Hypnomys
Hypnomys, otherwise known as Balearic giant dormice, is an extinct genus of dormouse (Gliridae) in the subfamily Leithiinae. Its species are considered examples of insular gigantism. They were endemic to the Balearic Islands in the western Mediterranean from the Early Pliocene until their extinction around 4,000 years ago. They first appeared in the fossil record on Mallorca during the Early Pliocene, presumably due to the Messinian salinity crisis causing a connection with mainland Europe. They later spread to Menorca, and a possible molar is also known from Ibiza. "Hypnomys" became extinct during the Holocene after human arrival on the  Balearics. They were one of only three native land mammals to the islands at the time of human arrival, alongside the shrew "Nesiotites" and goat-antelope "Myotragus". History of discovery. The first remains of "Hypnomys" were discovered in 1910 on the island of Mallorca in the Balearic Islands by British palaeontologist Dorothea Bate, with remains also found by Bate on Menorca a year later. Upon first examination, Bate considered the fossils to represent those of "Eliomys" or "Leithia", but in 1918 described the remains into the new genus "Hypnomys," describing two species, "H. morpheus" on Mallorca, and "H. mahonensis" on  Menorca. Evolutionary history. Mitochondrial DNA from "H. morpheus" indicates that "Hypnomys" is a member of the subfamily Leithiinae, and is more closely related to "Eliomys" (garden dormice) than to "Dryomys" (forest dormice) or "Muscardinus" (hazel dormouse). The divergence estimated by molecular clock between modern species of "Eliomys" and "Hypnomys" in a 2019 study was 13.67 million years ago. The ancestor of "Hypnomys" is often assumed to be the prehistoric "Eliomys" species "E. truci", known from the latest Miocene of the Iberian Peninsula. However, the molar complexity of the earliest dormice remains on Mallorca (presumably ancestral to "Hypnomys") and the deep  divergence between modern "Eliomys" and "Hypnomys" has led to suggestions that "Hypnomys" descended from a now extinct dormouse genus, possibly "Vasseuromys" or a closely related form. "Hypnomys" likely arrived in Mallorca during the Messinian salinity crisis (5.96–5.3 million years ago), an event when the Strait of Gibraltar closed and the Mediterranean evaporated, with the resulting sea level drop causing the exposure of the continental shelf, allowing dispersal from the Iberian Peninsula to the Balearic Islands, before the islands again became isolated following the reopening of the Straits of Gibraltar and the resulting Zanclean flood which refilled the Mediterranean approximately 5.3  million years ago, at the beginning of the Pliocene. Following this, the Balearic Islands were extremely remote, with no examples of terrestrial vertebrates arriving from the mainland in Mallorca and Menorca until human arrival during the late Holocene, allowing evolution to occur in long-term isolation. Although during the Early Pliocene some other mammals like hamsters and murids were present, by the Late Pliocene, "Hypnomys" represented one of three mammals present in Mallorca, alongside the goat-antelope "Myotragus" and the shrew "Nesiotites". "Hypnomys, Myotragus and Nesiotites" dispersed from Mallorca to Menorca during the Pliocene-Pleistocene transition as part of a  faunal turnover event replacing the fauna of Menorca, which had previously differed from Mallorca (containing species such as the giant rabbit "Nuralagus rex)", likely due to the islands being connected during episodes of low sea level as a result of Pleistocene glaciation. A tooth possibly belonging to "Hypnomys" is known from the Cova de ca na Reia site on Ibiza (Eivissa), of an uncertain Plio-Pleistocene age; however the assignment to "Hypnomys" is not definitive. Taxonomy. "Hypnomys" is divided into a number of species, spanning from the Pliocene to the Holocene. These are largely considered to be chronospecies (i.e. to  have sequentially evolved from the previous species). As with many extinct mammal species, they are largely distinguished by dental anatomy. Indeterminate remains of "Hypnomys" not assigned to species extend back to the Early Pliocene on Mallorca. The species "Hypnomys gollcheri" de Bruijn, 1966 from the Pleistocene of Malta has been assigned to the separate genus "Maltamys." Although "Hypnomys" was considered a subgenus of "Eliomys" by Zammit Maempel and de Bruijn, 1982 it has generally been considered distinct by other authors. The Late Pleistocene-Holocene Menorcan species "Hypnomys mahonensis" is considered to descend from a population of "Hypnomys morpheus" that migrated  to Menorca during the Late Pleistocene"," and is not descended from the preceding Menorcan species "Hypnomys eliomyoides." The only significant morphological difference between "H. mahonesis" and "H. morpheus" are the generally larger body size of the former, although the body size of "H. morpheus" varied substantially over the course of the glacial cycles, at times reaching sizes typical for "H. mahonesis." Description. The overall body size of "Hypnomys" is considerably larger than mainland dormice species, with the overall body size of the lineage gradually increasing over time. An articulated specimen of "Hypnomys" cf. "onicensis" measured in a 2010 study had  a head and body length of around 15 centimetres and a tail length of around 10 centimetres. A specimen of "H. morpheus" measured in the same study was found to have a head and body length of about 18 centimetres and a tail length of about 11.5 centimetres. The tail lengths are proportionally shorter than in species of "Eliomys". The specimen of "H. morpheus" was estimated to weigh between 173 and 284 grams. In comparison to species of "Eliomys", the skull and mandibles of "Hypnomys" species are substantially more robust, The robustness of the mandibles and zygomatic arches of the  skull indicate the presence of well developed masseter muscles. The limbs are also robust, with elongated zygopodiums (part of the limbs between the foot and elbow/knee) on both hind and forelimbs. Paleobiology. A 2016 bone histology study found that "H. onicensis" could live over 10 years, an exceptionally long lifespan in comparison to living "Eliomys", which can only live up to 5 years. Young juveniles were found to be already substantially larger than equivalently aged "Eliomys" individuals. Sexual maturity was also likely delayed in comparison to living "Eliomys". In a dental microwear study of "H. morpheus" the high number  of fine scratches on the teeth suggests that the species was more omniviorous than the garden dormouse, with the presence of pits on the teeth indicating the intake of hard food or grit. The lifestyle of "Hypnomys" has been debated. A 2010 study concluded that "H. morpheus" was more terrestrial than living dormice, based on morphological comparison of the bone proportions. However, a 2014 study disputed this, finding based on the proportions of the limb bones that "H. morpheus" was likely arboreal, and possibly also had fossorial (digging) capabilities. While the Balearic Islands lacked large terrestrial predators, "Hypnomys" was hunted  by birds of prey such as owls which are known to have inhabited the islands. Extinction. Like the two other endemic mammal genera on the Balearic islands, the shrew "Nesiotites" and the goat-antelope "Myotragus, Hypnomys" likely rapidly became extinct after human arrival in the Balearic islands during the mid-late 3rd millennium BC. The youngest current radiocarbon dates for "H. morpheus" are a few thousand years prior to human arrival, but later dates much closer to human arrival for "Nesiotites" and "Myotragus" suggest that it was also present at the time of arrival. Direct predation by humans is an  unlikely cause of extinction for "Hypnomys". Predators currently present on the Balearic Islands such as cats, weasels, martins and genets were introduced to the islands long after the extinction of the endemic mammals, and there is no compelling evidence for the early presence of dogs. The garden dormouse ("Eliomys quercinus") and wood mouse ("Apodemus sylvaticus") were early introductions to the islands and may have competed with "Hypnomys," though there is no concrete evidence that their existences overlapped. Diseases spread by introduced species may have contributed to the extinction.