A Miocene hyperdiverse crocodylian community reveals peculiar trophic dynamics in proto-Amazonian mega-wetlands

Abstract

Amazonia contains one of the world's richest biotas, but origins of this diversity remain obscure. Onset of the Amazon River drainage at approximately 10.5 Ma represented a major shift in Neotropical ecosystems, and proto-Amazonian biotas just prior to this pivotal episode are integral to understanding origins of Amazonian biodiversity, yet vertebrate fossil evidence is extraordinarily rare. Two new species-rich bonebeds from late Middle Miocene proto-Amazonian deposits of northeastern Peru document the same hyperdiverse assemblage of seven co-occurring crocodylian species. Besides the large-bodied Purussaurus and Mourasuchus , all other crocodylians are new taxa, including a stem caiman— Gnatusuchus pebasensis —bearing a massive shovel-shaped mandible, procumbent anterior and globular posterior teeth, and a mammal-like diastema. This unusual species is an extreme exemplar of a radiation of small caimans with crushing dentitions recording peculiar feeding strategies correlated with a peak in proto-Amazonian molluscan diversity and abundance. These faunas evolved within dysoxic marshes and swamps of the long-lived Pebas Mega-Wetland System and declined with inception of the transcontinental Amazon drainage, favouring diversification of longirostrine crocodylians and more modern generalist-feeding caimans. The rise and demise of distinctive, highly productive aquatic ecosystems substantially influenced evolution of Amazonian biodiversity hotspots of crocodylians and other organisms throughout the Neogene.

The late Miocene caimanine fauna (Crocodylia: Alligatoroidea) of the Urumaco Formation, Venezuela

“…A splenial symphysis is lacking in C. brevirostris (Fortier et al, 2014), but with the upper and lower jaw being in articulation, it cannot be determined whether the splenial partici-FIGURE 1. Simplified phylogenetic framework based on Scheyer et al (2013) and Salas-Gismondi et al (2015). The caimanine taxa present in the Urumaco Formation are marked in bold face.…”

“…As in MACN PV 5416, the fourth, ninth, and fourteenth maxillary alveoli are the largest in the series, but UCMP 39978 does not show strong lateral festooning of the skull margin in dorsal view (Rovereto, 1912;Langston, 1965). Salas-Gismondi et al (2015) erected a new caimanine species, Caiman wannlangstoni, from the late middle Miocene Pebas Formation of the Iquitos area in Peru, the holotype of which is a partial skull (MUSM 2377) with lateral margins that are strongly sinuous and distinctly diverging posteriorly in dorsal view. The authors indicate that specimen UCMP 39978 shows some affinities to C. wannlangstoni, but they treat its taxonomic status for the time being as a "La Venta Caiman" and a "distinct entity of uncertain taxonomic affinities."…”

“…Caiman latirostris (Daudin, 1802) and Caiman wannlangstoni (Salas-Gismondi, Flynn, Baby, Tejada-Lara, Wesselingh, and Antoine, 2015) The species Caiman lutescens was originally described as a species of Alligator from the Miocene "capas del Paraná" in northeastern Argentina FIGURE 4. Interpretative drawings of skull bone configuration in the rostro-orbital region of three small sized caimanines from the Urumaco Formation, in comparison to three living caimanines (anterior is to the top).…”

Reptiles Extintos - Volumen en Homenaje a Zulma Gasparini

“…Miocene record of crocodylians in the Pan-Amazonian region (sensu Hoorn et al, 2010) is characterized by the great taxonomic diversification of the caimanines clade (Langston, 1965;Riff et al, 2010;Bona et al, 2013a,b) with a wide geographic distribution of several genera (e.g., Mourasuchus, Purussaurus, Caiman) and local endemism at the species level (Bona et al, 2013a,b;Scheyer et al, 2013;Salas-Gismondi et al, 2015). It is known that during the Miocene continental vertebrates in South America reached huge body sizes (e.g., Cione et al, 2005;Vizcaíno et al, 2012;Vucetich et al, 2015).…”

“…The genus Mourasuchus is endemic to South America (Langston and Gasparini, 1997). One of the earliest mentions about these bizarre crocodylians was made by Langston (1965) (Brochu, 1999(Brochu, , 2010Aguilera et al, 2006;Bona, 2007;Bona et al, 2013a;Salas-Gismondi et al, 2015). This species was originally named by as Charandaisuchus nativus based on two posterior fragments of skull table.…”