2 PAPERS IN PALAEONTOLOGY 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 3 A B C D E F 4 PAPERS IN PALAEONTOLOGY FIG. 1. Holotype of Eocaiman cavernensis (AMNH FARB 3158). A, dorsal view of the skull. B, ventral view of the skull. C, left lateral view of the skull. D, dorsal view of the mandible. E, ventral view of the mandible. F, left lateral view of the mandible. Scale bar repre- sents 2 cm. Photographs by Mick Ellison. A B Ma W E 15 Santa Cruz Chenque 20 Colhué-Huapi Superpatagonian Miocene Leonian San 25 Upper Julian Puesto Almendra 30 ARGENTINA Oligocene Vera 35 Sarmiento Lower P. Almendra Rosado 40 Gran Barranca 45 Koluel-Kaike Senguer River Eocene ver Lake Lake Colhué-Huapi e 3 50 ? Chico Ri ut Musters Ro 45º 30' Las Flores 55 Sarmiento Comodoro ? Rio Chico Group Rivadavia Peñas Coloradas Gran Barranca Route 26 60 Chubut 46º S Gulf of Santa Cruz San Jorge Salamanca 70º Paleocene 68º 65 FIG. 2. A, maps depicting the location of the Gran Barranca site (red arrow) in South America and Argentina, where the fossil remains of Eocaiman cavernensis were collected during the first Scarritt Patagonian Expedition (1930–1931), led by George G. Simpson. B, schematic stratigraphy of the Cenozoic of central Patagonia, illustrating the middle Eocene age of the Gran Barrancan Member of the Salamanca Formation. Modified from Bellosi (2010). Beds, but still included it as part of the Gran Barrancan date the Gran Barrancan Member. They proposed that Member. the Barrancan age would range from 41.6 to 39.0 Ma, The precise age of these rocks has been controver- which corresponds to a Lutetian–Bartonian (middle sial. Simpson (1933a) first suggested an Eocene age for Eocene) age according to the International Commission the Notostylops Beds. Nevertheless, without a precise on Stratigraphy (Cohen et al. 2013; v2020/01). A Bar- date for the entire mammalian fauna (which represents tonian age was specifically obtained for the Simpson’s most of the fossils found in these rocks), it has been Y Tuff (= Notostylops Beds), with a mean age of referred to the Casamayoran South American Land 39.85 Ma estimated for its rocks. Mammal Age (SALMA), one of three SALMAs recog- nized in the Eocene of South America (Kay et al. 1999). Cifelli (1985) then divided the so-called MATERIAL AND METHOD Casamayoran age into two Eocene subages, Barrancan and Vacan, including the Notostylops Beds in the for- The genus Eocaiman mer (which is older than the Vacan). More recently, Re et al. (2010) used physical stratigraphy, 40Ar/39Ar The presence of E. cavernensis in nearly all morphology- dating analysis, and magnetic polarity stratigraphy to based phylogenetic studies of Caimaninae illustrates its GODOY ET AL.: REDESCRIPTION OF EOCAIMAN CAVERNENSIS 5 historical and phylogenetic significance. It represents the (Godoy et al. 2020). The scan resulted in 3515 slices, with type species of the genus Eocaiman and was described by a resolution of 1000 pixels, voxel size of 0.05517678 mm, Simpson (1933a) based on a single specimen (AMNH magnification ratio of 3.62471325, voltage of 180 kV, and FARB 3158), consisting of a partial skull and almost com- current of 200 lA. Initial visualization of slices, as well as plete lower jaws (Fig. 1). More recently, two other species virtual 3D reconstruction and rendering were performed were described for the genus. Bona (2007) erected using Volume Graphics VGStudio Max version 2.2, avail- E. palaeocenicus based on 13 specimens (MPEF-PV 1933 able at the Microscopy and Imaging Facility (AMNH). (holotype), 1935, 1936, MLP 90-II-12-117, 90-II-12-124, Segmentation of bones and selected structures was 93-XII-10-11, 93-XII-10-13, 95-XII-10-20, 95-XII-10-27, achieved with VGStudio Max version 3.0, available at the MACN-PV CH 1914, 1915, 1916 and 1627) from the vertebrate palaeontology laboratory of Dr Alan Turner, at Salamanca Formation (early Paleocene) of Argentina, all Stony Brook University. of which correspond to mandibular elements or teeth (see also Gasparini 1981, 1996). A third species, E. itaboraien- sis, was proposed by Pinheiro et al. (2013) based on four Phylogenetic analysis specimens (three partial left dentaries and one tooth; MCT 1791-R (holotype); MCT 1792-R; MCT 1793-R; The morphological data matrix used for the phylogenetic MCT 1794-R), from the Itaboraı Basin (late Paleocene) of analysis performed herein is a modified version of the data- Brazil. Therefore, E. cavernensis is stratigraphically the set presented by Cidade et al. (2020a), which is the most most recent of the three Eocaiman species, as well as the recent of a series of data matrices originating from the work only one with associated cranial material. of Brochu (1999, 2011). In terms of taxon sampling, we In addition, other specimens have tentatively been included Protocaiman peligrensis and excluded UCMP referred to the genus Eocaiman.