Etymology: The specific name comes from the Latin amicus for friend, honouring the community that has helped to protect the palaeontological site at Peirópolis, especially the members of the Associação dos Amigos do Sítio Paleontológico de Peirópolis (Association of the Friends of the Peirópolis Palaeontological Site).
Type locality: ‘Serra do Veadinho’, Municipality of Peirópolis, Minas Gerais State, south-eastern Brazil.
Diagnosis: Notosuchian that can be separated from all other members of this clade based on the following combination of characters (autapomorphies are indicated with an asterisk): the mandible is strongly anterodorsally projected; there is a symphyseal platform holding the teeth*; the surangular is anteriorly bifurcated; strong heterodont dentition both in size and shape, formed by teeth with circular and elliptical cross section; discrepancy of size of the first anterior pair of teeth that are much larger than all other mandibular teeth*; first anterior teeth strongly inclined anterodorsally; presence of small teeth on the medial side of the anterior portion of the dentary*; proportionally large number of the teeth, tightly packed, at the anterior symphysis*; presence of a symphyseal tooth battery.
Description and comparisons
The holotype of Labidiosuchus amicum consists of an incomplete lower jaw (DGM 1480-R) with the anterior part of the dentary complete, lacking most of the right mandibular ramus and the posterior end of the left mandibular ramus (Figs 1–4). The preserved part is about 70.5 mm long and clearly suggests that the jaw had a Y-shaped outline in dorsal view (Fig. 2).
Although the surface of some parts of this specimen is broken, there are no perceptible taphonomic deformations. The preservation of DGM 1480-R is the same as observed in other dinosaur bones collected in this area, also including other crocodylomorphs (e.g. Campos & Kellner, 1999; Kellner & Campos, 2000; Carvalho et al., 2004).
The dentaries are strongly bent dorsally with a concave dorsal and convex ventral margin, respectively. The anterior portion is projected anterodorsally at about 35°, similar to Adamantinasuchus navae (Nobre & Carvalho, 2006). Opposite dentaries are not fused to form an actual symphysis but one probably developed in ontogenetically older individuals. The contact region of opposite dentaries is longer than wide as in other notosuchians, differing from the putative notosuchians Anatosuchus and Simosuchus (Buckley et al., 2000; Sereno et al., 2003). The distal-most end of the dentaries forms a symphyseal platform, which consists of a horizontal shelf with a tightly packed set of teeth.
The lateral surface of the dentary ramus is flat and slightly compressed in the anterior to mid-sections, which can also be observed in Comahuesuchus, Chimaerasuchus, Malawisuchus, Mariliasuchus, Notosuchus, Simosuchus, the skull attributed to Sphagesaurus huenei (see Kellner et al., 2011 for details about the taxonomic status of Sphagesaurus), and Uruguaysuchus (Gasparini, 1971; Bonaparte, 1991; Wu & Sues, 1996; Gomani, 1997; Carvalho & Bertini, 1999; Buckley et al., 2000; Martinelli, 2003; Pol, 2003; Fiorelli & Calvo, 2008). The alveolar margins of the dentaries are flat and smooth and they do not project. All anterior-most alveoli are separated by thin septa; however, the distal-most alveoli are well separated by thick walls.
The mandibular fenestra is large and elliptical with the dorsal margin formed by the dorsal branch of the dentary and the anterior border of the surangular. The latter is bifurcated and has a lateral ramus contacting the dentary and a medial ramus directed to the splenial (Fig. 2), similar to Mariliasuchus and ‘Sphagesaurus’montealtensis (Carvalho & Bertini, 1999; Andrade & Bertini, 2008a). The anteromedial margin of the surangular is dorsally arched as in Comahuesuchus, Mariliasuchus, Notosuchus, Simosuchus, and ‘Sphagesaurus’montealtensis (Bonaparte, 1991; Carvalho & Bertini, 1999; Buckley et al., 2000; Andrade & Bertini, 2008a; Fiorelli & Calvo, 2008; Kellner et al., 2011), differing from the straight to near-straight margins observed in Anatosuchus, Araripesuchus gomesii, Malawisuchus, and Uruguaysuchus (Price, 1959; Gasparini, 1971; Gomani, 1997; Sereno et al., 2003).
The most striking feature of L. amicum is the dentition, with the teeth tightly packed in the symphyseal platform made by the dentaries. Altogether there are eight teeth present in this region, which is at a higher level relative to the remaining part of the lower jaw. As already noted, there is no taphonomic modification that could account either for the shape of the mandibular symphysis or for the particular position of the teeth. No tooth is complete, but almost every alveolus shows a partial tooth inside (Fig. 4). The first tooth (d1) is projected anterodorsally at about 35°, similar to the condition observed in Adamantinasuchus, Armadillosuchus, and Mariliasuchus (Carvalho & Bertini, 1999; Nobre & Carvalho, 2006; Marinho & Carvalho, 2009). However, in L. amicum this tooth is much larger than all others. Three teeth (d2−d4) follow, separated from each other by thin septa. The most anterior one (d2), the smallest of the jaw, is placed close to the midline. This tooth is unlikely to be a replacement tooth because it is far too small compared to the first tooth and located in the wrong position to be replacing the fourth tooth (d4), which is located posteriorly at the medial half of the jaw. The next tooth (d3) reaches the lateral margin of the dentary and has a rather elliptic transverse section (Fig. 4). Two other teeth (d5 and d6) follow, with elliptic and rounded transverse sections, respectively. These teeth are strongly obliquely implanted relative to the lateral margin of the dentary, with giroversion of about 50 and 40°, respectively. The oblique implantation of teeth is typical of the Sphagesauridae, although Mariliasuchus and Notosuchus also have some teeth with the main axis at an angle relative to the lateral margin of the jaw (Andrade & Bertini, 2008b, c). In Coringasuchus anisodontis the obliquely implanted teeth differ by their anterolingual to posterolabial direction (Kellner et al., 2009).
Posterior to those, a much larger tooth (d7) with a subcircular transverse section can be found, situated in a slightly dorsally projected part of the mandible. It is followed by a similar but slightly smaller tooth (d8), both being somewhat obliquely implanted.
The last tooth (d8) is positioned at the diverging point of the mandibular rami and can only be observed on the right side (Fig. 5). It is not fully broken and shows a well striated surface. Small ridges are located in a small posterolingual carina, differing from the robust ridges observed in sphagesaurids (Price, 1950; Pol, 2003). The tooth apex is acute like in Chimaerasuchus, Malawisuchus, and Uruguaysuchus (Gasparini, 1971; Wu & Sues, 1996; Gomani, 1997) with a single cusp. There is no constriction at the transitional region from the crown to the root as reported in some notosuchians, such as Candidodon, Malawisuchus, Mariliasuchus, and ‘Sphagesaurus huenei’ (Gomani, 1997; Andrade & Bertini, 2008b, c).