. Bulletin. Natural history; Natuurlijke historie. 54 PEABODY MUSEUM BULLETIN 41 above) found in all the compared taxa and specialized features unique to the Gastrochaenacea. The data of Figure 48 are abstracted diagrammati- cally in Figure 49 to show relative unweighted similarities with the Gas- trochaenacea. PERMOPHORIDAE ,>^ ,,, GRAMMYS11 DAE. GASTROCHAENIDAE HIATELLIDAE W ^ ISOFILIBRANCHIA Fig. 49. Abstract of the data compiled in Figure 48, showing unweighted similarities between the Gastrochaenidae and four possible ancestors. Each line connecting the Gastrochaenidae and another taxo
. Bulletin. Natural history; Natuurlijke historie. 54 PEABODY MUSEUM BULLETIN 41 above) found in all the compared taxa and specialized features unique to the Gastrochaenacea. The data of Figure 48 are abstracted diagrammati- cally in Figure 49 to show relative unweighted similarities with the Gas- trochaenacea. PERMOPHORIDAE ,>^ ,,, GRAMMYS11 DAE. GASTROCHAENIDAE HIATELLIDAE W ^ ISOFILIBRANCHIA Fig. 49. Abstract of the data compiled in Figure 48, showing unweighted similarities between the Gastrochaenidae and four possible ancestors. Each line connecting the Gastrochaenidae and another taxon represents one similarity from Figure 48. The two dotted lines represent possible but unverified similarities. Although the Hiatellacea and the Isofilibranchia resemble the gas- trochaenids in their representation by rock borers, these borers appear too early or too late in the fossil record or are too dissimilar in shell form to be likely gastrochaenid ancestors. The semiendolithic nestling Ordovician modiomorphid Corallidomus (Whitfield 1893) appears much too early to have given rise to the gastrochaenids. The Mytilacea are represented by Carboniferous through Permian forms morphologically similar to Lithophaga, but an endolithic habit has yet to be demonstrated for any Upper Paleozoic lithophagid (see Pojeta and Palmer 1976). Among other early lithophagids, the Fermian Lithodomina is too specialized in terms of its internal ligament, and the Jurassic/no/^^rna is too dissimilar in its large size and shell form to be likely gastrochaenid ancestors (see generic diagnoses in Soot-Ryen 1969 and Pojeta and Palmer 1976). Furthermore, data of soft anatomy and^hell mineralogy suggest that the Isofilibranchia are the least likely gastrochaenid ancestors among the possibilities considered in Figure 49. Many Mytilacea have an outer calcitic prismatic shell layer that differs mineralogically and microstructurally from the aragonitic outer prismatic layer observed in gastrochaenids (Obe
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