A new mammal from the Lower Cretaceous Jehol Biota and implications for eutherian evolution

Abstract

Here we report on a new Early Cretaceous eutherian represented by a partial skeleton from the Jiufotang Formation at Sihedang site, Lingyuan City, Liaoning Province that fills a crucial gap between the earliest eutherians from the Yixian Formation and later Cretaceous eutherians. The new specimen reveals, to our knowledge for the first time in eutherians, that the Meckelian cartilage was ossified but reduced in size, confirming a complete detachment of the middle ear from the lower jaw. Seven hyoid elements, including paired stylohyals, epihyals and thyrohyals and the single basihyal are preserved. For the inner ear the ossified primary lamina, base of the secondary lamina, ossified cochlear ganglion and secondary crus commune are present and the cochlear canal is coiled through 360°. In addition, plesiomorphic features of the dentition include weak conules, lack of pre- and post-cingula and less expanded protocones on the upper molars and height differential between the trigonid and talonid, a large protoconid and a small paraconid on the lower molars. The new taxon displays an alternating pattern of tooth replacement with P3 being the last upper premolar to erupt similar to the basal eutherian Juramaia . Parsimony analysis places the new taxon with Montanalestes , Sinodelphys and Ambolestes as a sister group to other eutherians. This article is part of the theme issue ‘The impact of Chinese palaeontology on evolutionary research’.

A new eutherian mammal from the Upper Cretaceous Bayinshire Formation of MongoliaCitation for this article: Lopatin, A. V. & Averianov, A. O. (2023) A new eutherian mammal from the Upper Cretaceous Bayinshire Formation of Mongolia. Journal of Vertebrate Paleontology . https://doi.org/10.1080/02724634.2023.2281478

“…On the Bayesian tree of the Wang et al matrix all Early Cretaceous Asiatic eutherians are considered to be stem therians. This result is repeated in our Bayesian analysis of the Wang et al (2022) matrix that includes Bayshinoryctes (Fig. 6B).…”

“…6A), based on the matrix from Averianov and Archibald (2016), is generally similar with the strict consensus tree produced by parsimony analysis of the same matrix and presented in that paper. In contrast, the parsimony and Bayesian trees generated from the same matrix by Wang et al (2022) are markedly different in the position of Metatheria. On the Bayesian tree of the Wang et al matrix all Early Cretaceous Asiatic eutherians are considered to be stem therians.…”

A stem therian mammal from the Lower Cretaceous of GermanyCitation for this article: Martin, T., Averianov, A.O., Schultz, J. A., and Schwermann, A. H. (2023) A stem therian mammal from the Lower Cretaceous of Germany. Journal of Vertebrate Paleontology . https://doi.org/10.1080/02724634.2023.2224848

“…The affinities of these taxa to eutherians have been questioned by some workers (Averianov, 2015; Sweetman et al, 2017). Eomaia was excluded from crown Theria in one analysis (O'Leary et al, 2013), but other workers have retained these fossils within eutherians (Bi et al, 2018; Martin, 2018; Wang et al, 2021). The same applies to the purported eutherians (so-called “Trinity therians”) from the Aptian-Albian Antlers Formation of the Trinity Group of Oklahoma and Texas and the Albian of the Cloverly Formation of Montana and Wyoming; according to Averianov (2015) they all belong to stem tribosphenidans.…”

“…The eutherian nature of Eomaia was not corroborated by a large analysis including molecular and morphological data (O'Leary et al, 2013). Juramaia , Eomaia , Sinodelphys , and Acristatherium were interpreted by some as stem therians (Averianov, 2015; Sweetman et al, 2017), but are still considered by others as eutherians or crown therians (Bi et al, 2018; Martin, 2018; Wang et al, 2021). According to King and Beck (2020), interpretation of Juramaia as a stem therian would be more consistent with its Middle to Late Jurassic age.…”

The nasopharynx as a window to half a billion years of evolutionary change to the upper respiratory tract

“…The opening of the posterior nares intraorally is a derived condition relative to other fish but is likely homoplastic relative to the condition of early amphibians (see Janvier, 2004) velar musculature as a distinct levator veli palatini is present among placental mammals but is not distinct from the palatopharyngeus among marsupials and monotremes (Edgeworth, 1935). This transition likely took place in the early Cretaceous period as the earliest placental mammal fossils date to this time (e.g., Ji et al, 2002;Wang et al, 2022).…”