Myotragus (Neo-Latin, derived from the Greek: , "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 western Mediterranean until its extinction around 4,300 years ago. The fossil record of Myotragus on the Balearic Islands extends over 5 million years back to the early Pliocene on Mallorca, thought to have arrived from the European mainland after the evaporation of the Mediterranean Sea during the Messinian Salinity Crisis at the end of the Miocene epoch (around 5.96-5.33 million years ago). Following the refilling of the Mediterranean at the beginning of the Pliocene, the Balearic Islands remained strongly isolated, allowing the evolution of Myotragus to occur over the next 5.3 million years with little outside influence.
Myotragus is represented by six sequential chronospecies showing progressive morphological change, including a reduction in body size, over the course of over 5 million years of evolution in isolation on the Balearics. The youngest and best-known species, M. balearicus, is noted for a number of unusual morphological adaptations, including shortened legs with stiffened feet, forward facing eyes suggestive of binocular vision, a reduced number of teeth as well as an evergrowing lower incisor, and a relatively long lifespan, which developed in an unusual ecosystem where only a few other mammal species were present, terrestrial predators were absent, and Myotragus functioned as the only major herbivore. M. balearicus became extinct when humans arrived in the Balearic Islands during the 3rd millennium BC (at minimum around 2282 BC), along with the large shrew Nesiotites and the giant dormouse Hypnomys, the only other terrestrial mammals native to the islands.
Early genetic research suggested that it was closely related to sheep of the genus Ovis; however, more recent research has indicated that its closest living relative is the takin (Budorcas taxicolor).
History of discovery and taxonomy The first remains of Myotragus were described by Dorothea Bate in 1909. Bate had been sent a letter by Robert Ashington Bullen, who informed her about a bone-bearing breccia deposit on the east of Mallorca, which prompted her to survey the island for Pleistocene aged cave deposits. Three such deposits were found, which yielded fragmentary remains of Myotragus, including a mostly complete skull, associated with a mandible and atlas vertebra, which was designated the type specimen of the new species and genus Myotragus balearicus. In 1915, Charles William Andrews described more material discovered in the intervening years, including material that had been discovered on Menorca.
Species Six sequential chronospecies of Myotragus have been named, representing 5 million years of gradual accumulated morphological change, including a reduction in body size and changes to the locomotor system, the teeth and the visual system. M. palomboi Bover, Quintana & Alcover, 2010 Early Pliocene, Mallorca M. pepgonellae Moyà-Solà & Pons-Moyà, 1982 Middle Pliocene, Mallorca M. antiquus Pons-Moyà, 1977 Late Pliocene (c. 3.6-2.6 Ma), Mallorca M. kopperi Moya & Pons, 1980 Early Pleistocene, Mallorca M. batei Crusafont & Angel, 1966 Middle Pleistocene, Mallorca, Menorca M. balearicus Bate, 1909 Late Pleistocene-Holocene, Mallorca, Menorca The species M. binigausensis Moyà-Solà and Pons-Moyà, 1980 from the Pleistocene of Menorca has been synonymised with M. batei. In 2005, a new genus Insulotragus, was erected by Bolver and Alcover, to house the earliest species that had previously been attributed to Myotragus, but this proposal was not widely accepted.
Evolution The closest fossil relatives of Myotragus are uncertain. A close relationship has been proposed to the genera Aragoral and Norbertia from the Late Miocene of mainland Europe, as well as the insular genera Ebusia from the Pliocene of Ibiza, and Nesogoral from the Early Pleistocene of Sardinia. A 2005 study of a partial mitochondrial genome suggested that Myotragus was the sister group to the genus Ovis, which includes sheep. However, analysis of a complete mitochondrial genome of M. balearicus published in 2019 found that its closest living relative is the takin (Budorcas taxicolor), native to the eastern Himalayas with an estimated divergence around 7.1 million years ago. A cladogram showing its position within Caprinae/Caprini is given below.
The ancestor of Myotragus likely arrived in the Balearic Islands during the Messinian stage of the late Miocene at a time at which the Strait of Gibraltar closed and the Mediterranean Sea evaporated, reducing sea level within the basin by 800–1200 metres, in an event called the Messinian salinity crisis (spanning from 5.96 to 5.33 million years ago), allowing dispersal from the Iberian Peninsula to the Balearics.
Later on, the re-opening of the straits and the resultant Zanclean flood, which refilled the Mediterranean at the beginning of the Pliocene around 5.3 million years ago isolated the animal populations on the islands. The Balearic Islands remained strongly isolated following the end of the Messinian Salinity Crisis, with no terrestrial vertebrate species dispersing to Mallorca and Menorca until modern human arrival on the islands during the Holocene, over 5 million years later. The changes in morphology Myotragus developed over the course of its evolution were probably driven by resource limitation on the relatively resource poor Balearics, with the lack of competitors leading to increased intraspecific competition, and the absence of effective predators meaning the population would periodically outstrip the carrying capacity of the islands, resulting in the overconsumption of vegetation faster than it could regrow, causing food resources to become heavily depleted, and as a consequence mass starvation, with only a small proportion of the population surviving a starvation episode, leading to strong selection pressure.
Myotragus initially only colonized the island of Mallorca. Only a handful of mammal species aside from Myotragus were able to colonise the island, including shrews, hamsters, dormice, murines (the subfamily including Old Worlds rats and mice) and rabbits. By the Late Pliocene, Myotragus represented one of only three genera of terrestrial mammal present on Mallorca, alongside the giant dormouse Hypnomys and the shrew Nesiotites, all of which would continue to be present on the island until the Holocene. On Menorca, a giant rabbit, Nuralagus rex evolved that covered the same niche as Myotragus in Mallorca. With the level of the sea falling due to glacial cycles during the Pleistocene, Mallorca and Menorca were periodically connected and the mammals of Mallorca, including Myotragus colonised Menorca, replacing the giant Menorcan rabbits. Following numerous cycles of connection and separation over the course of the Pleistocene, Menorca and Mallorca separated again at the beginning of the Holocene.
Description
The size of Myotragus varied between species, with later species on average smaller than earlier species as well as their mainland relatives, representing an example of insular dwarfism. The early species M. pepgonellae is estimated to have had a body mass of approximately , while the later M. kopperi is estimated to have been approximately . M. balearicus is estimated to have been approximately tall at the shoulder on average, with a 2004 study estimating an adult body mass of around , with another study estimating that it was around 18% the size of its late Miocene mainland relative/possible ancestor Aragoral. However, the size of adult M. balearicus individuals varied considerably, with one adult individual recorded as being only tall at the shoulder. Although Waldren (1999) wrote that "both sexes of [Myotragus balearicus] are in osteological appearance unusually similar", and that there are "no clear sexual differences [between the skeletons of females and males of M. balearicus]" Bover and Alcover (1999) wrote that M. balearicus was "a highly [sexually] dimorphic species at the adult stage". Assessing the presence of sized-based sexual dimorphism in M. balearicus is complicated by the high variance in size of both male and female individuals across time and space.
The limbs of M. balearicus, particularly the metapodial and phalange bones, are proportionally short in comparison to other caprines, with the tarsals, metatarsals and sesamoids being partially fused. The abdominal ribcage of M. balearicus is very broad in comparison to other caprines, suggesting that it hosted relatively large organs, with the rumen in particular probably being enlarged relative to other caprines.
Over the course of its evolution, the rostrum, the frontmost part of the skull forward of the eyesockets, of Myotragus became progressively shorter, resulting in the skull of M. balearicus being both very short and very deep. The eye sockets (orbits) of the skull of M. balearicus are roughly half the size those of other comparably sized caprines, and face forward as opposed to the sides as in most ungulates and in earlier Myotragus species, allowing for binocular vision. The skull had a pair of small, directly backward projecting horns that diverge little from each other, which are set relatively widely apart from each other on the skull and have a roughly circular (subcircular) cross section.Van der Geer AE . 2005. Island ruminants and the evolution of parallel functional structures. Quaternaire 2: 231–240
. There is little difference in horn core shape between male and female individuals, though it has been suggested that some of the skull bones (the basioccipital, basisphenoid and parietal bone) are thicker in male individuals, and that there are subtle differences in horn core morphology.W.H. Waldren. (1999). Indications of possible sexual dimorphism in the horn cores and certain cranial bone components of the insular, endemic ruminant Myotragus balearicus. Deinsea, 7(1), 383–400.
The species of Myotragus show a progressive reduction in the number of teeth through time, with M. balearicus having an adult dentition comprising a single evergrowing (hypselodont) incisor (a feature highly unusual among Bovidae, the broader group containing caprines, bovines, and antelopes), one premolar, and three molars in each half of the lower jaw, and two premolars and three molars in each half of the upper jaw. By contrast, the earliest species had three incisors, one canine, and two premolars in each half of the lower jaw, as is typical of most ruminants with the incisors not being ever-growing, and a third premolar present in each half of the upper jaw. The teeth in the later species are much more hypsodont (high crowned) than in earlier species.
Paleobiology
Diet