''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. Its species are considered examples of insular gigantism. They were endemic to the

Balearic Islands The Balearic Islands are an archipelago in the western Mediterranean Sea, near the eastern coast of the Iberian Peninsula. The archipelago forms a Provinces of Spain, province and Autonomous communities of Spain, autonomous community of Spain, ...

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 east by the Levant in West Asia, on the north by Anatolia in West Asia and Southern ...

from the

Early Pliocene Early may refer to: Places in the United States * Early, Iowa, a city * Early, Texas, a city * Early Branch, a stream in Missouri * Early County, Georgia * Fort Early, Georgia, an early 19th century fort Music * Early B, stage name of Jamaican d ...

until their extinction around the

3rd millennium BC File:3rd millennium BC montage.jpg, 400x400px, From top left clockwise: Pyramid of Djoser; Khufu; Great Pyramid of Giza, one of the Seven Wonders of the Ancient World; Cuneiform, a contract for the sale of a field and a house; Enheduana, a high pr ...

. They first appeared in the fossil record on Mallorca during the Early Pliocene (around 5 million years ago), presumably as a result to the evaporation of the Mediterranean sea during the

Messinian salinity crisis In the Messinian salinity crisis (also referred to as the Messinian event, and in its latest stage as the Lago Mare event) the Mediterranean Sea went into a cycle of partial or nearly complete desiccation (drying-up) throughout the latter part of ...

(5.96-5.33 million years ago) connecting the Balearic Islands with mainland Europe. They later spread to

Menorca Menorca or Minorca (from , 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 Mallorca. Its capital is Maó, situated on the isl ...

, and a possible molar is also known from

Ibiza Ibiza (; ; ; #Names and pronunciation, see below) or Iviza is a Spanish island in the Mediterranean Sea off the eastern coast of the Iberian Peninsula. It is 150 kilometres (93 miles) from the city of Valencia. It is the third largest of th ...

. ''Hypnomys'' became extinct during the late Holocene (around 4500-4000 years ago) likely shortly 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 Mallorca, or Majorca, is the largest of the Balearic Islands, which are part of Spain, and the List of islands in the Mediterranean#By area, seventh largest island in the Mediterranean Sea. The capital of the island, Palma, Majorca, Palma, i ...

in the Balearic Islands by British palaeontologist

Dorothea Bate Dorothea Minola Alice Bate (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 understandi ...

, with remains also found by Bate on

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

Leithia ''Leithia'' is an extinct genus of giant dormice from the Pleistocene of 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 e ...

'', but in 1918 described the remains into the new genus ''Hypnomys,'' describing two species, ''H. morpheus'' on Mallorca, and ''H. mahonensis'' on Menorca.

Taxonomy and evolutionary history

Mitochondrial DNA Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the D ...

from ''H. morpheus'' indicates that ''Hypnomys'' is a member of the subfamily Leithiinae, and closely related to the genus '' Eliomys,'' which contains (among others) the European garden dormouse (''Eliomys quercinus''). 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 nucleot ...

between modern species of ''Eliomys'' and ''Hypnomys'' in a 2019 study was 13.67 million years ago. Cladogram of dormice showing the placement of ''Hypnomys'' after Bover et al. 2020 and Petrova et al. 2024. 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 epoch (geology), 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 mea ...

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 is a narrow strait that connects the Atlantic Ocean to the Mediterranean Sea and separates Europe from Africa. The two continents are separated by 7.7 nautical miles (14.2 kilometers, 8.9 miles) at its narrowest point. Fe ...

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 islan ...

, 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 The Zanclean flood or Zanclean deluge is theorized to have refilled the Mediterranean Sea 5.33 million years ago. This flooding ended the Messinian salinity crisis and reconnected the Mediterranean Sea to the Atlantic Ocean, although it i ...

which refilled the Mediterranean approximately 5.3 million years ago, at the beginning of the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 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 and murids were present, by the

Late Pliocene Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * Late (The 77s album), ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudo ...

, ''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 The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

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

(Eivissa), of an uncertain Plio-Pleistocene age; however the assignment to ''Hypnomys'' is not definitive.

Species

''Hypnomys'' is divided into a number of species, spanning from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Holocene The Holocene () is the current geologic time scale, geological epoch, beginning approximately 11,700 years ago. It follows the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene to ...

. These are largely considered to be

chronospecies A chronospecies is a species derived from a 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 population that is p ...

(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 ( 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 the ge ...

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

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 and a tail length of around . A specimen of ''H. morpheus'' measured in the same study was found to have a head and body length of about and a tail length of about . The tail lengths are proportionally shorter than in species of ''Eliomys''. The specimen of ''H. morpheus'' was estimated to weigh between . 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 Dental microwear analysis is a method to infer diet and behavior in extinct animals, especially in fossil specimens. It has been used on a variety of taxa, including hominids, victoriapithecids, amphicyonids, canids, ursids, hyaenids, hyaenodont ...

study of ''H. morpheus'' the high number of fine scratches on the teeth suggests that the species was more omnivorous than the garden dormouse (which is heavily carnivorous), with the presence of pits on the teeth indicating the intake of hard food such as nuts and seeds, or grit, An analysis of the morphology of the lower jaw suggests that was probably efficient at gnawing and chewing. 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 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 (scansorial), but others are exclusively arboreal. The hab ...

, 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 (although not the same as) raptors, are hypercarnivorous bird species that actively predation, hunt and feed on other vertebrates (mainly mammals, reptiles and smaller birds). In addition to speed ...

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

. 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 The wood mouse (''Apodemus sylvaticus'') is a Muridae, 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 ...

(''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