Biology:Myotragus

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Short description: Extinct genus of mammals

Myotragus
Temporal range: Pliocene–Holocene 5.3–0.0046 Ma
Sóller. Museu Balear de Ciències Naturals. Myotragus balearicus (cropped).jpg
Skeleton of Myotragus balearicus
Scientific classification e
Domain: Eukaryota
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 "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.[1] 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 evaporation of the Mediterranean Sea during the Messinian Salinity Crisis.

Myotragus is represented by six sequential chronospecies representing 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, 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.

Early genetic research suggested that it was closely related to sheep of the genus Ovis;[2] however, more recent research has indicated that its closest living relative is the takin (Budorcas taxicolor).[3]

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.[4] In 1915, Charles William Andrews described more material discovered in the intervening years, including material that had been discovered on Menorca.[5]

Species

Six sequential chronospecies of Myotragus have been named,[6] 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.[7]

  • 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

The species M. binigausensis Moyà-Solà and Pons-Moyà, 1980 from the Pleistocene of Menorca has been synonymised with M. batei.[8]

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,[6] as well as the insular genera Ebusia from the Pliocene of Ibiza, and Nesogoral from the Early Pleistocene of Sardinia.[9] A 2005 study of a partial mitochondrial genome suggested that Myotragus was the sister group to the genus Ovis, which includes sheep.[2] 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.[3]

 
 

Bos

 
Caprini
 

Pantholops (Tibetan antelope)

 
 
 
 
 

†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 (spanning from 5.96 to 5.33 million years ago), allowing dispersal from the Iberian Peninsula to the Balearics.[10]

Later on, the re-opening of the straits and the refilling of the Mediterranean at the beginning of the Pliocene around 5.3 million years ago isolated the animal populations on the islands. 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 denudation of most vegetation and consequently mass starvation,[7][11][9] with only a small proportion of the population surviving a starvation episode, leading to strong selection pressure.[9]

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 and rabbits. By the Late Pliocene, Myotragus represented one of only three genera of 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.[12] On Menorca, a giant rabbit, Nuralagus rex evolved that covered the same niche as Myotragus in Mallorca.[13] 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 great Menorcan lagomorphs.[14] Both islands separated again at the beginning of the Holocene.

Description

Skull in dorsal view

The size of Myotragus varied between species, generally reducing with time. The early species M. pepgonellae is estimated to have had a body mass of approximately 60 kilograms (130 lb), while the later M. kopperi is estimated to have been approximately 23 kilograms (51 lb),[7] representing an example of insular dwarfism.[11] M. balearicus is estimated to have been approximately 50 centimetres (1.6 ft) tall at the shoulder,[15] with a 2004 study estimating an adult body mass of around 23–32 kg (51–71 lb).[7] The 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.[7] The skull had a pair of small posteriorly-directed horns.[15] The species of Myotragus show a sequential 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 bovids), 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.[16][6][17][18] 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[15] with the incisors not being ever-growing, and a third premolar present in each half of the upper jaw.[6] The teeth in the later species are much more hypsodont (high crowned) than in earlier species.[15] The limbs of M. balearicus are relatively short in comparison to other caprines, with the tarsals, metatarsals and sesamoids being partially fused.[19]

Paleobiology

Restoration

Diet

While tooth morphology and tooth texture suggests that some earlier Myotragus species may have been grazers or mixed feeders (consuming both grass and browse),[17] preserved coprolites of M. balearicus indicates that it was likely predominantly a browser, and heavily dependent on the native boxwood species Buxus balearica for a large part of its diet.[20][21] The increased hypsodonty over time of the teeth of Myotragus likely represents at least in part an adaptation to the increased consumption of abrasive food.[17]

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 M. balearicus is unlike any other mammal and similar to crocodilians in showing slow and adaptive rates, intermittently ceasing growth altogether, and reaching maximum size (somatic maturity) and probably sexual maturity at 12 years of age. This pattern of growth indicates that Myotragus, in the same way as extant ectothermic reptiles, adapted its metabolism to changing food and water availability, and ambient temperatures.[22] Newborn specimens of M. balearicus are estimated to have been approximately 15–18 centimetres (5.9–7.1 in) in height with a weight of about 700–900 grams (1.5–2.0 lb), approximately 2% the bodymass of a mature adult, much lower than that of a typical ruminant, in which newborns are usually over 4% the adult bodymass.[23]

Analysis of the high-crowned teeth of M. balearicus, shows that they grew more slowly than those of other caprines, with their last teeth erupting at approximately six years of age, likely as an adaption to their longevity.[11] Based on skeletochronology and dental durability analysis, some individuals of M. balearicus are likely to have reached a lifespan of 27 years, which is exceptionally long relative to its body size. The estimated mortality rates are substantially lower than those found for other members of Bovidae, with a large proportion of individuals surviving into old age.[24]

Movement and senses

An analysis of the phalanges (digits) of M. balearicus found that the bones of the foot were tightly bound by ligaments and inelastic. This suggests that it 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, with the ability to move quickly unnecessary on the largely predator-free islands. The proximal (closest to the ankle) and medial (intermediate between the furthest and closest) phalanges were likely orientated vertically relative to the ground surface, which reduced bending stresses.[19]

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, with the brain only being half the size of other comparably sized caprines. These likely represent optimisations to the animals energy budget, as neural tissue is energetically expensive to maintain, and much of it was unnecessary in a resource limited environment where there was little need to detect predators. The binocular vision of M. balearicus likely enhanced depth perception at the expense of the field of view. The outwards-facing eyes giving a large field of view typical of ungulates is thought to be an adaptation for vigilance against predators.[7]

Extinction

Diverse datings indicate that the three native terrestrial mammals of Mallorca and Menorca (Myotragus balearicus, the giant dormouse Hypnomys and the large shrew Nesiotites hidalgo) disappeared all in the same very short period of time, during the third millennium BC.[25] 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.[26][27]

In 1969 it was suggested that Myotragus shows signs of domestication[15] but a 2001 study found that there was no empirical evidence supporting the idea, with marks on Myotragus horns that were suggested to be made by humans were probably actually the result of gnawing on bones by other Myotragus, likely for their mineral content.[28]

See also

References

  1. ↑ Karolyn Shindler (2005). Discovering Dorothea : The Life of the Pioneering Fossil-Hunter Dorothea Bate. HarperCollins. ISBN 978-0002571388. 
  2. ↑ 2.0 2.1 Lalueza-Fox (6 December 2005). "Molecular dating of caprines using ancient DNA sequences of Myotragus balearicus, an extinct endemic Balearic mammal". BMC Evolutionary Biology 5: 70. doi:10.1186/1471-2148-5-70. PMID 16332256. 
  3. ↑ 3.0 3.1 Bover, Pere; Llamas, Bastien; Mitchell, Kieren J.; Thomson, Vicki A.; Alcover, Josep Antoni; Lalueza-Fox, Carles; Cooper, Alan; Pons, Joan (July 2019). "Unraveling the phylogenetic relationships of the extinct bovid Myotragus balearicus Bate 1909 from the Balearic Islands" (in en). Quaternary Science Reviews 215: 185–195. doi:10.1016/j.quascirev.2019.05.005. Bibcode: 2019QSRv..215..185B. https://linkinghub.elsevier.com/retrieve/pii/S0277379118306929. 
  4. ↑ Bate, Dorothea M. A. (September 1909). "I.—Preliminary Note on a New Artiodactyle from Majorca, Myotragus balearicus , gen. et sp. nov." (in en). Geological Magazine 6 (9): 385–388. doi:10.1017/S0016756800124665. ISSN 0016-7568. Bibcode: 1909GeoM....6..385B. https://www.cambridge.org/core/product/identifier/S0016756800124665/type/journal_article. 
  5. ↑ Andrews, Chas. W. (January 1915). "VI. A description of the skull and skeleton of a peculiarly modified rupicaprine antelope (Myotragus balearicus , Bate), with a notice of a new variety, M. balearicus var. major" (in en). Philosophical Transactions of the Royal Society of London. Series B, Containing Papers of a Biological Character 206 (325–334): 281–305. doi:10.1098/rstb.1915.0006. ISSN 0264-3960. Bibcode: 1915RSPTB.206..281A. 
  6. ↑ 6.0 6.1 6.2 6.3 Bover, Pere; Quintana, Josep; Alcover, Josep Antoni (November 2010). "A new species of Myotragus Bate, 1909 (Artiodactyla, Caprinae) from the Early Pliocene of Mallorca (Balearic Islands, western Mediterranean)" (in en). Geological Magazine 147 (6): 871–885. doi:10.1017/S0016756810000336. ISSN 0016-7568. Bibcode: 2010GeoM..147..871B. https://www.cambridge.org/core/product/identifier/S0016756810000336/type/journal_article. 
  7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 Köhler M.; Moyà-Solà S. (2004). "Reduction of Brain and Sense Organs in the Fossil Insular Bovid Myotragus". Brain, Behavior and Evolution 63 (3): 125–140. doi:10.1159/000076239. PMID 14726622. 
  8. ↑ Palombo, Maria Rita; Rozzi, Roberto; Bover, Pere (January 2013). "The endemic bovids from Sardinia and the Balearic Islands: State of the art" (in en). Geobios 46 (1–2): 127–142. doi:10.1016/j.geobios.2012.10.011. https://linkinghub.elsevier.com/retrieve/pii/S0016699513000120. 
  9. ↑ 9.0 9.1 9.2 Moyà-Solà, Salvador; Quintana Cardona, Josep; Köhler, Meike (2022-08-03). "Ebusia moralesi n. gen. nov. sp, a new endemic caprine (Bovidae, Mammalia) from the Neogene of Eivissa Island (Balearic Islands, Western Mediterranean): evolutionary implications" (in en). Historical Biology 34 (8): 1642–1659. doi:10.1080/08912963.2022.2060099. ISSN 0891-2963. 
  10. ↑ Mas, Guillem; Maillard, Agnès; Alcover, Josep A.; Fornós, Joan J.; Bover, Pere; Torres-Roig, Enric (2018-06-01). "Terrestrial colonization of the Balearic Islands: New evidence for the Mediterranean sea-level drawdown during the Messinian Salinity Crisis" (in en). Geology 46 (6): 527–530. doi:10.1130/G40260.1. ISSN 0091-7613. Bibcode: 2018Geo....46..527M. https://pubs.geoscienceworld.org/gsa/geology/article/46/6/527/530693/Terrestrial-colonization-of-the-Balearic-Islands. 
  11. ↑ 11.0 11.1 11.2 Jordana, Xavier; Köhler, Meike (February 2011). "Enamel microstructure in the fossil bovid Myotragus balearicus (Majorca, Spain): Implications for life-history evolution of dwarf mammals in insular ecosystems" (in en). Palaeogeography, Palaeoclimatology, Palaeoecology 300 (1–4): 59–66. doi:10.1016/j.palaeo.2010.12.008. https://linkinghub.elsevier.com/retrieve/pii/S0031018210007248. 
  12. ↑ Palombo, Maria Rita (January 2018). "Insular mammalian fauna dynamics and paleogeography: A lesson from the Western Mediterranean islands" (in en). Integrative Zoology 13 (1): 2–20. doi:10.1111/1749-4877.12275. ISSN 1749-4877. PMID 28688123. 
  13. ↑ Quintana, Josep; Kohler, Mike; Moya-Sola, Salvador (2011). "Nuralagus rex, gen. et sp. nov., an endemic insular giant rabbit from the Neogene of Menorca". Journal of Vertebrate Paleontology 31 (2): 231–240. doi:10.1080/02724634.2011.550367. 
  14. ↑ Pere Bover; Josep Quintanab; Josep Antoni Alcover (2008). "Three islands, three worlds: Paleogeography and evolution of the vertebrate fauna from the Balearic Islands". Quaternary International 182 (1): 135–144. doi:10.1016/j.quaint.2007.06.039. Bibcode: 2008QuInt.182..135B. 
  15. ↑ 15.0 15.1 15.2 15.3 15.4 Alexandra Van Der Geer; George Lyras; John De Vos (2010). Evolution of Island Mammals: Adaptation and Extinction of Placental Mammals on Islands. Wiley-Blackwell. pp. 132–146. ISBN 978-1405190091. 
  16. ↑ Bover, Pere; Alcover, Josep Antoni (November 1999). "The evolution and ontogeny of the dentition of Myotragus balearicus Bate, 1909 (Artiodactyla, Caprinae): evidence from new fossil data" (in en). Biological Journal of the Linnean Society 68 (3): 401–428. doi:10.1111/j.1095-8312.1999.tb01178.x. 
  17. ↑ 17.0 17.1 17.2 Winkler, Daniela Eileen; Schulz, Ellen; Calandra, Ivan; Gailer, Juan-Pablo; Landwehr, Christina; Kaiser, Thomas M. (January 2013). "Indications for a dietary change in the extinct Bovid genus Myotragus (Plio-Holocene, Mallorca, Spain)" (in en). Geobios 46 (1–2): 143–150. doi:10.1016/j.geobios.2012.10.010. https://linkinghub.elsevier.com/retrieve/pii/S0016699513000119. 
  18. ↑ Jordana, Xavier; Marín-Moratalla, Nekane; Moncunill-Solé, Blanca; Bover, Pere; Alcover, Josep Antoni; Köhler, Meike (2013-07-25). Laudet, Vincent. ed. "First Fossil Evidence for the Advance of Replacement Teeth Coupled with Life History Evolution along an Anagenetic Mammalian Lineage" (in en). PLOS ONE 8 (7): e70743. doi:10.1371/journal.pone.0070743. ISSN 1932-6203. PMID 23936247. 
  19. ↑ 19.0 19.1 Köhler, Meike; Moyà-Solà, Salvador (2001-08-22). "Phalangeal adaptations in the fossil insular goat Myotragus" (in en). Journal of Vertebrate Paleontology 21 (3): 621–624. doi:10.1671/0272-4634(2001)021[0621:PAITFI2.0.CO;2]. ISSN 0272-4634. http://www.tandfonline.com/doi/abs/10.1671/0272-4634%282001%29021%5B0621%3APAITFI%5D2.0.CO%3B2. 
  20. ↑ Alcover (1999). "The diet of Myotragus balearicus Bate 1909 (Artiodactyla: Caprinae), an extinct bovid from the Balearic Islands: evidence from coprolites". Biological Journal of the Linnean Society 66 (1): 57–74. doi:10.1111/j.1095-8312.1999.tb01917.x. 
  21. ↑ Welker, Frido; Duijm, Elza; van der Gaag, Kristiaan J.; van Geel, Bas; de Knijff, Peter; van Leeuwen, Jacqueline; Mol, Dick; van der Plicht, Johannes et al. (January 2014). "Analysis of coprolites from the extinct mountain goat Myotragus balearicus" (in en). Quaternary Research 81 (1): 106–116. doi:10.1016/j.yqres.2013.10.006. ISSN 0033-5894. Bibcode: 2014QuRes..81..106W. https://www.cambridge.org/core/product/identifier/S0033589400002106/type/journal_article. 
  22. ↑ Kohler, Meike; Moya-Sola, Salvador (2009). "Physiological and life history strategies of a fossil large mammal in a resource-limited environment". Proceedings of the National Academy of Sciences 106 (48): 20354–8. doi:10.1073/pnas.0813385106. PMID 19918076. Bibcode: 2009PNAS..10620354K. 
  23. ↑ P. Bover & J.A. Alcover. (1999). Estimating physical characteristics of neonate Myotragus balearicus (Artiodactyla, Caprinae). Deinsea, 7(1), 33–54.
  24. ↑ Jordana, Xavier; Marín-Moratalla, Nekane; DeMiguel, Daniel; Kaiser, Thomas M.; Köhler, Meike (2012-08-22). "Evidence of correlated evolution of hypsodonty and exceptional longevity in endemic insular mammals" (in en). Proceedings of the Royal Society B: Biological Sciences 279 (1741): 3339–3346. doi:10.1098/rspb.2012.0689. ISSN 0962-8452. PMID 22535784. 
  25. ↑ Pere Bover; Josep Antoni Alcover (2003). "Understanding Late Quaternary extinctions: the case of Myotragus balearicus (Bate, 1909)". Journal of Biogeography 30 (5): 771–781. doi:10.1046/j.1365-2699.2003.00872.x. 
  26. ↑ Bover, Pere; Valenzuela, Alejandro; Torres, Enric; Cooper, Alan; Pons, Joan; Alcover, Josep A (November 2016). "Closing the gap: New data on the last documented Myotragus and the first human evidence on Mallorca (Balearic Islands, Western Mediterranean Sea)" (in en). The Holocene 26 (11): 1887–1891. doi:10.1177/0959683616645945. ISSN 0959-6836. http://journals.sagepub.com/doi/10.1177/0959683616645945. 
  27. ↑ Valenzuela, Alejandro; Torres-Roig, Enric; Zoboli, Daniel; Pillola, Gian Luigi; Alcover, Josep Antoni (2021-11-29). "Asynchronous ecological upheavals on the Western Mediterranean islands: New insights on the extinction of their autochthonous small mammals" (in en). The Holocene 32 (3): 137–146. doi:10.1177/09596836211060491. ISSN 0959-6836. http://journals.sagepub.com/doi/10.1177/09596836211060491. 
  28. ↑ Damià Ramis; Pere Bover (2001). "A Review of the Evidence for Domestication of Myotragus balearicus Bate 1909 (Artiodactyla, Caprinae) in the Balearic Islands". Journal of Archaeological Science 28 (3): 265–282. doi:10.1006/jasc.2000.0548. 

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