科研之友
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摘要
New material of the Australian Lower Carboniferous rhizodontid Barameda provides additional information on poorly known aspects of rhizodontid anatomy, including the braincase and branchial arches, and indicates that two species of Barameda are present in the Mansfield Group, a larger poorly ossified species, B. decipiens, and a new species, Barameda mitchelli n. sp. based on the smaller well ossified specimens previously assigned to B. decipiens. Unusually, in a new pectoral fin specimen assigned to B. decipiens, the humerus lacks ossified entepicondylar and pectoral processes, strong projections present on the humeri of most sarcopterygian fish and tetrapods. Juveniles of rhizodontids and of Eusthenopteron (Tristichopteridae) also lack these processes (ossifying later in development), which along with poor overall ossification of the fin in juveniles and in B. decipiens, indicates that the pectoral fin of the latter has been affected by heterochronic processes. These observations suggest a degree of developmental variability (timing, degree of ossification) in the entepicondyle relative to the rest of the humerus, which could be extended to other posterior processes of pectoral fin elements such as the ulnare flange. This flange is present in tristichopterids but absent in rhizodontids and taxa closely related to the origin of tetrapods, such as Panderichthys.
- 出版日期2007-6-12
