Cranial shape variation in jacarean caimanines (Crocodylia, Alligatoroidea) and its implications in the taxonomic status of extinct species: The case of Melanosuchus fisheri

Abstract

Melanosuchus niger (Crocodylia, Alligatoroidea) is one of the six living caimanine species widely distributed throughout the Amazon River basin today. Although there is only one extant species of Melanosuchus, fossil material assigned to this genus, represented by M. fisheri, has been reported from the late Miocene in South America. However, the validity of this taxon has been questioned and a recent investigation indicates that the referred specimen of M. fisheri (MCZ 4336) actually belongs to Globidentosuchus brachyrostris, while those diagnostic characters present in the holotype (MCNC 243) fall into the spectrum of intraspecific variation of M. niger. Here, we compare the skull shape of the holotype of M. fisheri with the ontogenetic series of the four jacarean species (M. niger, Caiman yacare, Caiman crocodilus, and Caiman latirostris) using 2D-geometric morphometric analyses in two different views. The analyses indicate that MCNC 243 falls into the morphospace of M. niger and C. latirostris. Despite strong shape similarities between juveniles of C. latirostris and MCNC 243, further anatomical comparisons reveal notable differences between them. In contrast, no concrete anatomical differences can be found between MCNC 243 and M. niger, although shape analyses indicate that MCNC 243 is relatively robust for its size. Thus, this study is able to confirm that the genus Melanosuchus was present in the late Miocene, but it still remains unclear if MCNC 243 should be treated as a junior synonym or probably a sister species of M. niger. Its Miocene age favors the second option, but as the shape analyses were also not able to extract any diagnostic characters, it should be retained as Melanosuchus cf. niger.

Evidence for heterochrony in the cranial evolution of fossil crocodyliforms

“…snouts are relatively broader later in ontogeny; Bona & Desojo ; Foth et al . ). Nevertheless, quantitative investigations of possible heterochronic processes acting on the evolution of Crocodyliformes are rare (e.g.…”

“…; Watanabe & Slice ; Foth et al . ) allowed us to identify similarities between the morphological modifications during the ontogeny of Pissarrachampsa sera and the ontogenies of living taxa. For example, the best documented transformation is the relative reduction of the orbits, also found in living representatives of the three main lineages of Crocodylia: Gavialoidea, Crocodyloidea and Alligatoroidea (e.g.…”

“…; Foth et al . , ). Other common modifications previously reported include the mediolateral compression of the rostrum, although in Caiman latirostris the opposite process is observed (i.e.…”

Crocodylomorph cranial shape evolution and its relationship with body size and ecology

“…This is consistent with what was found in previous geometric morphometric studies (e.g. Pierce et al ., 2008; Sadleir & Makovicky, 2008; Pierce et al ., 2009; Piras et al ., 2009; Young et al ., 2010; Foth et al ., 2017; Wilberg, 2017; Godoy et al ., 2018), indicating that this region of the skull has the highest morphological variation. High variability in crocodylomorph snout length has long been acknowledged (Langston, 1973; Busbey, 1995; Brochu, 2001), even leading early taxonomists (e.g.…”

“…Previous works that use geometric morphometrics for studying crocodylomorph cranial shape have mostly focused on specific subgroups, especially crocodylians (Monteiro et al ., 1997; Pierce et al ., 2008; Sadleir & Makovicky, 2008; Piras et al ., 2009, 2010, 2014; Pearcy & Wijtten, 2011; Watanabe & Slice, 2014; Okamoto et al ., 2015; Clarac et al ., 2016; Salas-Gismondi et al ., 2016, 2018; Iijima, 2017; McCurry et al ., 2017 a ; Foth et al ., 2017; Bona et al ., 2018; Fernandez Blanco et al ., 2018; Morris et al ., 2019), but also thalattosuchians (Pierce et al ., 2009; Young et al ., 2010) and notosuchians (Godoy et al ., 2018). One important exception is the recent work of Wilberg (2017), that assessed cranial shape variation across Crocodyliformes (which is only slightly less inclusive than Crocodylomorpha; Irmis et al ., 2013), sampling a large number of species.…”