''Hypnomys'', otherwise known as Balearic giant dormice, is an extinct genus of

dormouse A dormouse is a rodent of the family Gliridae (this family is also variously called Myoxidae or Muscardinidae by different taxonomists). Dormice are nocturnal animals found in Africa, Asia, and Europe. They are named for their long, dormant hibe ...

(Gliridae) in the subfamily

Leithiinae Leithiinae is a subfamily of dormice. It is named after the ''Leithia'', an extinct genus of giant dormouse from the Pleistocene of Sicily. Classification Subfamily Leithiinae *Genus ''Chaetocauda'' ** Chinese dormouse, ''Chaetocauda sichuanensi ...

. Its species are considered examples of

insular gigantism Island gigantism, or insular gigantism, is a biological phenomenon in which the size of an animal species isolated on an island increases dramatically in comparison to its mainland relatives. Island gigantism is one aspect of the more general Fos ...

. They were endemic to the

Balearic Islands The Balearic Islands ( es, Islas Baleares ; or ca, Illes Balears ) are an archipelago in the Balearic Sea, near the eastern coast of the Iberian Peninsula. The archipelago is an autonomous community and a province of Spain; its capital is ...

in the western

Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the e ...

from the

Early Pliocene Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

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 The Messinian salinity crisis (MSC), also referred to as the Messinian event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation (dr ...

causing a connection with mainland Europe. They later spread to

Menorca Menorca or Minorca (from la, Insula Minor, , smaller island, later ''Minorica'') is one of the Balearic Islands located in the Mediterranean Sea belonging to Spain. Its name derives from its size, contrasting it with nearby Majorca. Its capi ...

, and a possible molar is also known from

Ibiza Ibiza (natively and officially in ca, Eivissa, ) is a Spanish island in the Mediterranean Sea off the eastern coast of the Iberian Peninsula. It is from the city of Valencia. It is the third largest of the Balearic Islands, in Spain. Its l ...

. ''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 ''Asoriculus'' is an extinct genus of terrestrial shrews in the subfamily Soricinae (red-toothed shrews) and tribe Nectogalini. The best known species, ''Asoriculus gibberodon'' was widespread in Europe from the Late Miocene to the Early Pleisto ...

'' and goat-antelope ''

Myotragus ''Myotragus'' (New Latin, Neo-Latin, derived from the Ancient Greek language, Greek: , and "Balearian mouse-goat"), is an extinct genus of goat-antelope in the tribe Caprini which lived on the Balearic Islands of Mallorca and Menorca in the we ...

''.

History of discovery

The first remains of ''Hypnomys'' were discovered in 1910 on the island of

Mallorca Mallorca, or Majorca, is the largest island in the Balearic Islands, which are part of Spain and located in the Mediterranean. The capital of the island, Palma, is also the capital of the autonomous community of the Balearic Islands. The Bal ...

in the Balearic Islands by British palaeontologist

Dorothea Bate Dorothea Minola Alice Bate FGS (8 November 1878 – 13 January 1951), also known as Dorothy Bate, was a Welsh palaeontologist and pioneer of archaeozoology. Her life's work was to find fossils of recently extinct mammals with a view to underst ...

, with remains also found by Bate on

a year later. Upon first examination, Bate considered the fossils to represent those of ''

Eliomys ''Eliomys'' ( grc-gre, Ἡλίομυς ''Hēlíomus'') is a genus of rodent in the family Gliridae. It contains the following extant species: * Asian garden dormouse, ''Eliomys melanurus'' * Maghreb garden dormouse, ''Eliomys munbyanus'' * Gard ...

'' or ''

Leithia ''Leithia'' is a genus of extinct giant dormice from the Mediterranean islands of Malta and Sicily. It is considered an example of island gigantism. ''Leithia melitensis'' is the largest known species of dormouse, living or extinct, being twi ...

'', 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 Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...

from ''H. morpheus'' indicates that ''Hypnomys'' is a member of the subfamily

, and is more closely related to ''

'' (garden dormice) than to ''

Dryomys ''Dryomys'' is a genus of dormouse. Collectively the members of the genus are referred to as forest dormice, although the type species also goes by the common name forest dormouse. Species The species within the genus ''Dryomys'' are: *''Dryomys ...

'' (forest dormice) or ''

Muscardinus The hazel dormouse or common dormouse (''Muscardinus avellanarius'') is a small mammal and the only living species in the genus ''Muscardinus''. Distribution and habitat The hazel dormouse is native to northern Europe and Asia Minor. It is the ...

'' (hazel dormouse). The divergence estimated by

molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleoti ...

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 The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

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

(5.96–5.3 million years ago), an event when the

Strait of Gibraltar The Strait of Gibraltar ( ar, مضيق جبل طارق, Maḍīq Jabal Ṭāriq; es, Estrecho de Gibraltar, Archaic: Pillars of Hercules), also known as the Straits of Gibraltar, is a narrow strait that connects the Atlantic Ocean to the Medi ...

closed and the Mediterranean evaporated, with the resulting sea level drop causing the exposure of the

continental shelf A continental shelf is a portion of a continent that is submerged under an area of relatively shallow water, known as a shelf sea. Much of these shelves were exposed by drops in sea level during glacial periods. The shelf surrounding an island ...

, 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 The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58 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 Hamsters are rodents (order Rodentia) belonging to the subfamily Cricetinae, which contains 19 species classified in seven genera.Fox, Sue. 2006. ''Hamsters''. T.F.H. Publications Inc. They have become established as popular small pets. The b ...

and

murids In Sufism, a ''murīd'' (Arabic مُرِيد 'one who seeks') is a novice committed to spiritual enlightenment by ''sulūk'' (traversing a path) under a spiritual guide, who may take the title murshid, '' pir'' or ''shaykh''. A ''sālik'' or Su ...

were present, by the

Late Pliocene Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

, ''Hypnomys'' represented one of three mammals present in Mallorca, alongside the goat-antelope ''

'' and the shrew ''

''. ''Hypnomys, Myotragus and Nesiotites'' dispersed from Mallorca to Menorca during the Pliocene-

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

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 The Quaternary glaciation, also known as the Pleistocene glaciation, is an alternating series of glacial and interglacial periods during the Quaternary period that began 2.58 Ma (million years ago) and is ongoing. Although geologists describe ...

. A tooth possibly belonging to ''Hypnomys'' is known from the Cova de ca na Reia site on

(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 The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

. These are largely considered to be

chronospecies A chronospecies is a species derived from a anagenesis, sequential development pattern that involves continual and uniform changes from an extinct ancestral form on an evolutionary scale. The sequence of alterations eventually produces a populatio ...

(i.e. to have sequentially evolved from the previous species). As with many extinct mammal species, they are largely distinguished by dental anatomy. * ''Hypnomys waldreni'' Reumer 1979 (Middle-Late Pliocene, Mallorca) * ''Hypnomys onicensis'' Reumer 1994 (formerly ''H. intermedius'' Reumer, 1981, renamed due to a species of ''Eliomys'' with the same name) (Early Pleistocene, Mallorca) * ''Hypnomys eliomyoides'' Agustí 1980 (Early Pleistocene, Menorca) * ''Hypnomys morpheus'' Bate 1918 (Middle Pleistocene-Holocene, Mallorca) * ''Hypnomys mahonensis'' Bate, 1918 (type) (Late Pleistocene-Holocene, Menorca) 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 In biology, a subgenus (plural: subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between t ...

of ''Eliomys'' by Zammit Maempel and de Bruijn, 1982 it has generally been considered distinct by other authors. The Late Pleistocene-Holocene Menorcan ''H. mahonesis'' is distinguished from ''H. morpheus'' by its simpler teeth morphology and generally larger body size, 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

Hypnomys morpheus&Eliomys quercinus skulls

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 Robustness is the property of being strong and healthy in constitution. When it is transposed into a system, it refers to the ability of tolerating perturbations that might affect the system’s functional body. In the same line ''robustness'' ca ...

. The robustness of the mandibles and

zygomatic arches In anatomy, the zygomatic arch, or cheek bone, is a part of the skull formed by the zygomatic process of the temporal bone (a bone extending forward from the side of the skull, over the opening of the ear) and the temporal process of the zygomati ...

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 Dental microwear analysis is a method to infer diet and behavior in extinct animals, especially in fossil specimens. Typically, the patterns of pits and scratches on the occlusal or buccal surface of the enamel are compared with patterns observed ...

study of ''H. morpheus'' the high number of fine scratches on the teeth suggests that the species was more omniviorous than the

garden dormouse The garden dormouse (''Eliomys quercinus'') is a rodent in the dormouse family. Characteristics The garden dormouse is gray or brown, with a whitish underside. It has black eye markings and large ears. Its hair is short, and its tail has a whi ...

, 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 Arboreal locomotion is the Animal locomotion, locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. Th ...

, and possibly also had fossorial (digging) capabilities. While the Balearic Islands lacked large terrestrial predators, ''Hypnomys'' was hunted by

birds of prey Birds of prey or predatory birds, also known as raptors, are hypercarnivorous bird species that actively hunt and feed on other vertebrates (mainly mammals, reptiles and other smaller birds). In addition to speed and strength, these predators ...

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 ''

'' and the goat-antelope ''

, Hypnomys'' likely rapidly became extinct after human arrival in the Balearic islands during the mid-late

3rd millennium BC The 3rd millennium BC spanned the years 3000 through 2001 BC. This period of time corresponds to the Early to Middle Bronze Age, characterized by the early empires in the Ancient Near East. In Ancient Egypt, the Early Dynastic Period is followe ...

. 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

(''Eliomys quercinus'') and

wood mouse The wood mouse (''Apodemus sylvaticus'') is a murid rodent native to Europe and northwestern Africa. It is closely related to the yellow-necked mouse (''Apodemus flavicollis'') but differs in that it has no band of yellow fur around the neck, ha ...

(''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.

References

{{Taxonbar, from=Q3787901 Rodent genera Prehistoric Europe Pleistocene Europe Holocene extinctions Dormice