CHARACTER DISPLACEMENT IN PRODUCTOID BRACHIOPODS FROM THE LATE PENNSYLVANIAN OF CENTRAL TEXAS

Biodiversity is partly dependent on the availability of resources and competition for those resources. Competition can potentially exclude weaker species, thus decreasing diversity. As competition negatively affects all competitors through energy loss, all organisms seek to avoid competition. Morphologically similar species often face the strongest competition from each other. Thus, competition may lead to character displacement, the changing of a physical trait away from the average state of competitors. Utilizing a new “niche,” or a different fraction of the same resource, allows a species to reduce competitive pressure. This natural selection for character displacement can drive speciation among intraspecific competitors, or limit the similarity of interspecific competitors. The fossil record precludes direct observation of competing organisms, but morphological differences are preserved. Morphology serves as a proxy for resource use both in the modern and deep time, and can be used to infer competition.

This study focused on two co-existing genera of “marginiferid” brachiopods, *Hystriculina* and *Kutorginella* (*Retaria*). These taxa are morphologically similar; both are small, transverse, sulcate productoids. Specimens were collected from three different, but environmentally similar, units: the Finis and Wayland Shales, and Pueblo Formation from the Virgil Series in central Texas. The purpose of this study was to determine if character displacement could be observed within these two taxa when co-occurring.

The Finis Shale fauna included both *Hystriculina* (62%) and *Retaria* (38%), as did that of the Wayland shale (33% *Hystriculina,* 77% *Retaria*). The Pueblo shale fauna contained only *Hystriculina*. When *Hystriculina* occurred without *Retaria* present, the volume, area (foot print), and corpus width standardized for size, all increased (p < 1.0E-11 for all). The presence of the larger *Retaria* appears to cause character displacement towards a smaller size in *Hystriculina*.

This trend indicates that *Hystriculina* and *Retaria* were competing for a similar resource base, and *Hystriculina* adapted to decrease this competition. This evidence of character displacement in the fossil record offers a deeper understanding of competition.