Myotragus

Balearic Islands cave goat Temporal range: Pliocene–Holocene
Myotragus balearicus skull from two angles
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Artiodactyla
Family:	Bovidae
Subfamily:	Caprinae
Tribe:	Caprini
Genus:	†Myotragus Bate, 1909
Type species
Myotragus balearicus Bate, 1909
Other species
M. palomboi Bover, Quintana & Alcover, 2010 M. pepgonellae Moyà-Solà & Pons-Moyà, 1982 M. antiquus Pons-Moyà, 1977 M. kopperi Moya & Pons, 1980 M. batei Crusafont & Angel, 1966

Myotragus (Neo-Latin, derived from the Greek: μῦς, τράγος and Βαλεαρίδες "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 western Mediterranean until its extinction around 4,500 years ago. The fossil record of Myotragus on the Balearic Islands extends over 5 million years back to the early Pliocene on Mallorca, where it presumably arrived after the major sea level drop during the Messinian Salinity Crisis. Myotragus is represented by a sequence of six sequential chronospecies representing a gradual change in morphology. The youngest and best known species, M. balearicus, is noted for a number of unusual morphological adaptions, including forward facing eyes suggestive of binocular vision, and a reptilian-like physiology. 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). M. balearicus became extinct when humans arrived in the Balearic Islands during the 3rd millennium BC.

History of discovery

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.

Description

Front half of the skeleton of Myotragus

Its eyes were not directed towards the sides, as are those of nearly all the herbivorous mammals, but towards the front like nearly all primates and carnivorans, granting them stereoscopic vision. The lower jaw contained two perennial-growth incisors, like rodents and lagomorphs, but not other ungulates. The lower jaw usually lacked other incisors, though some jaws have been found with vestigial second incisors. The upper jaw lacked incisors. The rest of its teeth were molars and premolars, adapted to the crushing of vegetable matter. The nose was short in comparison with the rest of the skull, similar to the noses of rabbits and hares. Finally, both sexes had at the top of the head two very short horns. It is possible these horns were longer, having short bone-bases and long horn-covers, but no complete horns have been found.

Myotragus balearicus was quite small in size (standing about 50 cm (20 in) at the shoulder) and weighed between 50 and 70 kg (110 and 150 lb). The legs were proportionately shorter than those of other related bovids, and less flexible, which did not make Myotragus balearicus exceptionally fast. This was not a serious problem because on the islands there were no predators except for some birds of prey, like the now-extirpated golden eagle, from which they probably hid in the vegetation. On the shoulders they had a pronounced hump, while the back was bent in the hindquarters. The legs, like many from the order Artiodactyla, had four fingers of which only two were used to walk. The tail was rather long in comparison to the rest of the body.

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, 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

Paleobiology

Restoration

Diet

Evidence from preserved coprolites of M. balearicus indicates that it was likely a browser, and heavily dependent on the native boxwood species Buxus balearica for a large part of its diet. The smooth texture of the coprolites indicates that the digestion was likely highly efficient.

Physiology and growth

The bone histology of M. balearicus shows lamellar-zonal tissue throughout the cortex, a feature otherwise typical of ectothermic reptiles. The growth of bones in Myotragus is unlike any other mammal and similar to crocodilians in showing slow and adaptive rates, intermittently ceasing growth altogether, and reaching somatic maturity by about 12 years. This pattern of growth indicates that Myotragus, in the same way as extant reptiles, adapted its metabolism to changing food and water availability, and ambient temperatures.

Movement

An analysis of the phalangeal bones of M. balearicus found that the bones of the foot were tightly bound by ligaments and inelastic. This suggests that Myotragus was obligately a slow walker with a reduced step length, and lacked the ability to jump. The likely reason for this is as an energy saving measure, as the shock absorbing mechanism in the foot bones of other caprines requires large amounts of muscle energy. The proximal and medial phalanges were likely orientated vertically relative to the ground surface, which reduced bending stresses.

Senses

The cranial endocast of M. balearicus indicates that the areas of the brain and structures associated with vision, sound and smell were strongly reduced when compared with other caprines. These likely represent optimisations to the animals energy budget, which were unnecessary in the absence of terrestrial predators.

Origins

Skull in oblique view

The unique characteristics of Myotragus balearicus are a consequence of a prolonged process of evolution on the islands (a clear example of island dwarfing). In this type of isolation, the ungulates tend to become smaller while rodents and lagomorphs increase their size, as happened to the Hypnomys, the giant dormouse that shared a habitat with Myotragus. Such species also tend to lose their fear reaction towards predators if none occur on the islands. A clear example of this is the loss of the capacity to run at high speed, the development of stereoscopic vision (which is useful to calculate distances, but not so to watch for predators) and the proportional reduction of the brain.

The analyses of DNA and the oldest fossils (Pliocene 5.3 million years ago) of the island of Majorca (Myotragus palomboi) indicate that Myotragus balearicus, in spite of being a browsing animal, descended originally from grazers. The closest fossil relatives of Myotragus have been suggested to be Aragoral mudejar and Norbertia hellenica from the Late Miocene of Europe. An analysis of complete mitochondrial genomes 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 is given below.

Bos   Caprini   Pantholops           †Bootherium (Helmeted muskox)     Ovibos (Musk ox)         Capricornis (Serow)     Naemorhedus (Goral)             Ovis (Sheep)               Oreamnos (Mountain goat)           Budorcas (Takin)           †Myotragus                 Rupicapra (Chamois)               Ammotragus (Barbary sheep)     Arabitragus (Arabian tahr)           Pseudois (Bharal)         Hemitragus (Himalayan tahr)     Capra (Markhor, ibexes, goats)

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, allowing a land connection between the Iberian Peninsula and the Balearics.

Later on, the opening of the straits and the massive salt water inflow isolated the animal populations, which diversified in the new Mediterranean islands created by tectonic forces. At the same time, climatic change replaced the vegetation of subtropical type with the present one of Mediterranean type, forcing Myotragus to develop drastic changes in its feeding and set of teeth.

Myotragus initially only colonized the island of Mallorca. On Ibiza a strange ecosystem without terrestrial mammals developed in which birds and bats were the main vertebrates, while in 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 Myotragus replaced the great Menorcan lagomorphs. Both islands separated again at the beginning of the Holocene.

Extinction

Front view of skull

Diverse datings indicate that the three native terrestrial mammals of Mallorca and Menorca (Myotragus balearicus, the giant dormouse Hypnomys morpheus and the large shrew Nesiotites hidalgo) disappeared all in the same very short period of time, during the third millennium BC. Historically there was debate as to whether the extinctions were caused by climate change, or whether they were exterminated by the first human settlers of the Balearic Islands.

The dominant theory is the one that postulates an extinction by human causes. Traditional methods had dated the first human colonization of the Balearic Islands towards 5000 BC or even before, but subsequent tests with modern methods of dating clearly indicate that there was no human presence before 3000 BC. This date agrees very closely with the fast decline of the three forms. The youngest remains of Myotragus date to around 2632 calibrated years BC, while the minimum date of human arrival on the Balearic Islands is currently 2282 BC. Extinction was likely rapid within less than 100 years of human arrival on the islands.

In 1969 it was suggested that Myotragus shows signs of domestication but was later found to have no empirical evidence supporting the idea.

See also

• List of extinct animals of Europe
• Messinian salinity crisis
• Schultz's rule
• Zanclean flood