Microbilobata, a new genus of earliest terebratulid brachiopod from the lower Silurian of Northwestern Canada: Implications for the origin of higher taxa
Title:
Microbilobata, a new genus of earliest terebratulid brachiopod from the lower Silurian of Northwestern Canada: Implications for the origin of higher taxa
Author:
Jin, Jisuo Chatterton, B. D. E.
Appeared in:
Historical biology
Paging:
Volume 11 (1996) nr. 1-4 pages 43-56
Year:
1996-06
Contents:
Microbilobata avalanchensis n. gen. and n. sp. from the Lower Silurian (upper Wenlock) carbonate rocks of the lower Delorme Group in the Avalanche Lake area, northwestern Canada, is described here as the earliest known terebratulid brachiopod. These small shells (less than 2 mm long) are subtriangular, anteriorly emarginate, possibly punctate, with both valves being sulcate at their anterior halves, the ventral sulcus bearing one prominent plica, and the dorsal sulcus marked by two plicae. Internally, M. avalanchensis has a centronelliform loop extending for about three fifths of the total shell length. The shells are silicified in carbonate rocks of mid to outer shelf origin. M. avalanchensis is relatively rare, with about 40 specimens so far found from samples collected at 58-60 m above the base of section AV5 in the Avalanche Lake area. Its presence in rocks of Wenlock age extends the earliest known terebratulids back about 16 million years from the oldest previously recorded terebratulids (earliest Devonian age). The small size and simple form of the new species suggest that heterochrony (progenesis) could have played a role in the origin of the Terebratulida. M. avalanchensis serves as a good example of Cope's Rule, indicating that the terebratulids evolved from a very small, unspecialized ancestor. The unusually small size of this taxon also offers one explanation as to why some ancestors or transitional forms of major taxonomic groups are extremely difficult to find in the fossil record.
Journal description