Morganucodon

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Morganucodon
Temporal range: Late Triassic-Middle Jurassic Rhaetian–Bathonian
Restored skull of Morganucodon oehleri.jpg
Scan and reconstruction of the M. oehleri holotype skull
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Cynodontia
Clade: Mammaliaformes
Order:† Morganucodonta
Family:† Morganucodontidae
Genus:† Morganucodon
Kühne, 1949
Type species
†Morganucodon watsoni
Kühne, 1949
Species
  • †M. watsoni(Kühne, 1949)
  • †M. oehleri(Rigney, 1963)
  • †M. heikuopengensis(Young, 1978)
  • †M. peyeri(Clemens, 1980)
  • †M. tardus(Butler and Sigogneau-Russell, 2016)

Morganucodon ("Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, Morganucodon is well represented by abundant and well preserved (though in the vast majority of cases disarticulated) material. Most of this comes from Glamorgan in Wales (Morganucodon watsoni), but fossils have also been found in Yunnan Province in China (Morganucodon oehleri) and various parts of Europe and North America. Some closely related animals ( Megazostrodon ) are known from exquisite fossils from South Africa. [1]

Contents

The name comes from a Latinization of Morganuc, the name for South Glamorgan in the Domesday Book, the county of Wales where it was discovered by Walter Georg Kühne, [2] giving the meaning "Glamorgan tooth".

History of discovery

Lower jaw of M. watsoni, Natural History Museum, London Morganucodon watsoni.JPG
Lower jaw of M. watsoni, Natural History Museum, London

In the summer of 1947, fieldwork was done at Duchy Quarry in Glamorgan in southern Wales. Grey conglomerate that formed fissure fill deposits within karstic voids in Carboniferous limestone was extracted. In 1949, Walter Georg Kühne noted the lower cheek tooth of a primitive mammal while examining samples of the rock. He named it Morganucodon watsoni, with the genus name being derived from Morganuc, which Kühne stated was the name of South Glamorgan in the Domesday Book, with the species name being in honour of D. M. S. Watson. [2] Additional remains of M. watsoni were described by Kühne in 1958. [3] Also in 1958, Kenneth Kermack and Frances Mussett described additional remains from Pant Quarry, about a mile from Duchy Quarry, that had been collected in 1956. [4] In August 1948, an expedition to Lufeng in Yunnan, China yielded a 1 in (2.5 cm) long skull. It was shortly sent to Beijing (then Peking) and then eventually sent out of China, and deposited with Kenneth Kermack at University College London in 1960. The specimen was preliminarily described in 1963 by Harold W. Rigney, who noted the similarity to Morganucodon from Britain, and considered it cogeneric, naming the new species Morganucodon oehleri in honor of the reverend Edgar T. Oehler, who had originally collected the specimen. [5] In 1978 C. C. Young described Eozostrodon heikuopengensis from the Hei Koa Peng locality near Lufeng, based on an associated skull and dentary, as well as a right maxilla and associated dentary. [6] A revision by William A. Clemens in 1979 assigned this species to Morganucodon, based on its close similarity to the two previously named species. [4] In 1980 Clemens named the species Morganucodon peyeri, from isolated teeth found in Late Triassic (Rhaetian) deposits near Hallau, Switzerland, with the species being named after paleontologist Bernhard Peyer. [7] In 1981, Kermack, Mussett and Rigney published an extensive monograph on the skull of Morganucodon. [8] In 2016 Percy Butler and Denise Sigogneau-Russell named the species Morganucodon tardus from an upper right molar (M34984) collected from the Watton Cliff locality near Eype in Dorset, England, dating to the late Bathonian stage of the Middle Jurassic. The species being named after the Latin tardus, late, in reference to it being the youngest member of the genus. [9]

Biology

Life restoration of M. oehleri Morganucodon.jpg
Life restoration of M. oehleri
Diagram of the skull of Morganucodon, with bones labelled Morganucodon skull.svg
Diagram of the skull of Morganucodon, with bones labelled

Morganucodon was a small, plantigrade animal. The tail was moderately long. According to Kemp (2005), "the skull was 2–3 cm in length and a presacral body length of about 10 cm [4 inches]. In general appearance, it would have looked like a shrew or mouse". [10] There is evidence that it had specialized glands used for grooming, which may indicate that, like present day mammals, it had fur. [11]

Like present day mammals of similar size and presumed habit, Morganucodon was likely nocturnal and spent the day in a burrow. There is no direct fossil evidence, but several lines of evidence point to a nocturnal bottleneck in the evolution of the mammal class, and almost all modern mammals of similar size to Morganucodon are still nocturnal. [12] [13] Likewise, burrowing was widespread both in non-mammalian cynodonts and in primitive mammals. [14] [15] The logics of phylogenetic bracketing would make Morganucodon nocturnal and burrowing too. Plant material from the conifer Hirmeriella was also found in the fissure fills, indicating Morganucudon lived in, or near, a forested area.

The diet appears to have been insects and other small animals, with a preference for hard prey such as beetles. [16] Like most modern mammal insectivores, it grew fairly quickly to adult size. [17] Its eggs were probably small and leathery, a condition still found in monotremes. [18]

The teeth grew in mammalian fashion, with deciduous teeth being replaced by permanent teeth that were retained throughout the rest of the animal's life. [19] The combination of rapid growth in juveniles and a toothless stage at infancy strongly suggests that Morganucodon raised its young by lactation; indeed, it may have been among the first animals to do so. [20] The molars in the adult had a series of raised humps and edges that fit into each other, allowing for efficient chewing. However, unlike the situation in most later mammals, the upper and lower molars did not occlude properly when they first met; as they wore against each other, however, their shapes were modified by wear to produce a precise fit. [21]

A 2020 study suggests that the metabolism of Morganucodon was significantly slower than that of comparably sized modern mammals, and that it had a life-span more similar to that of reptiles, with the oldest specimen having a lifespan of 14 years. Thus it likely did not possess the fully endothermic metabolism seen in current mammals. [22]

Species

Skull and jaws of M. oehleri Morganucodon.svg
Skull and jaws of M. oehleri
SpeciesAuthorYearStatusTemporal rangeLocationFormations
Morganucodon watsoniKuehne1949 Early Jurassic (Hettangian-Sinemurian)England, WalesVarious fissure fill deposits
Morganucodon heikuopengensisYoung1978Early Jurassic (Sinemurian)Yunnan, China Lufeng
Morganucodon oehleriRigney1963Early Jurassic (Hettangian)
Morganucodon peyeriClemens1980 Late Triassic (Rhaetian)Switzerland, France Klettgau, grès infraliasiques (Saint-Nicolas-de-Port)
Morganucodon tardusButler and Sigogneau-Russel2016 Middle Jurassic (Bathonian)England Forest Marble

Classification

Life restoration of M. watsoni Morganucodon watsoni life restoration.jpg
Life restoration of M. watsoni

Morganucodon is the type genus for the order Morganucodonta, a group of generally similar mammaliaforms known from the Late Triassic to Late Jurassic epochs, [23] [24] with one possible member ( Purbeckodon ) dating to the Early Cretaceous. [25] All were small and likely insectivorous. Morganucodon is the best preserved and best understood member of Morganucodonta.

There is currently controversy about whether or not to classify Morganucodon as a mammal or as a non-mammalian mammaliaform. Some researchers limit the term "mammal" to the crown group mammals, which would not include Morganucodon and its relatives. Others, however, define "mammals", as a group, by the possession of a special, secondarily evolved jaw joint between the dentary and the squamosal bones, which has replaced the primitive one between the articular and quadrate bones in all modern mammalian groups. Under this definition, Morganucodon would be a mammal. Nevertheless, its lower jaw retains some of the bones found in its non-mammalian ancestors in a very reduced form rather than being composed solely of the dentary. Furthermore, the primitive reptile-like jaw joint between the articular and quadrate bones, which in modern mammals has moved into the middle ear and become part of the ear ossicles as malleus and incus, is still to be found in Morganucodon. [26] Morganucodon also suckled (it may have been the earliest animal to do so), had only two sets of teeth and grew rapidly to adult size and stopped growing thereafter, all typical mammalian traits. [27]

Phylogeny [28]
Mammaliaformes  

See also

Related Research Articles

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Eozostrodon is an extinct morganucodont mammaliaform. It lived during the Rhaetian stage of the Late Triassic. Eozostrodon is known from disarticulated teeth from South West England and estimated to have been less than 10 cm (3.9 in) in head-body length, slightly smaller than the similar-proportioned Megazostrodon.

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Mammaliaformes is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent common ancestor of Morganucodonta and the crown group mammals; the latter is the clade originating with the most recent common ancestor of extant Monotremata, Marsupialia, and Placentalia. Besides Morganucodonta and the crown group mammals, Mammaliaformes includes Docodonta and Hadrocodium.

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References

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  2. 1 2 Walter G. Kühne, "On a Triconodont tooth of a new pattern from a Fissure-filling in South Glamorgan", Proceedings of the Zoological Society of London, volume 119 (1949–1950) pages 345–350
  3. ↑ Kühne, Walter Georg (1958-08-01). "Rhaetische Triconodonten aus Glamorgan, ihre Stellung zwischen den Klassen Reptilia und Mammalia und ihre Bedeutung für die REICHART'sche Theorie" [Rhaetian triconodonts from Glamorgan, their position between the classes Reptilia and Mammalia and their significance for REICHART's theory.]. Paläontologische Zeitschrift (in German). 32 (3): 197–235. doi:10.1007/BF02989032. ISSN   0031-0220. S2CID   128828761.
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  5. ↑ Rigney, Harold W. (March 1963). "A Specimen of Morganucodon from Yunnan". Nature. 197 (4872): 1122–1123. Bibcode:1963Natur.197.1122R. doi:10.1038/1971122a0. ISSN   0028-0836. S2CID   4204736.
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  20. ↑ Kielan-Jaworowska, Zofia; Cifelli, Richard L.; Luo, Zhe-Xi (2004). Mammals from the age of dinosaurs : origins, evolution, and structure. New York: Columbia University Press. pp. 148–153. ISBN   978-0231119184.
  21. ↑ Crompton, A. W.; Jenkins, F. Jr. (1968). "Molar occlusion in late Triassic mammals". Biological Reviews. 43 (4): 427–458. doi:10.1111/j.1469-185x.1968.tb00966.x. PMID   4886687. S2CID   1044399.
  22. ↑ Newham, Elis; et al. (2020). "Reptile-like physiology in Early Jurassic stem-mammals". Nature Communications. 11 (1): 5121. Bibcode:2020NatCo..11.5121N. doi: 10.1038/s41467-020-18898-4 . PMC   7550344 . PMID   33046697.
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  26. ↑ Kermack, K. A.; Mussett, Frances; Rigney, H. W. (1981). "The skull of Morganucodon". Zoological Journal of the Linnean Society. 71: 1–158. doi:10.1111/j.1096-3642.1981.tb01127.x.
  27. ↑ Mammals of the Mesozoic: The least mammal-like mammals
  28. ↑ Close, Roger A.; Friedman, Matt; Lloyd, Graeme T.; Benson, Roger BJ (2015). "Evidence for a mid-Jurassic adaptive radiation in mammals". Current Biology. 25 (16): 2137–2142. doi: 10.1016/j.cub.2015.06.047 . PMID   26190074.
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