Monday, March 5, 2012

[Paleontology • 1997] Zhangheotherium quinquecuspidens • A new symmetrodont mammal from China and its implications for mammalian evolution


The fossil remains of this small animal from the age of the dinosaurs show that it shares features of both modern mammals and their reptilian relatives, and that it lived near a lake with a rich diversity of vertebrates, insects, and plants.

Figure 1: Zhangheotherium quinquecuspidens (IVPP V7466, holotype).

Zhangheotherium quinquecuspidens Hu, Wang, Luo & Li, 1997

Etymology. Zhanghe, in honour of Zhang He, who collected and donated the holotype specimen to the Institute of Vertebrate Paleontology and Paleoanthropology; therium, beast (Greek); quinque, five (Latin); cuspis, point (Latin); dens, tooth (Latin), 
for the three main cusps plus two large accessory cuspules on the lower molars.

Locality. Jianshangou Valley (approximately 41° 41' 01" N, 120° 59' 30" E), about 32 km east of Chaoyang City, Liaoning Province, northeastern China1.

Horizon. The Jianshangou Beds, consisting primarily of shales, are the lowest lacustrine intercalation in the neutro-basic volcanic beds of the Yixian Formation.

Associated fauna. The Jianshangou Beds have yielded diverse fossil fish4, the birds Confuciusornis, Liaoningornis and Protarchaeopteryx, the theropod Sinosauropteryx, and diverse gastropods, bivalves, ostracods, conchostracans and insects.

Age. The age of the Yixian Formation is equivocal. Vertebrate faunal correlation and previous radiometric dates4 suggest that the Jianshangou Beds are either of the latest Jurassic age, or near the Jurassic–Cretaceous transition. An Early Cretaceous age was also suggested by invertebrate faunal correlation, and supported by a recent radiometric date. According to our most recent field investigation, this date should be regarded as an upper age limit for the Jianshangou Beds.







Conclusions
The nearly complete skeleton of Zhangheotherium quinquecuspidens has yielded new and more comprehensive anatomical information about the early therian mammals. New evidence on basicranial and postcranial anatomy from Zhangheotherium corroborates the hypothesis that symmetrodonts are a part of the basal therian radiation. Postcranial features of this new therian mammal support a sister-group relationship between multitubercualtes and therian mammals. A mobile clavicle–interclavicle joint that allows a wide range of movement of the forelimb has an ancient origin in the mammalian phylogeny. The abducted forelimb inferred for Zhangheotherium and other archaic therians suggests that early therian mammals lacked the more parasagittal forelimb posture of most living therians. The presence of a finger-like promontorium in Zhangheotherium indicates, albeit indirectly, that an uncoiled cochlea was present in symmetrodonts and that the coiled cochlea was a development later in therian evolution.


FIGURE 5. Phylogenetic relationships of Zhangheotherium quinquecuspidens.

Hu, Y., Wang, Y., Luo, Z., and Li, C. 1997. A new symmetrodont mammal from China and its implications for mammalian evolution. Nature 390: 137–142.

Hu, Y., Wang, Y., Li, C., and Luo, Z. 1998. Morphology of dentition and forelimb of ZhangheotheriumVertebrata PalAsiatica 36 (2): 102–125.



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