ON THE INVASION OF THE ANCESTORS OF THE TRIGONIOIDOIDEA (BIVALVIA) INTO THE NON-MARINE REALM—EVIDENCE FROM EASTERN TIBET, CHINA
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Abstract:
The new trigoniid taxon Baxoitrigonia baxoiensis gen. et sp. nov. of Tithonian(Late Jurassic) age is reported from the Baxoi(Basu) area in eastern Tibet, China. The external characters of this new taxon are fairly similar to those of the genus Iotrigonia, but it is characterized by a distinct median groove, and the anterior adductor muscle scar is similar to that of the Trigonioidoidea. This observation provides more direct evidence that the non-marine Trigonioidoidea(order Trigoniida) were derived from marine Trigoniida and not from the Unionoidea(order Unionida). This derivation correlates with the biostratigraphical sequence in eastern Tibet; Baxoitrigonia baxoiensis directly preceded the Jingxing fauna that comprises primal Trigonioidoidea that lived in non-marine environments. The invasion of the marine trigoniid ancestors of the Trigonioidoidea into non-marine environments, which took place in the peripherally isolated Lhorong-Baxoi region, and their evolution into non-marine Trigonioidoidea is a typical example of peripatric speciation.
