Athena Review Vol. 5, no. 1
Records of Life: Fossils as Original Sources
25. Jurassic and Cretaceous mammals
Origins of Eutherian and Metatherian mammals
Eutherian or placental mammals split from Metatherian or marsupial mammals sometime before 160 mya, probably in the Early Jurassic period (fig.1). This timing has been revealed by recent finds of the earliest eutherian mammal, Juramaia, whose physical characters have been analyzed by Luo et al (2011) to show that the split with metatherians probably occurred not long before the time of Juramaia (160-165 mya). This extends the first appearance of the eutherian–placental clade back by about 35 mya from the previous record, which had been based on the eutherian mammal Eumaia, dating from 125 mya (Ji et al 2002).
Juramaia sinensisJuramaia sinesis was a eutherian mammal from the Jurassic of China, found at the Daxigou site of Jianchang County of Liaoning Province in the Middle–Late Jurassic Tiaojishan formation, dated at 164–165 mya. The name means "Jurassic mother from China" in reference to placental affinities. The fossil preserved full dentition, an incomplete skull, the anterior part of the postcranial skeleton, and residual soft tissues including hairs.
Fig.1: chart showing the Jurassic and Cretacious timescale, correlated with current findings of therian or placental mammals in the center, and early metatherians at right (after Luo et al 2001, fig.3, and Archibald 2003, fig. x).
Juramaia (fig.2) weighed about 15–17 g, and was an insectivore based on tooth morphology. including shear surfaces of the upper molar, arranged in stepwise pattern, contrasting with those of other tribosphenic mammals. Among the earliest-known eutherians, Juramaia sinensis also differs from the later Eomaia (125 mya) in having a tworooted upper canine, and from Acristatherium (123 mya) in having different numbers of upper and lower incisors, a larger M3 and absence of diastemata between the anterior upper premolars. It also differs from Prokennalestes, Murtoilestes and Acristatherium in other features of dentition detailed by Luo et al. (2011).
The forelimb and shoulder girdle of
Juramaia show several eutherian traits, and lack such metatherian
features as the supra-scapular notch. The
phalanges or hand bones suggest an arboreal and scansorial (climbing) adaptation. These
habits are also corroborated by the forelimb features, which are
indicative of that habitat preference in extant mammals, such as the acute posterior angle of the scapula. Fig.2: Skull and anterior skeleton of Juramaia (Luo et al. 2011, fig.1).
In its habitat preference, Juramaia is considered similar to the Early Cretaceous eutherian Eomaia, to the Cretaceous and Early Cenozoic metatherians, and to living scansorial or arboreal didelphids. The earliest-known eutherians Juramaia and Eomaia and the earliest metatherian Sinodelphys are thus each climbing and arboreal mammals, and differ from contemporary Mesozoic mammals, most which are terrestrial. This suggests that the phylogenetic split of eutherians and metatherians and their earliest evolution are accompanied by major ecomorphological diversification, notably scansorial adaptation, which made it possible for therians to exploit arboreal niches.
Juramaia, dated about 160 mya, establishes a significantly earlier geological time for the split of the metatherian–marsupial and the eutherian–placental lineages than previously shown by the fossil record. The previously earliest eutherian record is Eomaia and the metatherian record is Sinodelphys, both about 125 mya. The next oldest eutherian with a direct geochronological dating is Acristatherium at 123 mya)
Because Juramaia is unambiguously placed on the placental side of the marsupial–placental divergence, the marsupial–placental divergence must have occurred before Juramaia. Marsupials and Placentals collectively make up 99.9% of all living mammals and are very important in terrestrial ecosystems, especially after the Cretaceous/Tertiary extinction of non-avian dinosaurs. Timing of the divergence of marsupials and placentals is critical for calibrating rates of evolution in therian mammals, and are important for molecular evolutionary studies and comparative genomics.
Eomaia
Eomaia scansoria was a eutherian (placental) mammal whose fossil skeleton was discovered in the Lower Cretaceous Yixian Formation of northeastern China, dated at 125 million years ago (mya) . The name Eomaia scansoria means "scansorial (climbing) dawn mother". At the time of its discovery (Ji et al. 2002) this was the earliest eutherian mammal known, and extended the date of t
he oldest eutherian records with skull and skeleton by about 40–50 million year. Eomaia (like the earlier Juromaia, described above) has limb and foot features known only from scansorial (climbing) and arboreal
(tree-living) extant mammals, in contrast to the terrestrial or cursorial
(running) features of other Cretaceous eutherians. Fig.3: Skeleton of Eomaia scansoria (Ji et al. 2002, fig.1).
The age of Eomaia scansoria (125 mya) correlates with the Mid Barremian stage of the Early Cretaceous period (fig.1). In various features of its dentition detailed by Ji et al. (2002), Eomaia differs from other Early and Middle Cretaceous eutherians including Prokennalestes, Murtoilestes, and Montanalestes. E. scaznsoria also differs from the early metatherian Deltatheridium, which is about contemporary at 125 mya (fig.1). in having a typical eutherian dental formula, 5.1.5.3/4.1.5.3 (incisors, canine, premolars, molars)
Acristatherium
The basal eutherian mammal, Acristatherium yanensis is from the Early Cretaceous (Early Aptian) Jehol biota. It was discovered in the Lujiatun Bed of the Yixian Formation.in China, and dated at 123.2 mya (Hu et al 2009) . The taxon is based on a partial skull 25 mm long, preserved in three dimensions. Its right upper and lower dentitions are nearly complete and it has a dental formula 4.1.5.3/3.1.5.3.
The new mammal reveals several craniodental characteristics of Early Cretaceous eutherians previously unknown in fossil records of therians, such as a possible vestige of the septomaxilla, a feature only otherwise seen in nonmammalian therapsids (fig.4).
The craniodental features of the new taxon are
compared with those of other Early Cretaceous eutherians and
therians in a phylogenetic analyses by Hu et al. (2009). They place A. yanensis as the most basal eutherian
in the selected group (which includes Eomaia but not the subsequently reported, significantly earlier Juromaia). Fig.4: Skull of Acristatherium yanensis, with bones labelled (Hu et al. 2009, fig.1).
The morphological differences between Acristatherium and Eomaia indicate that Acristatherium retains more primitive features than Eomaia, and that eutherians already had a significant degree of generic diversification ca 125 mya
References:
Hu, Yaoming, Jin Meng, Chuankui Li and Yuanqing Wang 2009 New basal eutherian mammal from the Early Cretaceous Jehol biota, Liaoning, China Royal society
Ji, Quang, Zhe-Xi Luo, Chong-Xi Yuan, , John R. Wible, Jian-Ping Zhang, & Justin A. Georgi, 2002. The earliest known eutherian mammal. Nature 416.
Luo, Zhe-Xi, Chong-Xi Yuan, Qing-Jin Meng & Qiang Ji 2011 A Jurassic eutherian mammal and divergence of marsupials and placentals Nature 476
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