''Teinolophos'', from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

τείνω (''teínō''), meaning "extend", and λόφος (''lóphos''), meaning "crest", is a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of teinolophid

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

that lived during the late

Barremian The Barremian is an age in the geologic timescale (or a chronostratigraphic stage) between 125.77 Ma (million years ago) and 121.4 ± 1.0 Ma (Historically, this stage was placed at 129.4 million to approximately 125 million years ago) It is a ...

age Age or AGE may refer to: Time and its effects * Age, the amount of time someone has been alive or something has existed ** East Asian age reckoning, an Asian system of marking age starting at 1 * Ageing or aging, the process of becoming older ...

of the

Early Cretaceous The Early Cretaceous (geochronology, geochronological name) or the Lower Cretaceous (chronostratigraphy, chronostratigraphic name) is the earlier or lower of the two major divisions of the Cretaceous. It is usually considered to stretch from 143.1 ...

. It is known from four specimens, each consisting of a partial lower jawbone collected from the Wonthaggi Formation at Flat Rocks, Victoria,

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

. The generic name, ''Teinolophos'', means "extended ridge", a reference to its tooth structure. The specific name honours the artist Peter Trusler. Originally, ''Teinolophos'' was thought to be a eupantothere. Further research revealed similarities to '' Steropodon'', except in size: the animal was around 10 cm long. It is often listed as a steropodontid, though it may be more basal.Thomas H. Rich, James A. Hopson, Pamela G. Gill, Peter Trusler, Sally Rogers-Davidson, Steve Morton, Richard L. Cifelli, David Pickering, Lesley Kool, Karen Siu, Flame A. Burgmann, Tim Senden, Alistair R. Evans, Barbara E. Wagstaff, Doris Seegets-Villiers, Ian J. Corfe, Timothy F. Flannery, Ken Walker, Anne M. Musser, Michael Archer, Rebecca Pian and Patricia Vickers-Rich (2016). "The mandible and dentition of the Early Cretaceous monotreme Teinolophos trusleri". Alcheringa: An Australasian Journal of Palaeontology. 40(4): 475–501. https://doi.org/10.1080/03115518.2016.1180034 ''Teinolophos'' is deeply divergent within monotreme evolution, so in 2022 it was proposed to move it into its own family, Teinolophidae. '' Stirtodon'' may be a close relative of ''Steropodon'' within Teinolophidae. The

holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

is a partial left

dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...

known as NMV P208231. An age of approximately 123 million years makes this the earliest known monotreme. The lower molar is broadly similar in morphology to the m2 of ''Steropodon''. The trigonid is compressed and the talonid has no basin. The dentary is about one sixth the size of ''Steropodon'', and wear facets indicate an "orthal" occlusion with the upper molars.

Description

Jaw

The construction of the lower jaw differs from existing monotremes. Among the contrasts are the condyle, which is well above the tooth row (instead of at about the same height); and the ascending ramus, which is also higher. Also different is that ''Teinolophos'' probably had a strong bite. A unique feature for known toothed monotremes is that the trigonid is tall, while the talonid is set much lower. This is more like the general mammalian arrangement. The molar is double-rooted, which is

plesiomorph In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral Phenotypic trait, character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorph ...

ic when compared to ornithorhynchids, but is a shared characteristic with ''Steropodon'' and '' Kollikodon''. Subsequent monotreme molars are multi-rooted. Unlike modern monotremes, ''Teinolophos'' lacked a beak.

Ears

Unlike modern monotremes, which have suspended ear bones much like

placental Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguished ...

s and

marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...

s, ''Teinolophos'' still had them connected to the jaw via the

Meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left; also known as Meckelian cartilages), above which the incus and malleus are located. Meckel's cartilage arises from the fir ...

. This reinforces the idea that the modern ear condition evolved independently among monotremes and

theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...

ns.

Evolution

One study interpreted ''Teinolophos'' as a basal platypus.Rowe, T., Rich, T. H., Vickers-Rich, P., Springer, M., & Woodburne, M. O. (2008). The oldest platypus and its bearing on divergence timing of the platypus and echidna clades. Proceedings of the National Academy of Sciences, 105(4), 1238-1242. doi:10.1073/pnas.0706385105

References

Bibliography

* Rich ''et al.'' "Early Cretaceous Mammals from Flat Rocks, Victoria, Australia". Records of the Queen Victoria Museum, Launceston (1999). * Rich, T. H., ''et al.'' "Monotreme nature of the Australian Early Cretaceous mammal Teinolophos". ''Acta Palaeontologica Polonica'' 46(1). 2001 Pages 113–118. * Rowe, T., ''et al.'' "The oldest platypus and its bearing on divergence timing of the platypus and echidna clades". ''Proceedings of the National Academy of Sciences'' 105(4). 2008 Pages 1238–1242.

External links

Life in the Shadows, Non-reptilian life in Mesozoic Australia

"Prehistoric jawbone reveals evolution repeating itself"
10 February 2005
"The oldest platypus and its bearing on divergence timing of the platypus and echidna clades"
2008 {{Taxonbar, from=Q133024 Prehistoric monotremes Prehistoric mammal genera Aptian life Cretaceous mammals of Australia Fossils of Australia Fossil taxa described in 1999 Taxa named by Tom Rich