morganucodon

''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.

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,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 giving the meaning "Glamorgan tooth".

History of discovery

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. Additional remains of ''M. watsoni'' were described by Kühne in 1958. 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. In August 1948, an expedition to Lufeng in Yunnan, China yielded a 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. 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. A revision by William A. Clemens in 1979 assigned this species to ''Morganucodon,'' based on its close similarity to the two previously named species. 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. In 1981, Kermack, Mussett and Rigney published an extensive monograph on the skull of ''Morganucodon''. 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.

Biology

''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 inches In general appearance, it would have looked like a shrew or mouse". There is evidence that it had specialized glands used for grooming, which may indicate that, like present day mammals, it had fur.

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. Likewise, burrowing was widespread both in non-mammalian cynodonts and in primitive mammals. 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. Like most modern mammal insectivores, it grew fairly quickly to adult size. Its eggs were probably small and leathery, a condition still found in monotremes.

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.Alexander F. H. van Nievelt and Kathleen K. Smith, "To replace or not to replace: the significance of reduced functional tooth replacement in marsupial and placental mammals", ''Paleobiology'', Volume 31, Issue 2 (June 2005) pages 324–346 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. 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.

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.

Species

Classification

''Morganucodon'' is the type genus for the order Morganucodonta, a group of generally similar mammaliaforms known from the Late Triassic to Late Jurassic epochs, with one possible member ('' Purbeckodon'') dating to the Early Cretaceous. 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''. ''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.Mammals of the Mesozoic: The least mammal-like mammals
/ref>

;Phylogeny

See also

* Evolution of mammals

References

{{Taxonbar, from=Q131813

Morganucodonta
Late Triassic synapsids
Triassic synapsids of Europe
Jurassic synapsids
Transitional fossils
Rhaetian first appearances
Middle Jurassic extinctions
Fossil taxa described in 1949
Taxa named by Walter Georg Kühne