Yinotheria

Evolutionary history

According to genetic studies Yinotheria first diverge from other mammals around 220 to 210 million years ago, at some point in the Triassic or Early Jurassic. The oldest known fossils are a bit younger, dating around 168 to 163 million years in the Middle Jurassic. These fossils are the genera Pseudotribos of China, Shuotherium of both China and England, Itatodon of Siberia and Paritatodon of Kyrgyzstan and England. These belong to the family Shuotheriidae, and they are the only known northern hemisphere group of yinotherians.

Around the same time as Shuotheriidae, the infraclass Australosphenida first appeared. The family Henosferidae occupied in the southern hemisphere in Argentina and Madagascar with genera Henosferus, Ambondro, and Asfaltomylos. This suggests that this family could have been more widespread and diverse in Gondwana during that time, although due to their fragile state some species might have been destroyed due to geological events.

In the Early Cretaceous in the region that is now known as Australasia the family Ausktribosphenidae and the first monotremes appeared. Despite being found in the same region of the world and same time period, recent work has found that the older Henosferidae is the sister taxon to Monotremata, with Ausktribosphenidae being the next sister taxa in Australosphenida. Ausktribosphenidae includes the genera Bishops and Ausktribosphenos.

A monotreme fossil of a jaw fragment 110 million years old were found at Lightning Ridge, New South Wales. These fragments, from the species Steropodon galmani, are the oldest known fossils of monotremes. Fossils from the genera Kollikodon, Teinolophos, and Obdurodon have also been discovered. In 1991, a fossil tooth of a 61-million-year-old platypus was found in southern Argentina (since named Monotrematum, though it is now considered to be an Obdurodon species). (See fossil monotremes below.) Molecular clock and fossil dating give a wide range of dates for the split between echidnas and platypuses, with one survey putting the split at 19–48 million years ago, but another putting it at 17–89 million years ago. All these dates are more recent than the oldest known platypus fossils, suggesting that both the short-beaked and long-beaked echidna species are derived from a platypus-like ancestor.

Prototheria

Originally monotremes were classified as a subclass of mammals known as Prototheria. The names Prototheria, Metatheria and Eutheria (meaning "first beasts", "changed beasts", and "true beasts", respectively) refer to the three mammalian groupings which have living representatives. Each of the three may be defined as a total clade containing a living crown-group (respectively the Monotremata, Marsupialia and Placentalia) plus any fossil species which are more closely related to that crown-group than to any other living animals.

The threefold division of living mammals into monotremes, marsupials and placentals was already well established when Thomas Huxley proposed the names Metatheria and Eutheria to incorporate the two latter groups in 1880. Initially treated as subclasses, Metatheria and Eutheria are by convention now grouped as infraclasses of the subclass Theria, and in more recent proposals have been demoted further (to cohorts or even magnorders), as cladistic reappraisals of the relationships between living and fossil mammals have suggested that the Theria itself should be reduced in rank.

Prototheria, on the other hand, was generally recognised as a subclass until quite recently, on the basis of an hypothesis which defined the group by two supposed synapomorphies: (1) formation of the side wall of the braincase from a bone called the anterior lamina, contrasting with the alisphenoid in therians; and (2) a linear alignment of molar cusps, contrasting with a triangular arrangement in therians. These characters appeared to unite monotremes with a range of Mesozoic fossil orders (Morganucodonta, Triconodonta, Docodonta and Multituberculata) in a broader clade for which the name Prototheria was retained, and of which monotremes were thought to be only the last surviving branch (Benton 2005: 300, 306).

Australosphenida hypothesis and Yinotheria

The evidence which was held to support Prototheria is now universally discounted. In the first place, examination of embryos has revealed that the development of the braincase wall is essentially identical in therians and in 'prototherians': the anterior lamina simply fuses with the alisphenoid in therians, and therefore the 'prototherian' condition of the braincase wall is primitive for all mammals while the therian condition can be derived from it. Additionally, the linear alignment of molar cusps is also primitive for all mammals. Therefore, neither of these states can supply a uniquely shared derived character which would support a 'prototherian' grouping of orders in contradistinction to Theria (Kemp 1983).

In a further reappraisal, the molars of embryonic and fossil monotremes (living monotreme adults are toothless) appear to demonstrate an ancestral pattern of cusps which is similar to the triangular arrangement observed in therians. Some peculiarities of this dentition support an alternative grouping of monotremes with certain recently discovered fossil forms into a proposed new clade known as the Australosphenida, and also suggest that the triangular array of cusps may have evolved independently in australosphenidans and therians (Luo et al. 2001, 2002). Australosphenida is characterized by the shared presence of a cingulum on the outer front corner of the lower molars, a short and broad talonid, a relatively low trigonid, and a triangulated last lower premolar.

The Australosphenida hypothesis remains controversial; for example, there seems to be a presence of lingual cingula in various non-australosphenidan mammals and some work has shown the possibility of Eutheria being the sister group to Australosphenida, without monotremes. As a result, some taxonomists (e.g. McKenna & Bell 1997) prefer to maintain the name Prototheria as a fitting contrast to the other group of living mammals, the Theria. In theory, the Prototheria is taxonomically redundant, since Monotremata is currently the only order which can still be confidently included, but its retention might be justified if new fossil evidence, or a re-examination of known fossils, enables extinct relatives of the monotremes to be identified and placed within a wider grouping.

When systematic work was performed, it was also found that Australosphenida is the sister taxon to Shuotheriidae, an obscure group of Mesozoic mammals that found in what is now China. Kielan-Jaworowska, Cifelli & Luo 2002 had this to say regarding the Shuotheriidae, particularly Shuotherium:

In our view, the most compelling evidence as to the affinities of Shuotherium lies in the structure of the last premolar, which shares striking similarities to that of Australosphenida

Lower molar structure of Shuotherium and Australosphenida is obviously quite different, and for this reason we do not place Shuotherium within this Gondwanan clade. Based on the limited evidence available, however, we suggest that Shuotherium is a viable sister-taxon to Australosphenida.

Yinotheria is named for this grouping.

Taxonomy

In comparison to Metatheria and Eutheria, where there seems to be a better understanding on the relationships among taxa with substantial fossil evidence, Yinotheria has few fossils; mostly consists of (with few exceptions) the jawbones and teeth. In addition the group seem to have not been as diverse in their evolutionary history, in comparison to members of both Metatheria and Eutheria. Future analysis and better fossil remains could affect the membership of Yinotheria as well as rearranging and revising the relationships of stem-monotremes and crowned monotremes.

Phylogeny

Below is a simplified tree on Averianov et. al, 2014 after Woodburn, 2003 and Ashwell, 2013