2. Geological setting

The Neogene fossil record in Argentina is principally from the Ituzaingó Formation (De Alba Reference De Alba1953; Herbst Reference Herbst1971), in a level informally called “Mesopotamiense” or “Conglomerado osífero” (sensu Frenguelli, Reference Frenguelli1920; see Cione et al. Reference Cione, Casciotta, Azpelicueta, Barla and Cozzuol2005). This unit emerges from Ituzaingó (Corrientes province) to Paraná (Entre Ríos province) (Fig. 1) and over the margins of Paraná River, from the vicinity of Paraná further north. The most explored localities include are La Celina [S 31° 37′ 37″, W 60° 20′ 04″], Toma Vieja [S 31° 38′ 42·5″, W 60° 28′06″] and Villa Urquiza [S 31° 38′ 42·5″, W 60° 22′ 50·5″], Entre Ríos Province (Brandoni Reference Brandoni2011; Brandoni & Scillato-Yané Reference Brandoni and Scillato-Yané2007) (Fig. 1). This level overlies the marine Paraná Formation (Bravard Reference Bravard1858) and is characterised by the presence of levels of quartz gravel, clay clasts and numerous isolated fragments of continental and marine vertebrates (Brandoni & Scillato-Yané Reference Brandoni and Scillato-Yané2007; Brandoni Reference Brandoni2011) (Fig. 2).

Modified from Brandoni & Scillato-Yané (Reference Brandoni and Scillato-Yané2007).

Figure 2 Integrated stratigraphic column of the Ituzaingó Formation in the area of Paraná, Entre Ríos province. Scale represents the maximum thickness of the “Conglomerado Osífero”.

Based on the evidence of stratigraphic relations and the fossil vertebrates recorded, Cione et al. (Reference Cione, Azpelicueta, Bond, Carlini, Casciotta, Cozzuol, de la Fuente, Gasparini, Goin, Noriega, Scillato-Yané, Soibelzon, Tonni, Verzi, Vucetich, Aceñolaza and Herbst2000) proposed a correlation of this unit with the late Miocene Huayquerian (for South America) and the Tortonian of the international scale.

3. Systematic palaeontology

Crocodylia Gmelin, 1789, sensu Benton & Clark, Reference Benton, Clark and Benton1988

Alligatoridae Cuvier, 1807, sensu Norell et al., Reference Norell, Clark and Hutchison1994

Caimaninae Brochu, 2003 (following Norell Reference Norell1988)

Mourasuchus Price, Reference Price1964

Type species.M. amazonensis Price, Reference Price1964.

Emended diagnosis.Mourasuchus is diagnosed by the following unambiguous synaphomorphies: dentary linear between the fourth and tenth alveoli (Brochu Reference Brochu, Rowe, Brochu and Kishi1999; character 68-2) and orbits smaller than the infratemporal fenestrae, reduced supratemporal fenestrae (character 165-2). This genus it is also characterised by the following combination of characters: an extremely wide, compressed and long rostrum related to the very small skull table; lateral border of rostrum straight without festooning; supratemporal surrounded anteriorly mainly by postorbitals; prefrontals contacting in the midline, so that nasals do not contact the frontal in dorsal view; wide external nares, larger than other cranial openings but entirely surrounded by premaxillae; laterotemporal arcades depressed, so lateral surfaces of jugal, quadrate and quadratojugal face dorsally; supraoccipital with large dorsal exposure and deep insertion into skull table; slender U-shaped mandibles that curve from first to fifth alveoli and then are linear posteriorly to sixth alveolus; very gracile mandibular symphysis extended only at level of first alveolus; small conical teeth with tiny lateral carinae; and upper and lower tooth rows with more than 40 teeth. Emended from Price (Reference Price1964), Langston (Reference Langston1965) and Bocquentin & Souza-Filho (Reference Bocquentin and Souza-Filho1990).

Mourasuchus nativus (Gasparini Reference Gasparini1985)

1. 1985 Carandaisuchus nativus Gasparini, 51; Fig. 1.

2. 1990 Mourasuchus nativus (Gasparini Reference Gasparini1985) Boquentin & Souza Filho, p. 231, figs 2–4.

Holotype. MLP 73-IV-15-8. Skull table. (Fig. 3A, B).

Figure 3 Mourasuchus nativus: MLP 73-IV-15-8, holotype in (A) dorsal and (B) lateral views; MLP 73-IV-15-9, fragment of posterior sector of skull in (C) dorsal, (D) occipital and (E) lateral views; UFAC-2515 in (F) lateral view. Abbreviations: bs = basisphenoid; bo = basioccipital; cap = capitate process; cb = caudal bridge of laterosphenoid; cf = carotid foramen; exo = exoccipital; cr A = crest A; cr B = crest B; cr cot = cotylar crest of laterosphenoid; exo p = exoccipital process; f = frontal; fnSO = foramen for trigeminal supraorbital nerve; fqch = foramen for cranioquadrate channel; fV = trigeminal foramen; fV₂ = maxillary trigeminal foramen; fV₃ = mandibular trigeminal foramen; gV = trigeminal groove; fVtymp = foramen for trigeminal tympanic branch; lb = lateral bridge of laterosphenoid; lcb = laterocaudal bridge of quadrate; ls = laterosphenoid; p = parietal; po = postorbital; pt = pterigoid; ptf = postemporal fenestra; q = quadrate; so = supraoccipital; sq = squamosal; tr = transversal ridge; IX-X-XI = openings for glosopharyngeal (IX), vagus (X) and accessory (XI) nerves; XII 1, 2 = openings for respective branches of hipoglosal nerve.

Referredmaterial. MLP 73-IV-15-9, posterior sector of skull; UFAC-1397, left maxilla; UFAC-1424, posterior sector of skull and left mandibular ramus; UFAC-1431-1477-1666-2515-3530-3717-4259-4885-4925, posterior sector of skull; UFAC-1484, left mandibular ramus; UFAC-1485, right mandibular ramus; UFAC-1495, right maxilla.

Emended diagnosis.Mourasuchus nativus is diagnosed by the following features: transverse robust ridge at the posterior sector of skull table with developed squamosal eminences in adult stages; lateral bridge of quadrate dividing the trigeminal foramen into two separate openings for the passage of the maxillary (V2) and mandibular (V3) branches of the trigeminal nerve; large postemporal fenestrae and large opening for the passage of tympanic ramus of trigeminal nerve, exposed on lateral view and aligned with trigeminal foramen. It differs from Mourasuchus amazonensis, Mourasuchus arendsi and Mourasuchus atopus by presence of middle crest on posterior dorsal surface of parietal and from Mourasuchus atopus by presence of a pronounced notch at lateral edge of jugals. Mourasuchus nativus also shows exoccipitals not projected ventrally at occipital tubera. Emended from Gasparini (Reference Gasparini1985) and Bocquentin & Souza Filho (Reference Bocquentin and Souza-Filho1990).

Occurrence. The holotype and MLP 73-IV-15-9 come from the banks of the Paraná River, in the area of Paraná (Entre Ríos province, Argentina; Fig. 1); Ituzaingó Formation (Herbst Reference Herbst1971), Upper Miocene (Cione et al. Reference Cione, Azpelicueta, Bond, Carlini, Casciotta, Cozzuol, de la Fuente, Gasparini, Goin, Noriega, Scillato-Yané, Soibelzon, Tonni, Verzi, Vucetich, Aceñolaza and Herbst2000; Brandoni & Scillato-Yané Reference Brandoni and Scillato-Yané2007; Brandoni Reference Brandoni2011) (Fig. 2).

Referred material housed in UFAC comes from the Niterói site, right bank of the Acre River, between the cities of Rio Branco and Senador Guiomard [S 10° 08′ 30·0″, W 67° 48′ 46· 3″); Solimões Formation, Upper Miocene.

Description. A complete description of the cranial osteology of this species is provided by Bona et al. (Reference Bona, Degrange and Fernández2012). In dorsal view (Fig. 3A, C), the skull table shows straight and parallel lateral margins that in the posterior sector are laterally oriented because of the squamosal projections at the paroccipital processes. The posterior margin of the skull table in M. nativus approximates a half-hexahedron, a feature shared with Caiman. Supratemporal fenestrae show different sizes and shapes that vary during ontogeny. In young specimens (MLP-73-IV-15-9, UFAC-2515-1666-4925), they are tri-lobed with a middle, anterolateral and posterolateral lobes. In adult specimens, these fenestrae are relatively small and their outlines vary from circular (MLP-73-IV-15-8) to tri-lobed (UFAC-1424-1431-4259-4885). Those curvatures of the margins of the supratemporal fenestra communicate with grooves over the skull roof, interpreted by Bona et al. (Reference Bona, Degrange and Fernández2012) as vascular marks. The medial and posterolateral ones are related to grooves that surround the squamosal eminences, more pronounced in larger specimens (see Bona et al. Reference Bona, Degrange and Fernández2012 figs 2, 6). As in other young caimanines, in MLP-73-IV-15-9 the anterior opening of the post-temporal canal is exposed in dorsal view in the supratemporal fossa and obliquely oriented. In MLP-73-IV-15-8 and in all UFAC specimens this opening is vertically oriented and covered by the dermal skull roof, a synapomorphy of the crown group caimans (Brochu Reference Brochu, Rowe, Brochu and Kishi1999: 68). The infratemporal fenestra is not preserved in the Argentinian material of M. nativus but in UFAC-1424 (Bocquentin & Souza-Filho Reference Bocquentin and Souza-Filho1990, p. 231, fig. 3A), both infratemporal fenestrae are partially preserved. They are wider than long, probably triangular shaped and wider than the skull table width. As observed in that figure, the quadratojugal of M. nativus forms the posterior part of the lateral margin of the fenestra, and the lateral margin of jugal presents a marked notch. In contrast to the condition in most caimans, in both specimens of M. nativus the ornamentation is reduced, with bone surfaces particularly smooth, with crests and isolate cells and pits.

In occipital view (Fig. 3D), the skull table surface is particularly high because of the abrupt elevation of parietals, squamosals and supraoccipital. These bones all form a robust transversal crest situated over the posterior margin of the skull table. At this ridge, the squamosals form two bumps (or eminences). While their size varies with skull size, the posterior ridge is developed in all young and adult specimens (Fig. 3A–C). CT scan images show that these bumps are solid and not internally affected by the paratympanic sinus system (Bona et al. Reference Bona, Degrange and Fernández2012, fig. 4). The occipital sector dorsal to the foramen magnum is high. As Gasparini (Reference Gasparini1985) pointed out, the dorsal margin of the squamosal descends abruptly at the paroccipital processes. The supraoccipital is also high and narrow, and delimits the huge post-temporal fenestrae medially. These fenestrae are large, similar in size to the supratemporal fenestrae, bounded ventrally by the exoccipitals, dorsally and laterally by the squamosals and medially by the supraoccipitals (Fig. 3D). The basioccipital plate (sensu Iordansky, Reference Iordansky, Gans and Parsons1973) is rectangular in shape. The ventral margin (basioccipital-basiphenoid suture) is horizontal and straight and, as in other crocodylians, it delimits the medial (odd and anterior) and lateral (pair and posterior) openings of the Eustachian tube (medial pharyngeal and pharyngotympanic recesses, respectively; Witmer et al. Reference Witmer, Ridgely, Dufeau, Semones, Endo and Frey2008). In contrast to other caimanines, the exoccipitals are not projected ventrally at the basioccipital tubera (Fig. 3D), extending ventrally only to the level of the ventral border of the occipital condyle. The basioccipital plate also bears the marked medial crest and, in association with the tuberosity, serves in all crocodylians as attachment for the tendons of the M. basioccipitovertebralis (=M. rectus capitis anterior) and the M. occipitotransversalis profundus (M. longissimus capitis; Iordansky Reference Iordansky, Gans and Parsons1973, p. 226).

In lateral view (Fig. 3B, E, F), the laterosphenoid forms the anterolateral wall of the cerebral cavity. As in other crocodylians, the laterosphenoid body forms the rostral border of the trigeminal foramen, which is posteriorly delimited by the prootic (Holliday & Witmer Reference Holliday and Witmer2009). Dorsally, the postorbital process of the laterosphenoid is sutured caudally to the quadrate and dorsorostrally with the frontal. The laterosphenoid bears a marked longitudinal oblique crest (cranial crest of laterosphenoid, Fig. 3E, F), for the attachment of the pseudotemporalis muscle (Iordansky Reference Iordansky1964; Holliday & Witmer Reference Holliday and Witmer2007; Bona & Desojo Reference Bona and Desojo2011), that in M. nativus is more vertically oriented than in other caimanines. The crest extends from the dorsal surface of the postorbital process of laterosphenoid ventral and caudally over the cranial margin of the laterosphenoid lateral bridge. In MLP-73-IV-15-9 the lateral and caudal bridges of the laterosphenoid are preserved. As in other caimanines (e.g. Caiman, Paleosuchus and Melanosuchus), the caudal bridge is typically short, robust and encloses the supraorbital branch of the trigeminal nerve (Holliday & Witmer Reference Holliday and Witmer2009; Bona & Desojo Reference Bona and Desojo2011). The supraorbital nerve runs dorsally into a well-marked groove (Fig. 3F). The lateral bridge of this bone is complete and ventrally sutured with pterygoid. It forms the lateral bony limit of the cavum epiptericum which provides the passage of the ophthalmic ramus of trigeminal nerve (V1) and bears an accentuated groove for the maxillary nerve (V2). The trigeminal fossa is huge, caudo-rostrally oriented and bound by laterosphenoid (rostrally), quadrate (caudally) and pterygoid (ventrally). In M. nativus, the quadrate forms a laterocaudal bridge, caudal to the lateral bridge of the laterosphenoid and ventrally sutured to this bone (Fig. 3F). It is clear that this bridge divides the maxillary (V2) from mandibular (V3) trigeminal branches, and that V3 branch is caudoventrally oriented. Bona et al. (Reference Bona, Degrange and Fernández2012) pointed out that the presence of this bridge is correlated with the huge opening of the trigeminal fossa in this species. This bony structure lies just at the area of attachment of the adductor mandibulae externus muscle (see Holliday & Witmer Reference Holliday and Witmer2007; Bona & Desojo Reference Bona and Desojo2011) and probably serves as a bony surface of attachment of this muscle. The quadrate in M. nativus is also more laterally facing and crests ‘‘A’’ and ‘‘B’’ (Iordansky Reference Iordansky1964) are more laterally and caudally oriented than in other caimanines, respectively. The re-orientation of these crests and the trigeminal V3 branches are features probably related to a different orientation of adductor muscles, and osteologically correlates to the presence of a flat and elongated skull in this species (see Bona et al. Reference Bona, Degrange and Fernández2012).

Although the prootic is not broadly exposed on the lateral braincase wall in most living crocodylians, in M. nativus it is more exposed than in other Caimaninae. At the posterior sector of the trigeminal foramen, it forms the caudal margin of the trigeminal foramen and the medial wall of the trigeminal fossa. The prootic also forms the rostral margin of the foramen for the passage of the tympanic trigeminal branch (Holliday & Witmer Reference Holliday and Witmer2009), which in MLP-73-IV-15-9 is particularly large (almost equal to the trigeminal foramen), completely exposed in lateral view and horizontally aligned with the trigeminal foramen (Fig. 3E).

Observations and phylogenetic relationships. One of the earliest discussions about these bizarre crocodylians was made by Langston (Reference Langston1965), who described and named Nettosuchus atopus from the middle Miocene of Colombia, erecting the family Nettosuchidae to include it. At approximately the same time, Price (Reference Price1964) described Mourasuchus amazonensis from the Upper Miocene of Brazil. Langston (Reference Langston1966) argued that Nettosuchus Langston, Reference Langston1965 is a junior synonym of Mourasuchus Price, Reference Price1964, understanding its similarities with alligatorids. Recent cladistic analyses place this taxon among caimanines (Brochu Reference Brochu, Rowe, Brochu and Kishi1999, Reference Brochu2003, Reference Brochu2010; Aguilera et al. Reference Aguilera, Riff and Boquentin-Villanueva2006; Bona Reference Bona2007; Brochu Reference Brochu2010; Fig. 4). Although Mourasuchus is endemic to South America (Langston & Gasparini Reference Langston, Gasparini, Kay, Madden, Ciffelli and Flinn1997), the sister group relationships with North American Eocene Orthogenysuchus olseni Mook, Reference Mook1924 supported by those phylogenetic studies deserve consideration (Brochu Reference Brochu, Rowe, Brochu and Kishi1999; Langston Reference Langston2008; Brochu Reference Brochu2010; Riff et al. Reference Riff, Romano, Oliveira, Aguilera and Hoorn2010; Fig. 4). Moreover, all phylogenetic analyses link Purussaurus with Mourasuchus and Orthogenysuchus. If so, the Purussaurus and Mourasuchus lineages would go back to the Eocene and multiple dispersal events between North and South America would be necessary to explain the biogeographic history of this clade (Brochu Reference Brochu, Rowe, Brochu and Kishi1999, Reference Brochu2010). Future revisions of Orthogenysuchus will clarify this situation.

Figure 4 Strict consensus cladogram of 30 most parsimonious trees of 265 steps showing Mourasuchus species relationships and the phylogenetic position of Caiman gasparinae. Derived character states for nodes 1–15 and non-caimanines species are listed in Appendix 4. White and black circles indicate homoplastic and non-homoplastic characters states, respectively.

Beyond basic descriptive work (Price Reference Price1964; Langston Reference Langston1965, Reference Langston2008; Boquentin-Villanueva Reference Bocquetin-Villanueva1984; Gasparini Reference Gasparini1985), no detailed comparative anatomical or phylogenetic analyses of Mourasuchus species have been made. Charandaisuchus nativus was erected by Gasparini (Reference Gasparini1985) as a nettosuchid (following Langston Reference Langston1965), based on two posterior fragments of skull table. Later, Boquentin & Souza-Filho (Reference Bocquentin and Souza-Filho1990) described more cranial material of this taxon and considered Charandaisuchus a junior synonym of Mourasuchus. Bona et al. (Reference Bona, Degrange and Fernández2012) provide a detailed study of the cranial anatomy of M. nativus and morphological features that clarify the phylogenetic relationships of the species.

To explore the phylogenetic relationships of Mourasuchus nativus, a maximum parsimony analysis was carried out (Fig. 4). Mourasuchus monophyly is supported by two unambiguous synapomorphies: the dentary is linear between the fourth and tenth alveoli (Brochu Reference Brochu, Rowe, Brochu and Kishi1999, character 68-2) and the orbits are smaller than the infratemporal fenestrae, with reduced supratemporal fenestrae (character 165-2; Appendix 1). Character 68-2 is also present in some non-alliagatoroids (e.g Tomistoma, Thoracosaurus, Eosuchus, Gavialis, Gryposuchus, Hesperogavialis) but it is not homoplastic among alligatoroids (Fig. 4). Based on current morphological data, Morasuchus nativus is the sister taxon of M. amazonensis+M. arendsi+M. atopus, a clade unambiguously supported by the presence of a marked knob at the anterior-medial margin of the orbits (character 167-1, Appendix 1). This hypothesis assumes that all species of Mourasuchus were already differentiated at the Middle Miocene in South America. The M. atopus+M. amazonensis clade is unambiguously supported by the presence of a supratemporal fenestra surrounded anteriorly by the postorbital. (character 166-1; Fig. 4; Appendix 1). Exploring the phylogenetic position of Mourasuchus nativus within alligatoroids, it can be found that this species could be apomorphy based, defined by lateral edge of suborbital fenestra bowed medially (character 105-1; Brochu Reference Brochu, Rowe, Brochu and Kishi1999), a character that is homoplastic within the group (Fig. 4, nodes 11 & 16; Appendix 5; Brochu Reference Brochu, Rowe, Brochu and Kishi1999).

Caiman Spix, 1825

Type species.Caiman fissipes Spix, Reference Spix1825.

Diagnosis. Snout not acute but blunted and anteriorly depressed; extensive inferior “molar” row, blunted and flattened; feet with divided fingers (fissipalmati) or frequently with semi palmated fingers, fourth tooth on each side of the lower jaw occluding in an real fossa in the maxilla (Translated from the original diagnosis). [Original diagnosis: “Rostrum non acutum sed obtusum, supra depressun, largiusculum; arcus maxillaris inferior patens, obtusus, applanatus; pedes posteriores vel fissi vel frequenter semipalmati; dens inferior utrinque quartus in fossam maxillae superioris recondendus” Spix (1825, p. 3)]

Observations. This Caiman type species is Caiman fissipes Spix, Reference Spix1825 (=Crocodilus latirostris Daudin, Reference Daudin1802) by subsequent designation by Schmidt (Reference Schmidt1928). Osteological characters given by Spix (Reference Spix1825) are not exclusive to the genus and they are shared by almost all alligatorids and caimanines. Although phylogenetic proposals based on molecular data support the monophyly of Caiman, (e.g. DNA sequences of the mitochondrial cytochrome b gene, the nuclear Recombination Activating Gene 1 and the myelocytomatosis oncogene; Hrbek et al. Reference Hrbek, Vasconcelos, Rebelo and Farias2008), cladistic analyses based on morphology propose the paraphyly of this genus (Norell Reference Norell1988; Poe Reference Poe1997; Brochu Reference Brochu, Rowe, Brochu and Kishi1999, Reference Brochu2004, Reference Brochu2010; Reference Brochu2011; this paper; Fig. 4). Jacarea Gray, Reference Gray1844 is defined by Brochu (Reference Brochu, Rowe, Brochu and Kishi1999: 74) as “a node based group comprising the last common ancestor of Caiman crocodilus, C. yacare, C. latirostris and Melanosuchus niger and all of its descendents”, based on four unambiguous synapomorphies i.e. anterior half of axis neural spine slopes anteriorly (character 11-1), iliac blade rounded with modest dorsal indentation (character 28-1), articular-surangular suture with anterior process ventral to lingual foramen (character 44-2) and lateral edge of suborbital fenestra bowed medially (character 105-1). As stated above, character 105-1 is homoplastic among alligatoroids (Brochu Reference Brochu, Rowe, Brochu and Kishi1999; this paper).

The sister-group relationship between Caiman latirostris and Melanosuchus (not completely supported in the present cladistic analysis, Fig. 4) is actually supported by only one unambiguous synapomorphy (i.e. rostral ridges very prominent; Brochu Reference Brochu, Rowe, Brochu and Kishi1999, Reference Brochu2011). Given the lack of support in combined cladistic analyses as well as the weak support obtained from morphological data set, re-naming the genus (with the older name Jacaretinga, e.g. Norell Reference Norell1988) or synonymising Melanosuchus with Caiman (Poe Reference Poe1997) is premature (Brochu Reference Brochu, Rowe, Brochu and Kishi1999). On the other hand, the unnamed clade “C. yacare+C. crocodilus” (Brochu Reference Brochu, Rowe, Brochu and Kishi1999, p. 75) is supported by two synapomorphies: surangular with spur bordering the dentary tooth row lingually for at least one alveolus length; and occlusion pit between the 7th and 8th maxillary teeth (Brochu Reference Brochu, Rowe, Brochu and Kishi1999; Fig. 4; characters 61-0 and 78-1). Given the fragmentary nature of fossil species of Caiman, the phylogenetic relationships including all Caiman taxa and an emended diagnosis of the genus have to be made on the basis of future morphological information. Detailed and accurate anatomical descriptions of extant species considering the intra-specific variability are scarce (Bona & Desojo Reference Bona and Desojo2011). These kinds of studies should be done to obtain new morphological data in order to clarify this situation and to define Caiman and most of its species osteologically. The monophyly of Caiman could be a taxonomical decision, considering Melanosuchus and Caiman as the same genus. Within Caimaninae, extant Caiman species and those described in this contribution share morphological features, such as the relative size of the temporal fenestrae and orbits (a plesiomorpic character whitin among alligatorids; character 165-1) and the pattern of relative size of premaxilla–maxilla alveoli and diastemata (e.g. all Caiman species have five premaxillary alveoli, and the 3rd and 4th maxillary alveoli are the biggest of the maxillary tooth row). Although these characters are also shared with Melanosuchus, Caiman species do not have the vomer exposed in palatal view and almost all lack conspicuous rostral crests (except the broad snouted Caiman latirostris).

Caiman australis (Burmeister Reference Burmeister1885)

1. 1858 Crocodilus australis Bravard.

2. 1883 Crocodilus australis (Bravard Reference Bravard1858); Burmeister Reference Burmeister1883, pp. 148–150 (in part).

3. 1887 Crocodilus meridionalis Scalabrini, p. 37.

4. 1887 Alligator paranensis Scalabrini, p. 37.

5. 1887 Proalligator australis (Bravard Reference Bravard1858); Ambrosetti, pp. 420–426 (in part).

6. 1898 Alligator australis (Bravard Reference Bravard1858); Ameghino, p. 240.

7. 1912 Alligator australis (Bravard Reference Bravard1858); Rovereto, p. 341–346 (in part); fig. 2; plate XVI, 1a,b.

8. 1932 Proalligator australis (Bravard Reference Bravard1858); Rusconi, p. 197.

9. 1933 Proalligator australis (Bravard Reference Bravard1858); Rusconi, p. 59, figs 1, 2.

10. 1936 Proalligator australis (Burmeister Reference Burmeister1885); Patterson, p. 47–48 (in part).

11. 1965 Proalligator australis (Burmeister Reference Burmeister1885); Langston, p. 177–118.

Holotype. MACN PV 258, complete left maxilla (Fig. 5A–D).

Figure 5 Caiman australis: MACN PV 258, Holotype in (A) dorsal (B) palatal, (C) medial and (D) lateral views. Abbreviations: mx-js=maxilla–jugal suture; mx-la s=maxilla–lacrymal suture; mx-ns=maxilla–nasal suture; mx-pmx s=maxilla–premaxilla suture; po vest rec=recessus postvestibularis; sof=suborbital fenestra; 3°pmx, 4°pmx=3° and 4° premaxillary alveoli.

Emended diagnosis.Caiman with a narrow snout, maxilla longer and narrower than extant Caiman species. 3rd and 4th alveoli are the largest of the series and similar in size. Maxillary interalveolar spaces reduced. Lateral margin of maxilla less festooned than in other Caiman species, in dorsal and lateral view. Maxillary ornamentation with predominance of prominent and elongated grooves and bumps.

Occurrence. Banks of Paraná River, in the area of Paraná (Entre Ríos province, Argentina; Fig. 1); Ituzaingó Formation (Herbst Reference Herbst1971), Upper Miocene (Brandoni & Scillato-Yané Reference Brandoni and Scillato-Yané2007, Brandoni Reference Brandoni2011).

Description. In dorsal view, the natural limits of the maxilla are preserved (Fig. 5A). While the rostral and medial contacts of the maxilla with the premaxilla and nasal are preserved, only the medial part of the suture (for contact with lacrimal) is present caudally. The dorsal surface is ornamented with isolated pits and grooves, mainly longitudinally oriented that delimit small bumps. This ornamentation differs from that present in most of the skull bones of extant caimans, which are characterised by conspicuous cells. In this view, the lateral margin of the maxilla is practically straight, tapering forward. Maxillary proportions differ from other Caiman species: it is narrow and elongated, suggesting a narrow-snouted specimen. In palatal view (Fig. 5B), the medial maxillary contact is not preserved. The first to the ninth alveoli are preserved. The first, second and fifth to ninth alveoli are similar in size, and the third and fourth alveoli are the largest of the maxillary tooth row, a characteristic shared with other Caiman species. The interalveolar spaces are reduced except between the sixth and seventh alveoli. In this specimen, the rostral margin of suborbital fenestra is preserved, showing that it is extended rostrally to the level of the 8th alveolus. In medial view (Fig. 5C), maxillary recesses for the nasal sinuses that pneumatise the rostrum in extant crocodylians are less complex than in other Caiman species, such as C. latirostris (e.g recessus caviconchalis, Witmer Reference Witmer1995; Bona & Desojo Reference Bona and Desojo2011).

Observations. Based on fragmentary and disassociated material, Bravard (Reference Bravard1858, Reference Bravard1860) erected the name Crocodilus australis to include all the cranial and postcranial fossil remains of non-longirostrine crocodylians from the “Mesopotamiense”. Unfortunately, Bravard (Reference Bravard1858; reprinted by Burmeister Reference Burmeister1883) did not provide any illustration or detailed description of those materials, and both papers had limited distribution. Although parts of the “Catalogue” provided by Bravard (Reference Bravard1860) are in the British Museum of Natural History, the sections on fossil crocodylians appear to be missing (Langston Reference Langston1965). An expanded detailed description of Crocodilus australis was given later by Burmeister (Reference Burmeister1885), based on maxillary, dental, and vertebral material. Although Burmeister neither numbered this material nor specified a type, the description of the maxilla matches precisely to MACN PV 258 (Fig. 5). Based on an isolated fragment of premaxilla, dentary, teeth and postcranial elements, Ambrosetti (Reference Ambrosetti1887) erected the genus Proalligator to include this species. He inferred that the fragment of premaxilla belonged to the same specimen of Bravard and noted that the depressions for occlusion of the dentary teeth, present in both cranial specimens, indicated that Crocodilus australis was not a “crocodilino” but an “alligator” (Ambrosetti Reference Ambrosetti1887, p. 420). Unfortunately, he gave no argument to justify a new generic name.

The first formal diagnosis and discussion of characters of Crocodilus australis was proposed by Rovereto (Reference Rovereto1912) (as Alligator australis; for a detailed comment about the taxonomic history of this species, see Rusconi Reference Rusconi1933; Langston Reference Langston1965; Gasparini Reference Gasparini1973). Rovereto (Reference Rovereto1912) indicated that certain features of the teeth that differentiate C. australis (i.e., the size and proportions of the maxilla) described by several authors as characteristic of this species, were erroneously attributed. Langston (Reference Langston1965, p. 118) provided an emended diagnosis of this taxon: “Alligatorids of presumed Caiman habitus with rostrum relatively wider than Caiman sclerops but narrower than in Caiman latirostris; sculpture pronounced, consisting of ridges and deep furrows instead of tips; facial canthi not strongly developed, festooning of maxilla not pronounced. Teeth relatively large, widely spaced, and less differentiated in size than in living species…” In our opinion, the proportions of MACN PV 258, indicate that this specimen had a rostrum longer and narrower than extant Caiman species, including Caiman crocodilus. These general proportions fit with those present in extant species of Paleosuchus, but in Caiman australis the snout is low and not quadrangular in section as in Paleosuchus. Although any observation related to the tooth morphology of this species cannot be made at present, we agree with Langston (Reference Langston1965) and Gasparini (Reference Gasparini1973) that in Caiman australis, the maxillary alveoli are less differentiated in size but the interalveolar spaces in MACN PV 258 are reduced.

Taxonomic assignation referrals should be based preferably on apomorphies than on the presence of a unique combination of characters. Nevertheless, fossil specimens are often fragmentary, especially those of Caiman. This is the case of C. australis, which is known from a maxillary left fragment. Available morphological information used to reconstruct phylogenetic relationships among alligatorids caimanines (Brochu Reference Brochu, Rowe, Brochu and Kishi1999, Reference Brochu2010, Reference Brochu2011; this paper) is insufficient to propose the phylogenetic position of this species within alligatorids. However, the occlusion pattern of the mandibular tooth and the presence of the larger 3rd and 4th maxillary alveoli are features that allow us to consider this species as an alligatorid Caiman (Gasparini Reference Gasparini1973, Reference Gasparini1981). Even though the relative size of the maxillary alveoli is similar to that of Melanosuchus, Caiman australis differs from it by the proportions of the snout and the absence of a conspicuous maxillary crest.

Several cranial and postcranial elements have been assigned to Caiman australis but never in association with maxillary material (e.g. Ambrosetti Reference Ambrosetti1887; Rovereto Reference Rovereto1912; Rusconi Reference Rusconi1933; Sáez Reference Sáez1928; Gasparini Reference Gasparini1973). This includes several partial dentaries with particular features, such as similar alveolar sizes, reduced interalveolar spaces, symphyseal mandibular section that is low and narrow with the symphysis extending to the fourth and fifth alveoli, which are seen in MACN PV 5533, 5535, 5537 and 5588. Furthermore, the hollow between the sixth and seventh maxillary alveoli for occlusion of the corresponding mandibular teeth in MACN PV 258 is coincident with hypertrophy of the twelfth dentary teeth preserved in the material MLP-73-IV-15-2 assigned by Gasparini (Reference Gasparini1973) to Caiman australis. This material has the alveolus more laterally situated than in other Caiman species. The left dentary fragment MACN PV 5588 has an implanted 4th tooth which is less curved than in extant Caiman extant species, with distinctively spaced and pronounced longitudinal striations.

Caiman australis is one of at least five species of this genus recorded in the late Miocene in Argentina, supporting the hypothesis of the great diversification of Caiman in these latitudes during the late Miocene (see below).

Caiman gasparinae Bona & Paulina Carabajal (Reference Bona and Paulina Carabajal2013).

1. 1887 Crocodylus paranensis Scalabrini, p. 37.

2. 1912 Alligator? ameghinoi Rovereto, pp. 360–367 (en parte), fig. 16.

3. 1933 Xenosuchus paranensis Rusconi, pp. 67–80 (en parte), fig. 9.

Holotype. MLP-73-IV-15-1; skull represented by a rostrum with articulated fragments of premaxillae, maxillae, nasals, left lacrmal and a partial braincase lacking the basicranium (Fig. 6A–F).

Modified from Bona & Paulina Carabajal (Reference Bona and Paulina Carabajal2013).

Figure 6 Caiman gasparinae: MLP-73-IV-15-1 (A–F) and MACN PV 5555 (G): snout in dorsal (A), left lateral (B) and ventral (C) views; braincase in dorsal (D), left lateral (E) and occipital (F) views; fragment of right premaxilla in dorsal (G) view. Abbreviations: exo=exoccipital–opistotic complex or otoccipital; f=frontal; fm=foramen magno; m=maxillar; 1°–6° m=1°–6° maxillar; n=nasal; na=narina; p=parietal; pm=premaxilla; pm–m c=premaxillary–maxillary curvature; po=postorbital; ptf=postemporal foramen; q= quadrate; so=supraoccipital; sq=squamosal; 1°–5° pm=1°–5° premaxillar alveoli; IX, X, XI=openings for glosopharyngeal (IX), vagus (X) and accessory (XI) nerves; XII₁, XII₂=openings for respective branches of hipoglosal nerve.

Referred material. MACN PV 5555; fragment of right premaxilla (Fig. 6G).

Occurrence. Banks of Paraná River, in the area of Paraná (Entre Ríos province, Argentina; Fig. 1); Ituzaingó Formation (Herbst Reference Herbst1971), upper Miocene (Brandoni & Scillato-Yané Reference Brandoni and Scillato-Yané2007; Brandoni Reference Brandoni2011).

Description. A detailed description and a taxonomic treatment of this taxon is provided by Bona & Paulina Carabajal (Reference Bona and Paulina Carabajal2013). The following description, mainly based on specimen MLP-73-IV-15-1 (Fig 6A–F), summarises the main cranial morphology of this species and its particular morphological features. The general morphology of the skull is similar to that present in other Caiman species (outline and relative dimension and contacts of skull bones). The cranial bones are characterised by crests and depressions forming an irregular surface of marked ridges delimiting interconnected pits. There are also some ornamental bumps (bony convexities) on the maxilla and the skull table. This kind of ornamentation is not the typical pattern observed in extant caimans. In both MLP-73-IV-15-1 and MACN PV 5555 (Fig. 6A, B, G) the irregular ornamentation extends further rostrally than in other Caiman species.

The rostral portion preserved in MLP-73-IV-15-1 belongs to a large-sized and broad-snouted caimanine, with a low and short rostrum. As in other caimans, a marked curvature at premaxilla-maxilla suture is present in dorsal and lateral views (Fig. 6A, B). The external naris is a single, sub-circular, dorsally-oriented opening that is broadly separated from the anterior margin of premaxilla, as seen in MACN PV 5555. In lateral view, both premaxillae have a lower dorsal surface behind the naris. In dorsal view, they are laterally expanded. In MLP 73-IV-15-1, the nasals do not reach the external naris because of the relatively large rostral contact of premaxilla at the midline (this feature varies intraspecifically in Caiman; Bona & Desojo Reference Bona and Desojo2011). The nasal is sub-rectangular, similar to that present in C. yacare, with straight and parallel lateral margins tapering rostral and caudally. In spite of its broad snouted condition, in this species the nasals are relatively narrow. Maxillary crests are also absent and only four isolated bumps are observed in the left maxilla, which are elongate and obliquely oriented (caudomedially–rostrolaterally; Fig 6A). In palatal view (Fig. 6C), five premaxillary and six maxillary alveoli are preserved on the left side of the rostrum. In MLP-73-IV-15-1, the interalveolar spaces of the premaxilla are reduced in size, except for the 3rd, which is larger than the biggest premaxillary alveolus, the 4th. The premaxillary–maxillary alveoli follow the general Caiman–Melanosuchus pattern (Fig. 6C), with five premaxillary alveoli where the 4th is larger than the 3. Furthermore, the 3rd and 4th maxillary alveoli are larger than the others.

On the skull table, the supratemporal fenestrae are relatively small, with dermal bones of the skull roof overhanging their rims, a morphological feature present in adult specimens of the crown group caimans (Brochu Reference Brochu, Rowe, Brochu and Kishi1999). As in Caiman and Melanosuchus, the orbits are larger than the infratemporal fenestrae and the supratemporal fenestrae are smaller, though not obliterated. The lateral margins of the skull table diverge caudally. Its dorsal surface is deeply concave at the midline, especially on the frontal and the supraoccipital. Ornamental protuberances are also present in this section of the skull along at the caudal and caudomedial margins of the orbits (Fig. 6D). The edges of the orbit and the lateral and caudal margins of the skull table are strongly thickened. In lateral view, the descending process of the postorbital, at the postorbital pillar, is subtriangular in cross-section. The pyramidal corpus of the laterosphenoid (Holliday & Witmer Reference Holliday and Witmer2009) is Y-shaped, with the rostral margin slightly concave and the postorbital process narrow. In occipital view, exoccipital-ophisthotic complex and squamosals form a high and dorsoventrally concave occipital table with a curved dorsal margin (Fig. 6F). Lateral to the foramen magnum, the three foramina for the opening of cranial nerves XII₁, XII₂ and X–XI are visible. They show a pattern similar to that present in caimans (Bona & Desojo Reference Bona and Desojo2011). In contrast to other caimans, in MLP-73-IV-15-1 the carotid foramen is visible only in lateral view (Fig. 6F). The carotid canal has a similar trajectory as in other caimans (Bona et al. Reference Bona, Degrange and Fernández2012). Nevertheless, unlike another caimans, this foramen is bound by the crista tuberalis posteriorly and not anteriorly, an autapomorphic character of this species (Bona & Paulina Carabajal, in press).

Observations.Alligator? ameghinoi Rovereto, Reference Rovereto1912 was based on isolated large cranial and postcranial remains. The syntype is formed by three vertebrae, a premaxilla and a mandibular fragment. Rovereto (Reference Rovereto1912) provided good illustrations of this material that allows us to identify the right premaxilla (p. 364, fig. 16a) as MCN 5555, described in this contribution. Later, Rusconi (Reference Rusconi1933) erected the genus Xenosuchus to include all the large-sized alligatorids from the Neogene of Paraná. In the diagnosis, this author mentioned a series of characters related to the huge proportions of the bones and teeth, and some general features shared by other alligatorids (e.g. “five teeth in premaxilla”; Rusconi Reference Rusconi1933, p. 67). This author assigned two species to this genus: Xenosuchus paranensis (Scalabrini, Reference Scalabrini1887), with the subspecies X. paranensis ameghinoi, and X. lutescens (Rovereto, Reference Rovereto1912). Xenosuchus paranensis was described, based on dentaries, vertebrae, humeri and a premaxilla. Although the premaxillary fragment was not described and only schematically drawn following Rusconi (Reference Rusconi1933, p. 81, fig. 9), we recognise that the premaxilla corresponds to MACN PV 5555, as well as part of the syntype of A.? ameghinoi figured by Rovereto (Reference Rovereto1912): a fragment of right premaxilla with four alveoli preserved (2nd–5th) and external naris separated from the rostral edge of the snout. Unfortunately, it cannot be established whether the premaxilla figured by these authors corresponds to that described by Scalabrini (Reference Scalabrini1887) as Crocodilus paranensis, but we agree with Langston (Reference Langston1965, p. 121): “Rusconi who evidently did not examine the specimen states that by “hueso incisivo” Scalabrini meant premaxilla. But if it is so the statement that only part of a right bone contained eight teeth is curious indeed. He observes that if the bone in question had been a maxilla (a reasonable assumption) the great size of what Scalabrini supposed to be the third premaxillary alveolus would have no significance. If parts of premaxilla and maxilla were involved together, the description is equally meaningless. From the typological viewpoint therefore Crocodilus paranensis seems to be a nomen vanum… I follow Patterson in regarding all original paranensis specimens as Caiman.”

Later, Gasparini (Reference Gasparini1973) assigned MLP-73-IV-15-1 to Caiman latirostris but gave no description of the specimen or any justification for that assignation. Both MLP 73-IV-15-1 and MACN PV 5555 belong to same large-sized taxon, with external naris separated from the rostral edge of the snout. Based on the skull morphology of MLP-73-IV-15-1, both specimens can be referred to as an alligatorid species, with parietals excluded from the posterior edge of the skull table (Brochu Reference Brochu, Rowe, Brochu and Kishi1999, character 82-3; modified from Norell Reference Norell1988, character 11). Although in the present cladistic analysis this character is an ambiguous synapomorphy for this clade, Caiman gasparinae is linked with caimanines by shared dorsal edges of orbits upturned (103-1, homoplastic) and medial parietal wall of supratemporal fenestra bearing foramina (104-1). Both features are preserved in MLP-73-IV-15-1. As is shown in Figure 4, Caiman gasparinae is included in the clade Jacarea (Brochu Reference Brochu, Rowe, Brochu and Kishi1999), supported by four synaphomorphies (44-2, 105-1, 143-1, 153-2). The incisive foramen (153-2) is not preserved in either MLP-73-IV-15-1 or MACN PV 5555, but canthi rostralii are very prominent in Caiman gasparinae (143-1). Nevertheless, unlike the broad snouted C. latirostris, in C. gasparinae the canthi are short, with prominent bumps and no conspicuous crests on the maxillary surface (Fig. 6).

This species differs from other large forms, such as Purussaurus, by: the outline, size and morphology of the narial openings; the shallow premaxillary height in lateral view; the relative proportions of orbits and temporal fenestrae; and the skull table outline. The snout features (i.e. narial position, bone proportions and sculpturing) differentiate this taxon from other extant Caiman species. In particular, it differs from C. latirostris in not having conspicuous maxillary crests and the different proportion of the snout bones (i.e. in Caiman latirostris, the nasals are wider). Although this species is a broad-snouted caimanine, it presents relatively narrowed nasals, differing from the broad snouted C. latirostris, in which nasals are proportionally wider with lateral convex margins (and not parallel and straight as in Caiman gasparinae). The posterior section of the skull also shows peculiar morphological conditions in the position of the opening of lateral carotids, which are laterally situated and do not open on the occipital table, general feature in caimans. This taxon represents one of the largest known Caiman species, pertaining to the numerous huge mandibular and postcranial remains found in the area of Paraná.

Caiman latirostris (Daudin Reference Daudin1802)

1. 1802 Crocodilus latirostris Daudin, p. 417.

2. 1825 Caiman fissipes Spix, fig. 3.

3. 1912 Alligator australis (Bravard Reference Bravard1858); Rovereto, p. 341 (in part), fig. 1a.

4. 1936 Caiman paranensis (Scalabrini Reference Scalabrini1887); Patterson, p. 50 (in part).

5. 1912 Alligator lutescens Rovereto, p. 346 (in part), fig. 4a.

6. 1933 Proalligator australis (Bravard Reference Bravard1858); Rusconi, p. 59 (in part), fig. 10.

7. 1933 Xenosuchus lutescens (Rovereto Reference Rovereto1912); Rusconi, p. 80 (in part), fig. 11a

Referred material. MACN PV 5416, left premaxilla and maxilla; MACN PV 5576, left premaxilla; MLP 73-IV-15-16, fragment of left premaxilla; MLP 73-IV-15-12 fragment of braincase.

Occurrence. From the late Miocene, banks of Paraná River, in the area of Paraná Entre Ríos province, Argentina (Fig. 1); Ituzaingó Formation (Herbst Reference Herbst1971), (Brandoni Reference Brandoni2011; Brandoni & Scillato-Yané Reference Brandoni and Scillato-Yané2007; Fig. 2) to the Recent (NE of Argentina, Paraguay, Bolivia, N of Uruguay and S – SE of Brazil).

Description. A complete description of the osteology of skull and mandible of this species was published by Bona & Desojo (Reference Bona and Desojo2011) based on extant specimens. The following description is based on those Miocene materials assigned to C. latirostris in this contribution. MACN PV 5416 is a fragment of a left rostrum from a large, broad-snouted specimen (Fig. 7A–D). As in extant specimens, the premaxillary–maxillary curvature is marked in lateral view (Fig. 7A). Nevertheless, this curvature is moderate in MACN PV 5416 in dorsal view (Fig. 7B). The skull of extant specimens of C. latirostris has a triangular outline with the lateral margins converging rostrally and, although the premaxillary–maxillary curvature is practically absent in dorsal view, the lateral margin of premaxilla becomes rounded, as in the huge MACN PV 5416 (Bona & Desojo Reference Bona and Desojo2011, fig. 2A, B). In this specimen, the dorsal surface of the snout shows two pronounced maxillary crests. In C. latirostris and Melanosuchus, the posterior one extends continuously and caudo-rostrally over the dorsal surface of prefrontal, lacrimal and maxilla, from the medial margin of the orbit to almost the lateral margin of the maxilla. In MACN PV 5416, this crest is continuous over the maxillary surface and interrupted posteriorly only because both the lacrimal and prefrontal are not preserved in this specimen. In MACN PV 5416 and MACN PV 5576 (Fig. 7B, E), the premaxillae delimit the narial openings and are confluent, as in other caimanines.

Figure 7 Caiman latirostris: MACN 5416, left fragment of rostrum in (A) lateral, (B) dorsal, (C) palatal and (D) medial views; MACN 5576, left premaxilla in (E) dorsal and (F) palatal views. Abbreviations: f1°t=foramen for 1° mandibular tooth; mx=maxilla; mx cr=maxillary crest; n=nasal; pmx=premaxilla; 3°pmx–4°pmx=3° and 4° premaxillary alveoli; 4° mx=4° maxillary alveolus.

Similar to most specimens of Caiman latirostris, the nasals contact the posterior margin of the external naris in MACN PV 5416. Nevertheless, this condition varies intraspecifically but not ontogenetically and, in some extant specimens of C. latirostris, the premaxillae contact each other dorsomedially, excluding nasals from the posterior margin of the external naris (Bona & Desojo Reference Bona and Desojo2011). Contrary to the condition present in some Caiman species (e.g. C. yacare), in C. latirostris the caudolateral margin of the naris forms a narrow edge, which is also present in MACN PV 5416, MACN PV 5576 and MLP 73-IV-15-16. In lateral view (Fig. 7A), the ventral margin of maxilla is convex with a rostral and a caudal curvature, as in other caimanines. In palatal view (Fig. 7C), the first ten maxillary alveoli are preserved. As in other caimans, their size increases from first to fourth, decreases from fourth to fifth and then increases from seventh to ninth. As in C. latirostris and contrary to the condition observed in Melanosuchus, in palatal view the maxillae contact each other along the middle line, so the vomer is not exposed on the palatal table. In MACN PV 5416, MACN PV 5416, MACN PV 5576 and MLP 73-IV-15-16, the premaxilla delimits the incisive foramen, which in C. latirostris is heart- or teardrop-shaped (as in MACN PV 5576 and MACN PV 5416 respectively; Fig. 7C, F). In MACN PV 5416 and MACN PV 5576, the anterior ventral surface of the premaxilla has a pit that receives the first mandibular tooth. Similar to some extant specimens of C. latirostris, this tooth pierces the palatal surface of the premaxilla (but never its dorsal anterior surface, as in other caimanines such as C. yacare, C. crocodilus, Melanosuchus or Mourasuchus). In caimans, the presence of a perforation of 1st mandibular tooth in the premaxilla varies within species, but not ontogenetically. In medial view (Fig. 7 D), the maxillary recesses for the nasal sinuses (Witmer Reference Witmer1995) are complex with many diverticula as in Caiman latirostris.

MLP 73-IV-15-12 (Fig. 8) is a braincase fragment. In dorsal view, the medial section of the posterior margin of the skull table is straight and perpendicular to the sagittal plane (as in extant caimanines; i.e. Caiman, Melanosuchus, Paleosuchus; Fig. 8B). The rostral opening of the temporal canal is seen through the supratemporal fenestra and is obliquely oriented, showing that this fragment belongs to a juvenile specimen (Brochu Reference Brochu, Rowe, Brochu and Kishi1999). The parietal delimits the supratemporal fenestra medially and, together with the supraoccipital, forms a shallow depression with two smooth lateral ridges that extend from the medial edge of the supratemporal fenestra to the posterior margin of the skull table (as in C. latirostris).

Figure 8 Caiman latirostris: MLP 73-IV-15-12, braincase fragment in (A) occipital, (B) dorsal and (C) lateral views. Abbreviations: afpt = anterior foramen of postemporal canal; bo = basioccipital; bsf = basisphenoid; cb = caudal bridge of laterosphenoid; cf = carotid foramen; exo = exoccipital; fov = trigeminal fossa; fv = trigemimal foramen; lb = lateral bridge of laterosphenoid; pa = parietal; ptf = postemporal fenestra; q = quadrate; so = supraoccipital; sq = squamosal; stf = supratemporal fossa; IX-X-XI = openings for glosopharyngeal (IX), vagus (X) and accessory (XI) nerves; XII₁, XII₂ = openings for respective branches of hipoglosal nerve.

As in other caimanines, the supraoccipital extends over the skull table and the dorsal surface is slightly concave with a poorly pronounced median crest. This crest is more pronounced in adult specimens (Bona & Desojo Reference Bona and Desojo2011). In occipital view (Fig. 8A) the supraoccipital is wider than high, pentagonal shaped and forms the ventromedial margin of the reduced post-temporal fenestra. Exoccipitals are sutured at the midline, delimiting the foramen magnum dorsally and dorsolaterally and, as in caimanines, they are projected ventrally to the basioccipital tubera (Brochu Reference Brochu, Rowe, Brochu and Kishi1999). Laterally to the foramen magnum, the three horizontally aligned foramina for the passage of cranial nerves IX, X, XI, and XII are preserved at the left side (Fig. 8A). As in C. latirostris from medial to lateral they correspond to the separated opening for XII2 and XII1 branches and the single opening for nerves IX, X, and XI. Characteristic of this species, this last foramen is the largest and is horizontally oriented. Ventrally to all these foramina, the carotid foramen (for the passage of the internal carotid arteries) is also preserved. As in other caimans, the basioccipital is hexagonal and forms the occipital condyle and the medioventral margin of the foramen magnum. In lateral view (Fig. 8C), the laterosphenoid forms the rostro-lateral wall of the preserved braincase and extends dorsally, forming the anteroventral area of the medial wall of the supratemporal fossa. Together with the prootic, it delimits the trigeminal opening in equal proportions, forming the anterior margin. The lateral and caudal laterosphenoid bridges are preserved (Holliday & Witmer Reference Holliday and Witmer2009). As in C. latirostris, the lateral bridge contacts the pterygoid ventrally by a relatively long suture (Bona & Desojo Reference Bona and Desojo2011). The broad laterosphenoid caudal bridge for the passage of the supraorbital nerve (the first branch of the trigeminal maxillary nerve; Holliday & Witmer Reference Holliday and Witmer2009) articulates with the quadrate. The quadrate surface preserved in this specimen has the rostral part of the “crest A” (Iordansky Reference Iordansky1964) for the attachment of adductor muscles. As in extant specimens of C. latirostris and contrary to other caimanis such as C. yacare, C. crocodilus and Melanosuchus, this part of the crest reaches the laterosphenoid-quadrate suture and is situated ventrally to the opening for the supraorbital nerve (Bona & Desojo Reference Bona and Desojo2011).

Observations. As stated above, Rovereto (Reference Rovereto1912, p. 342) defined Proalligator australis, mainly based on the morphology of the left maxilla (MACN PV 258). Based on the size of the materials, he assigned to this species two mandibular fragments and a left premaxilla (the latter figured in Rovereto Reference Rovereto1912, fig. 1a). His detailed description and illustration of the premaxilla suggests that the premaxilla of MACN PV 5576 (Fig. 7E, F) may be assigned to C. latirostris (by the presence of a narrow ridge at the posterior margin of nares; 1st mandibular tooth piercing only the palatal surface of premaxilla). In the same paper, Rovereto (Reference Rovereto1912, p. 346) erected A. lutescens on the basis of the large size of several isolated cranial and postcranial fragments. Among these, there are two skull fragments that can be identified as MACN PV 5416 and 13551 (Figs 7A–D, 9). Although these materials were not associated, this author assumed that, given their size, they belong to the same species. Even more, he points out that while the rostral fragment (MACN PV 5416) was similar in morphology to Caiman latirostris (see below), the skull table (MACN PV 13551) had some peculiar features. Rovereto (Reference Rovereto1912, pp. 346–349) referred to this material as follows: “The most noticeable remain of this species … is a skull table … Comparing this material with the largest skulls of Caiman latirostris that I could access, there are remarkable distinctions in the dimensions and some differences in the general aspect” [From the original: “El trozo mas vistoso de esta especie … es una parte superior de cráneo … Comparando este conjunto con el que corresponde al más grande de los cráneos que he podido proporcionarme del Caiman latirostris, se observan notables diferencias en las dimensiones y algunas en la configuración general.”] He provided a brief description of some of the diagnostic characters of this species: a longitudinal depression of the skull table at the middle line; frontal short with its cranial end poorly extended between pre-frontals; and the probable presence of long nasals (missing in the material). We accept the skull table description given by Rovereto as the original diagnosis of A. lutescens and the material MACN PV 13551 as the holotype (see Langston Reference Langston1965, p. 121). Regarding the large size of specimens, Rusconi (Reference Rusconi1933) erected de genus Xenosuchus, with X. paranensis and X. lutescens (considered synonyms by Patterson 1937). Unfortunately, Rusconi could not see the skull table figured and described by Rovereto (Reference Rovereto1912), and chose the rostral fragment (MACN PV 5416) as the lectotype of X. lutescens, misreporting the number of the material as “4516” (Gasparini Reference Gasparini1973).

A comparative study of the cranial morphology in Caiman latirostris (Bona & Desojo Reference Bona and Desojo2011) showed that in some specimens, the tip of the first mandibular tooth pierces the palatal surface of the premaxilla, but never at its dorsal anterior surface, as in other caimanines such as C. yacare, Melanosuchus and Mourasuchus (the condition seen in MACN PV 5416, 5576, MLP-73-IV-15-16). Similar to Caiman crocodilus and Melanosuchus, but contrary to other caimanines such as C. yacare, the rostral part of the quadrate crest “A” (Iordansky Reference Iordansky1964) in C. latirostris is situated ventral to the opening for the supraorbital nerve (MLP 73-IV-15-12, Fig. 8C). The presence of a conspicuous and continuous second rostral crest at the maxilla and the kind of tooth pierce in the premaxilla allows the assignment of MACN PV 5416 to C. latirostris. Following Gasparini (Reference Gasparini1973, Reference Gasparini1981), we regard Alligator lutescens in part (Rovereto Reference Rovereto1912, p. 346, fig. 4a) and Xenosuchus lutescens in part (Rusconi Reference Rusconi1933, 80, fig. 11a) as its junior synonyms. This species was represented in the late Miocene by specimens larger than the extant ones forms (e.g. MACN 5416 belongs to a specimen with an anterior snout width of 19 cm and a total cranial length of approximately 45 cm).

Caiman cf. lutescens Langston (Reference Langston1965) from the Middle Miocene of Colombia is represented by an incomplete skull lacking most of the occipital region, braincase, cranial table and right temporal arcade (UCMP 39978, Langston Reference Langston1965, p. 75, figs 32–34; pl 2). Following the detailed description and illustrations of this material provided by Langston (Reference Langston1965), this specimen shows a Caiman-like general morphology, sharing some similarities with C. latirostris, such as the presence of rostral canthi, palatines widened rostrally and a triangular dorsal outline of the skull (the latter condition is present in young specimens of this species; Bona & Desojo Reference Bona and Desojo2011). Nevertheless, the relatively short snout of this specimen (e.g. short and wide nasals and maxillae), the presence of broad narial external openings and the absence of information of the skull table and brain case morphology lead us to assign UCMP 39978 to C. latirostris and, thus, to synonymise Caiman cf. lutescens is inappropriate. As stated above, the type material of Alligator lutescens Rovereto (Reference Rovereto1912) corresponds to a skull table with distinctive diagnostic characters (see below). Langston (Reference Langston1965) used this material to reconstruct this part of the skull of Caiman cf. lutescens but there is not enough evidence to justify that assumption (see Brochu Reference Brochu, Rowe, Brochu and Kishi1999). MACN PV 13551 (Fig. 9) corresponds to a big sized adult caimanine, with supraoccipital extended on the skull table. As in Purussaurus, the skull table has a U- or V- like posterior margin with a table surface deeply convex along its sagittal line, relatively broad interorbital space, orbits probably not extended rostro-caudally, frontal extremely short with a reduced rostral process and squamosals laterally elevated with moderated caudo-lateral bumps. Nevertheless, contrary to Purussaurus and as in Caiman and Melanosuchus, supratemporal fenestrae are relatively small.

Figure 9 Caiman lutescens: MACN 13552, skull table in (A) dorsal and (B) occipital views. Abbreviations: fr=frontal; pa=parietal; pfr=prefrontal; po=postorbital; so=supraoccipital; sq=squamosal.

The presence of C. lutescens was mentioned for the Upper Miocene of Venezuela by the record of a left rostral fragment (Sánchez-Villagra & Aguilera Reference Sánchez-Villagra and Aguilera2006, fig. 3P, Q; Scheyer & Moreno-Bernal Reference Scheyer, Moreno-Bernal, Sánchez-Villagra, Aguilera and Carlini2010). Although its general morphology is similar to C. latirostris, the skull table configuration of this specimen is unknown so it cannot be assigned to C. lutescens. Future analysis of this material will clarify the geographic distribution of C. latirostris in the Upper Miocene. From the available fossil evidence, we conclude that C. lutescens (only known from a skull table) and C. latirostris were found in the Upper Miocene in the area of Paraná. Caiman. latirostris is one of the extant crocodylian species with the largest temporal distribution and represented in the Miocene of South America by large-sized specimens.

Caiman cf. yacare

Referred material. MLP 73-IV-15-5, MLP 73-IV-15-6, right dentary fragments; MLP 73-IV-5-17, MACN PV 5417 fragments of right maxilla.

Occurrence. From the late Miocene; banks of Paraná River, in the area of Paraná Entre Ríos province, Argentina (Fig. 1); Ituzaingó Formation (Herbst Reference Herbst1971), (Brandoni & Scillato-Yané Reference Brandoni and Scillato-Yané2007; Brandoni Reference Brandoni2011; Fig. 2) to the Recent (centre and NE of Argentina, Paraguay, Bolivia, and part of western Brazil).

Description. The following description is based on fragmentary material that exhibits morphological similarities with the extant species C. yacare. MLP 73-IV-15-5 and MLP 73-IV-15-6, correspond to rostral (symphyseal) fragments of dentary that belong to three different individuals. Similar to C. yacare, in these specimens the symphysis extends back to the anterior margin of the 5th alveolus, and is oriented such that it forms a 20° (MLP 73-IV-15-5) or 21° (MLP 73-IV-15-6) angles with the medial mandibular margin (Fig. 10B). In dorsal view, the preserved alveoli show a similar pattern of that of C. yacare. In MLP 73-IV-15-5 (Fig. 10B), the posterior part of the first to the 11th and the anterior part of 12th alveoli are preserved. The first and fourth alveoli are the largest, the fifth to seventh alveoli decrease in length, then the eighth to the tenth alveoli increase again and the 11th alveolus is large. As in C. yacare, the fragments of the premaxilla here assigned to Caiman cf. yacare shows the lateral and laterocaudal margin of the nostril with a broad edge and a lateral margin sub-parallel to the lateral margin of premaxilla. It is remarkable that MLP 73-IV-15-15 belongs to a large specimen with a lateral width equal to 10·4 cm (see comments below).

Figure 10 (A) Caiman yacare MLP R 5045, anterior sector of mandible in dorsal view. (B) Caiman cf. yacare MLP 73 IV 15 5, right dentary fragment in dorsal view. Abbreviations: d=dentary; sp=splenial; 4°d=4° dentary alveolus.

Observations. The first mention of Caiman cf. yacare in the “Mesopotamiense” was made by Gasparini (Reference Gasparini1973), based on several mandibular remains, and was later accepted in other works (Gasparini Reference Gasparini1981, Reference Gasparini1996; Cione et al. Reference Cione, Azpelicueta, Bond, Carlini, Casciotta, Cozzuol, de la Fuente, Gasparini, Goin, Noriega, Scillato-Yané, Soibelzon, Tonni, Verzi, Vucetich, Aceñolaza and Herbst2000; Piña & Argañaraz Reference Piña, Argañaraz, Aceñolaza and Herbst2000). All the dentary fragments here assigned to Caiman cf. yacare shared morphological features with Caiman yacare (symphysis extended at posterior part of 5th alveolus and forming a 20°–30° angle with the medial margin of the mandible). The isolated fragments of the premaxilla have a broad ridge at the lateral and laterocaudal margin of nostrils (a feature present in extant specimens of this species). The first mention to a late Miocene occurrence of the extant C. yacare was made by Fortier et al. (Reference Fortier, Brochu and Souza Filho2009), based on a skull from the Niterói outcrops of the Solimões Formation in northern Brazil. The record of dentary remains, together with these premaxillary fragments from the Parana area, reinforce the conclusion that C. yacare was already differentiated, but also show that it had a widespread distribution in the late Miocene. Moreover, also it shows that the Miocene specimens would have reached larger sizes than the extant ones.