Evolutionary novelty in a rat with no molars

Abstract

Rodents are important ecological components of virtually every terrestrial ecosystem. Their success is a result of their gnawing incisors, battery of grinding molars and diastema that spatially and functionally separates the incisors from the molars. Until now these traits defined all rodents. Here, we describe a new species and genus of shrew-rat from Sulawesi Island, Indonesia that is distinguished from all other rodents by the absence of cheek teeth. Moreover, rather than gnawing incisors, this animal has bicuspid upper incisors, also unique among the more than 2200 species of rodents. Stomach contents from a single specimen suggest that the species consumes only earthworms. We posit that by specializing on soft-bodied prey, this species has had no need to process food by chewing, allowing its dentition to evolve for the sole purpose of procuring food. Thus, the removal of functional constraints, often considered a source of evolutionary innovations, may also lead to the loss of the very same traits that fuelled evolutionary diversification in the past.

Biostratigraphical and palaeoecological implications of the small mammal assemblage from the late early Miocene of Montalvos2, Teruel Basin, Spain

“…Subsequent workers in the field have commented on its unusual long tubular diastema with weak lower incisors that end below the m2 in combination with the complex fragile high-crested cheek teeth. In this context, it is of interest that the extant endemic shrew rats from Sulawesi and the Philippines which feed on soft-bodied invertebrates have mandibles that are very similar to those of Melissiodon (Musser 1990;Esselstyn et al 2012). The combination of a long and weak mandible and high-crested cheek teeth in Melissiodon suggests that these murids were also feeding on insects (beetles?…”

High resolution images for 'Morphological and geographic definitions of the Sulawesian shrew rats Echiothrix leucura and E. centrosa (Muridae, Murinae), and description of a new species of sucking louse (Phthiraptera, Anoplura).' (Bulletin of the American Museum of Natural History, no. 391)

“…Large ears may also help detect scraping movement of prey as they move through and beneath decaying leaves on the forest floor. The front claws are not as large relative to body size and don't seem to be as adapted for digging as those seen in species of either Melasmothrix and Tateomys (Musser, 1982) or Paucidentomys (Esselstyn et al, 2012), the other Sulawesian shrew rats; the claws of Echiothrix are stout and appear sufficiently robust to excavate earthworms from moist subsurface soil layers and to dig into old tree falls lying on the forest floor where the wet wood has decayed to a soft and spongy texture. One female E. centrosa, caught in a Conibear trap set on a decaying limb bridging the Sungai Sadaunta, had soil and debris matted on the fur beneath the chin and on parts of the front legs.…”

“…Among murines, this osteological design of the pterygoid-alisphenoid region is repeated only in the Philippine shrew rat Rhynchomys (see the cranial illustrations in Musser and Heaney [1992: 78] and Balete et al [2007: 293]) and the Sulawesian shrew rat Paucidentomys (Esselstyn, et al, 2012). The best way to appreciate this severe modification is to first describe the pterygoid region in the Sulawesian Maxomys dollmani, which expresses a pattern common to nearly all other murines, especially those from the Indomalayan region, Sulawesi, Philippines, New Guinea, and Australia (see the cranial illustrations in Musser, 1982Musser, , 1991Musser and Newcomb, 1983;Musser and Holden, 1991;Musser and Heaney, 1992;Flannery, 1995;Musser et al, 2008;Musser and Lunde, 2009;Heaney et al, 2012;Balete et al, 2012), as well as species in Europe and Africa (see the cranial drawings in Happold, 2013).…”

“…4). By contrast, the montane Sulawesian shrew rats Melasmothrix naso, Tateomys rhinogradoides, and Paucidentomys vermidax, all consumers of earthworms, bear elongate curved claws that are very long relative to size of the foot (Musser, 1982;Esselstyn et al, 2012).…”

“…Echiothrix and and the two species of this genus defined here, along with four other genera and five species of described shrew rats-Melasmsothrix naso, Tateomys rhinogradoides and T. macrocercus (Musser, 1982), Sommeromys macrorhinos (Musser and Durden, 2002), and Paucidentomys vermidax (Esselstyn et al, 2012)-and two additional undescribed genera are among the 15 known genera and more than 50 species of murids endemic to the island of Sulawesi (Musser, in press). Of these shrew rats, the diversity of species in Echiothrix, their range over the island, and their associated ecologies are feebly documented and require serious inquiry.…”