Eutheria : Mesozoic Eutheria

Taxa on This Page

1. Acristatherium X
2. Eomaia X
3. Eutheria
4. Juramaia X

Life reconstruction of the very early eutherian mammal Eomaia scansoria, from the Barremian of China. Total length about 10 cm. Artwork by Nobu Tamura. Wikipedia, GNU Free Documentation/Creative Commons Attribution Share Alife

For a long time, indeed for almost two thirds of the Mesozoic era, the Eutheria were tiny, shrew-like mammals, all of whom would have looked a lot like Eomaia, shown above. Although they continued to evolve through the many millions of years of the dinosaurs reign, they never became very large or diverse, or in any way distinct from the rest of the mesozoic mammals. Indeed, they showed no sign of assuming the dinosaurs' mantle and becoming the masters of the world. It was only, it seems, sheer luck (bad for the dinosaurs, good for the eutharians) that resulted in a combination of factors - acid rain, supervolcanos, asteroid imapct, sea level fall, collapse of the marine food chain, and no doubt others as well - that brought about the dramatic change from Mesozoic to Cenozoic, from age of reptiles to age of mammals.

Scientists disagree as to whether the divergence into the ancetsors of modern mammalian groups occured early on, during the middle Cretaceous, or only suddenly, with the extinction of the dinosaurs. If the former than many Mesozoic eutherians were ancestral to modern groups, if the latter, they represent a long stem lineage where nothing much happens. Whatever the interpretation, it is agreed that the genera mentioned on this page represent the very early ancestral line of eutharians. MAK111202

Descriptions

Eutheria : people > possums. Archibald et al. (2001). Daulestes, Eomaia. 

Range: middle Jurassic

Phylogeny: Theria Palaeoryctidae Metatheria + * : Acristatherium + Eomaia + JuramaiaEomaia + (Adapisoriculidae + (Zhelestidae + (Asioryctitheria + (Zalambdalestidae + (Leptictida + Palaeoryctidae + Crown Group Placentalia)))))

Characters: dental formula dental formula, 5/4,1/1,5/5,3/3; lower canine with two roots; premolars 5/5; upper molars triangular in occlusal view; postcingulum (if present), postmetaconular crista, & postmetacrista plus metastylar lobe form three en echelon ridges from lingual to labial along posterior margin of molar; metacone & paracone of approximately equal size; all living forms placental, but placenta probably not primitively present. ATW

Comments: In Evolutionary systematics, Eutheria is a fancy way of saying Placental Mammal. In cladistics, Eutheria is the total group of placental mammals: that is, all mammals more closely related to placentals than any other mammals. However, early eutherians seem not to have been placentals at all, as they possessed abone, the prebubic bone, also found in marsupials, that meant there was no room for a placenta. It ised to be thought that eutherians appeared during the lower Barremian (early Cretaceous), represented for example by the well known early form Eomaia scansoria ("Dawn mother"). The recent discovery of Juramaia sinensis ("Jurassic mother") pushes their origin back to the Middle Jurassic, and shows that all of the main lines of mammal eveolution co-existed at this time. For some years, the ancestors of the main Cenozoic placental mammal groups were sought among the tiny shrew-like fossils of Cretaceous mamals, and this early divergebnce wa supported by molecular sequencing (Bininda-Emonds et al 2007). But in the same year that paper came out, a new cladistic analysis, all known fossil Mesozoic eutherians - both non-placental and true placental - lie outside the placental crown group (Wible et al. 2007). And so it goes, back and forth. We have decided to go with Wible et al. 2007, until the next paper comes along to overturn everything again. MAK120713.

Links: Eutheria; Placental Mammals; Basal Eutheria; Eutheria; Supplementary information; Earliest Known Ancestor of Placental Mammals Discovered; News at Carnegie Museum of Natural History; photos_display.asp?ID=7271; Australia's Lost Kingdoms - Tingamarra; Fossil sites of Australia - Murgon; Echte und Höhere Säuger German); f01 Living mammals are placentals (eutheria), marsupials, and ...; Xenarth.; TAED 3.0; Link to NCBI Front Page; Eutheria (Höhere Säuger); IMGT Marie-Paule page- IMGT repertoire; Xenarth. (excellent summary); Placental mammals (Eutheria) follows current molecular phylogenies): Eutheria Tree of Life); Eutheria - Wikipedia (Wikipedia); Eutheria (Trevor Dykes); Basal Eutheria Mikko's Phylogeny); Eutheria--Derived mammals VPP); Placental mammals (Eutheria) 4 superordinal groupings); Xenarth. brief, but useful lecturenotes); Outlines- Part 4 - Eutheria prettier, but along the same lines the last). ATW030807.

Juramaia sinensis

Range: Daxigou site, Jianchang, from the Tiaojishan Formation; Callovian/Bathonian of western Liaoning, China.

Phylogeny: basal Eutheria

Comments: The earliest eutherian mammal, known from an articulated and nearly complete skeleton including incomplete skull preserved with full dentition. It's discovery shows that placentals evolved some 35 million years earlier than previously thought. (Luo et al 2011)

Links: Wikipedia

Acristatherium yanensis Hu, Meng, Li & Wang, 2009;

Range: Yanzigou, Shangyuan, Beipiao, Liaoning, China; Lujiatun bed of the Yixian Formation, Early Aptian, Jehol biota, China.

Phylogeny: basal Eutheria

Characters: dental formula 4.1.5.3/3.1.5.3.

Comments: based on a partial skull (25 mm or one inch long) that is preserved in three dimensions. Several craniodental characteristics of Early Cretaceous eutherians previously unknown in fossil records of therians, such as a possible vestige of the septomaxilla. Phylogenetic analyses place A. yanensis as the most basal eutherian in the selected group. (Hu et al 2009)

Links: Wikipedia

Eomaia scansoria Ji, Luo, Yuan, Wible, Zhang & Georgi, 2002.

Range: Yixian Formation, Liaoning Province, China, Barremian (Early Cretaceous)

Phylogeny: basal Eutheria

Comments: The name Eomaia means "dawn mother". Prior to discovery of Juramaia the oldest known eutherian. Almost complete specimen about 10 centimetres in length, estimate of the body weight 20-25 grams. Although the fossil's skull is squashed flat, its teeth, tiny foot bones, cartilages and even its fur are visible. While possessing a number of eutherian features, Eomaia is not a true placental mammal as it lacks some features that are specific to placentals. It also has epipubic bones extending forwards from the pelvis, which are not found in any placental, but are found in all other mammals - non-placental eutherians, marsupials, monotremes and mammaliformes – and even in the cynodont therapsids that are closest to mammals. Their function is to stiffen the body during locomotion. This stiffening would be harmful in pregnant placentals, whose abdomens need to expand - Wikipedia

Graphic:Eomaia scansoria displayed in the Hong Kong Science Museum, showing the exquisite preservation. Photo by Laikayiu, Wikipedia, GNU Free Documentation/Creative Commons Attribution Share Alife