A TEST OF AMBOCOELIID (BRACHIOPODA) SPINOSITY AS AN ADAPTATION FOR SOFT SUBSTRATES USING ABUNDANCE PATTERNS THROUGH TRANSGRESSIVE-REGRESSIVE CYCLES AND WITHIN PYRITIC SHALE BEDS

            Brachiopods exhibit many morphological features that are interpreted as adaptations for living on a soft substrate; including thin, spinose, and concavo-convex shells.  Using ambocoeliid brachiopods from the Hamilton Group, Middle Devonian of New York, the hypothesis of spines as an adaptation for soft substrates is tested by examining the occurrence and abundance of these brachiopods in varying depositional environments as well as along an onshore – offshore gradient.  Crurispina nana possesses numerous short spines that are present on both the pedicle and brachial valves.  Ambocoelia umbonata, while similar in size and shape to C. nana, is smooth-shelled and more generalistic.  The abundance of these taxa is compared through transgressive-regressive cycles and within pyritic shale beds.  Transgressive facies represent periods when sediment influx basinward was reduced as sediment was trapped in estuaries and deltaic complexes, causing sediment starvation and formation of hardgrounds in offshore settings.  Regressive facies represent periods when sediment influx increased basinward, often recognized as thick sedimentary sequences involving sedimentary dilution of fossiliferous strata in offshore settings.  Pyritic shales are inferred to form under conditions were bioturbation is frequent, and commonly contain pyritized burrow steinkerns.  Examination of the abundance of C. nana and A. umbonata in different depositional environments and through transgressive-regressive cycles suggests that C. nana occurs preferentially in environments with soft substrates (pyritic shale beds and regressive facies).  Furthermore, C. nana has only been observed in fine-grained offshore settings, while A. umbonata occurs along an onshore – offshore gradient including both coarse- and fine-grained sediment.  These observations may imply competition in soft-substrate environments between the generalistic A. umbonata and the apparently specialized C. nana.  Results support the hypothesis that spinosity in C. nana is an adaptation for inhabiting soft substrates.