Diego H. Verzi

Phylogenetic affinities of the late Miocene echimyid †Pampamys and the age of Thrichomys (Rodentia, Hystricognathi)

Abstract Phylogenetic affinities of the late Miocene echimyid †Pampamys emmonsae Verzi, Vucetich, and Montalvo, 1995 (Huayquerian South American Land Mammal Age, central Argentina), were analyzed. In the most-parsimonious tree obtained, subfamilies of Echimyidae were nonmonophyletic (except for Dactylomyinae). Two major clades were recovered. One of them included the living fossorial Eumysopinae and the extinct †Theridomysops. The other clade grouped the terrestrial eumysopines Thrichomys (punaré) and Proechimys–Trinomys (spiny rats), and the arboreal eumysopines Mesomys (spiny tree-rats) + Echimyinae–Dactylomyinae. †Pampamys was the sister genus of Thrichomys, suggesting the Huayquerian South American Land Mammal Age (>6.0 million years ago [mya] by biochronology) as a minimum age for the origin of the living genus. Both major echimyid clades recognized here are represented by simplified-molared species in the Huayquerian South American Land Mammal Age. This would be related to the expansion of open environments during the late Miocene, and the geographical bias of the fossiliferous Huayquerian deposits exposed mostly in southern South America.

The Role of Evolutionary Integration in the Morphological Evolution of the Skull of Caviomorph Rodents (Rodentia: Hystricomorpha)

The mammalian cranium is a complex structure composed by three partially independent modules: face, cranial base and cranial vault. At the same time, it interacts with the mandible by sharing the masticatory function. Since these units develop and work together, their function and evolution may occur through correlated changes. Here, we assessed the patterns of evolutionary shape variation and covariation (i.e. integration) of cranial modules and mandible among the highly ecomorphologically diverse caviomorph rodents, and the potential evolutionary consequences on the morphological evolution of this clade. Three-dimensional geometric morphometrics was used to describe cranio-mandibular shape. The phylogenetic signal and evolutionary allometric component of morphometric variables were analyzed; in addition, evolutionary covariation among cranial modules and mandible was assessed using phylogenetic comparative methods. Significant phylogenetic signal and evolutionary allometry were detected. Large covariance values, involving coordinated breadth increase as the main shape change, were recorded between cranial vault and base, followed by cranial vault and face, and face and mandible. Since the basicranium may be the main cranial integrator, the overall widening of the cranial base, derived from the enlargement of the auditory bullae, could be influencing the integrated evolution of skull. In caviomorphs, the cranio-mandibular morphological evolution would be the outcome of a tight covariation among the modular units, and this could be driven by several factors such as allometry and specializations to environmental niches.

A new Echimyidae (Rodentia, Hystricomorpha) from the late Miocene of southern South America

ABSTRACT n A new genus of echimyid rodent, †Ullumys, from the uppermost Miocene of northwestern Argentina is described. It includes two species, †U. pattoni, sp. nov., and †Ullumys intermedius, nov. comb. †Ullumys pattoni is known from a skull fragment and the corresponding right hemimandible coming from levels of Loma de Las Tapias Formation, underlying a tuff dated at 7.0 ± 0.9 Ma (Huayquerian age, San Juan Province). †Ullumys intermedius is represented by a left hemimandible from the ‘Araucanense’ of Valle de Santa María (Huayquerian age, Catamarca Province). A phylogenetic analysis in the context of octodontoids linked †Ullumys to the clade subtended by the extinct echimyids †Pampamys and †Eumysops and the living Thrichomys. †Ullumys has a peculiar craniomandibular morphology, shared only with †Eumysops among the Octodontoidea, involving specializations to open environments such as large and posteriorly extended orbits and related low mandibular condyles. Phylogenetic relationships of †Ullumys support the hypothesis that echimyids recorded since the latest Miocene in southern South America, and linked to those currently inhabiting Brazilian open biomes, represent a marginal sample of the great diversity evolving primarily in northern tropical areas.