010) proposed the name ‘Simpson’s Y Tuff’ for the group. for the specific rock layer originally known as Notostylops GODOY ET AL.: REDESCRIPTION OF EOCAIMAN CAVERNENSIS 1207 A B C D E F 1208 PAPERS IN PALAEONTOLOGY, VOLUME 7 FIG. 1. Holotype of Eocaiman cavernensis (AMNH FARB