THE INVISIBLE INARTICULATE HYPOTHESIS: UNVEILING WIDESPREAD OCCURRENCES OF LINGULID BRACHIOPODS IN THE CENOZOIC FOSSIL RECORD

Whereas lingulid brachiopods occur today in many low-latitude marine benthic ecosystems, their documented Cenozoic fossil record is remarkably limited. Their scarcity in the post-Paleozoic fossil record may reflect either a taphonomic megabias or a real secular decline in their ecological and evolutionary importance. To evaluate this issue, we have sampled Miocene and Pliocene units of the Atlantic Coastal Plain. These units, while famous for diversity and abundance of carbonate (e.g., mollusks, arthropods, corals) and phosphatic (e.g., fish teeth, epifaunal discinid brachiopods) macrofossils, are not known for their lingulid brachiopod fauna.

A total of 38 samples of shell material from multiple localities of the Yorktown (Pliocene) and Eastover (Miocene) formations were evaluated for the presence of lingulid brachiopods. Fine sediment fractions (< 2mm) of residual shell debris were screened for the presence of phosphatic material. In 11 out of 38 samples, fragments of lingulid valves identifiable to the genus level were found (posterior shell beaks with unique internal septa details assignable to the genus Glottidia). In 30 out of 38 samples, shell fragments likely to represent lingulids were found. This assignment is strongly supported both mineralogically and microstructurally by comparative electron microscopy-based analyses of the recovered shell fragments and live-collected specimens of Glottidia palmeri.

This study suggests that lingulid brachiopods may be widespread in the late Cenozoic marine fossil record, but remain largely undetectable due to a severe taphonomic bias. This prediction, which we refer to as the “Invisible Inarticulate Hypothesis”, can be translated into the following testable statement: “lingulids are preserved in many low-to-mid latitude Cenozoic fossil assemblages, but their remains are represented, nearly exclusively, by tiny shell fragments”.

Cenozoic lingulids are likely to be severely underrepresented in occurrence-based paleontological databases such as PBDB. As important, the Cenozoic fossil record of lingulid brachiopods may be more accessible than previously thought and, if analyzed using fragment-oriented approaches, should offer a wealth of useable data.