THE BRACHIOPOD TRAP: WHAT THEIR OLDEST (UPPER ORDOVICIAN, OHIO) FAILED ESCAPE BURROWS TELL US ABOUT THE EVOLUTION OF BURROWING IN LINGULIDS

Infaunal organisms living in shallow marine settings are vulnerable to exhumation during storms or entombment by storm-deposited sediments. Cambrian­–Early Ordovician lingulids included epifaunal as well as possible infaunal forms. However, many epifaunal forms became extinct during the Middle Ordovician, and Late Ordovician lingulids were similar in their infaunal habits and marginal habitats. Modern infaunal lingulids are able to reorient themselves after burial in sediments, but it is unclear when this ability evolved. Initial burrowing of juvenile lingulids, as well as re-burrowing of exhumed modern lingulids involves digging downwards and then back up in a u-shape, but successful escape burrowing involves the instinct to move upwards only. Previous documentation of lingulid escape burrows dates only as far back as the Triassic.

Thousands of Pseudolingula sp. from the upper Fairview Formation in the spillway of Harsha Lake, Cleremont County, Ohio, have been found beneath disarticulated Rafinesquina shells in a storm deposit. Some lingulids are also observed beneath other concave-downwards shell fragments in the same bed. The Rafinesquina shells are partially spalled, revealing clusters of lingulids concentrated just beneath the shell near the centers of the valves. Thin-sections of individual Rafinesquina specimens and associated sediment reveal burrows beneath the lingulids. Intriguingly, in some cases they appear to have followed each other’s routes upwards.

The ability to escape burial requires an instinct to burrow upwards. When these brachiopods encountered the Rafinesquina shells, their burrows inevitably turned upward in the direction of the highest point beneath the Rafinesquina shell, where the clusters of Pseudolingula were observed initially. In several instances the lingulids were apparently millimeters away from bypassing the obstacle, but took a fatal turn towards the highest part of the overlying shell instead.

The entrapment of Pseudolingula at the end of their burrows beneath Rafinesquina suggests that lingulid brachiopods have long had the ability to burrow upwards and re-establish themselves after burial during a storm event, even though these particular lingulids were unfortunate enough to encounter a physical barrier to their upward progress.