Demosponges and hexactinellid sponges from the Lower Devonian Ross Formation of west-central Tennessee

Abstract

A fauna of eight taxa of demosponges and hexactinellid sponges has been collected from the Lower Devonian (Lochkovian) Ross Formation, largely out of the upper Birdsong Shale Member in Benton, Decatur, and Perry Counties in west-central Tennessee. The Upper Birdsong Shale (“bryozoan zone”) in which the sponges are most common appears to have been deposited below normal wave base in a quiet marine environment, and represents a terrigenous clastic sediment influx onto a carbonate shelf that had existed in the area from at least the middle Silurian. Benton Quarry in Benton County was the most productive locality for fossil sponges.The new demosponge genera and species Ginkgospongia foliata and Coniculospongia radiata occur with the new species Haplistion lobatum and skeletal mats of fine spicules, along with moderately rare specimens of Hindia sphaeroidalis Duncan. The new hexactinellid genus and species Stiodermiella amanita and Stiodermiella tetragona are characterized by peculiar ornamented papillose, swollen spicules that produce a massive, armored layer on the upper part of the sponge. The latter are associated with the new hexactinellid species Twenhofelella bulbulus, which has relatively normal-appearing hexactines, and with an indeterminate hexactinellid genus, which has spinose hexactines in irregular orientation in a small, platelike fragment. Root tufts of probable hexactine origin also occur.Swollen spicules in Stiodermiella are reminiscent of swollen spicules in the family Stiodermatidae Finks, largely from the Permian of western Texas, but elements of the family are also known from Lower Carboniferous to Permian rocks in Europe and North America.