Eric Gustafson

The Early Eocene Decapod Crustacean Fauna of the Lookingglass Formation, Oregon

Introduction

The early eocene decapod crustacean fauna of the lookingglass formation, oregon. Explore Oregon's Early Eocene decapod crustacean fossils from the Lookingglass Formation. Discover the Pacific Northwest's earliest Cenozoic crab fauna, well-preserved with unique genera.

Abstract

The Tenmile Member of the Lookingglass Formation (Lower Eocene) of southwest Oregon produces a diverse marine invertebrate fossil fauna including at least eleven genera of decapod crustaceans. This is the earliest Cenozoic crab fauna known from the Pacific Northwest, and is one of the earliest on the west coast of North America. The fossiliferous sediments are generally fine-grained mudstones without much compaction, with evidence suggesting a low-energy environment in relatively shallow water and a mild warm climate. The abundance of complete crabs at some localities is exceptional, and suggests repeated mass mortality caused by environmental conditions. Modern relatives of several of the taxa are burrowers. The fossils are often well-preserved in concretions. Preservation in concretions preferentially involves decaying crab corpses; other taxonomic groups are underrepresented in concretions. The fauna includes an unusual number of raninid crabs, in three genera (Raninoides vaderensis, Rogueus orri, and Doraranina manleyi) of which the latter two genera are only known from this formation. A scyllarid, Llajassus caesius, is one of a very few of this group known as fossils. Dominant in numbers is the euryplacid crab Orbitoplax weaveri, with specimens numbering in the thousands. Other crustaceans include the ghost shrimp Ctenocheles hokoensis, Panopeus baldwini (the largest crab in the fauna), Eriosachila orri, Palaeopinnixa rathbunae, and rare examples of Archaeozius occidentalis and Marycarcinus hannae. The abundance of specimens allows examination of several taxa as populations rather than individuals as is frequently the case in fossil crabs.

Review

The paper, "The Early Eocene Decapod Crustacean Fauna of the Lookingglass Formation, Oregon," presents a significant and timely contribution to our understanding of early Cenozoic marine invertebrate life. By detailing the diverse decapod crustacean fauna from the Tenmile Member of the Lookingglass Formation, the authors establish the earliest known Cenozoic crab assemblage from the Pacific Northwest, and one of the earliest identified on the west coast of North America. This foundational work provides a critical faunal baseline for deciphering the paleobiogeography and evolutionary patterns of decapods during a pivotal warm period in Earth's history, shedding light on a previously underrepresented region. The research reveals a remarkably rich assemblage, comprising at least eleven genera of decapod crustaceans. Notably, the fauna includes an unusual abundance of raninid crabs, with two genera (*Rogueus orri* and *Doraranina manleyi*) unique to this formation, alongside the rare scyllarid *Llajassus caesius*. Dominating the collection numerically is the euryplacid crab *Orbitoplax weaveri*, with thousands of specimens, a rarity for fossil assemblages. The authors also provide valuable insights into the paleoenvironment, characterized by fine-grained mudstones suggesting a low-energy, shallow-water setting under a mild warm climate. Exceptional preservation, often within concretions, points to repeated mass mortality events, preferentially involving decaying crab corpses, and crucially facilitates population-level studies of several taxa – a rare and highly valuable opportunity in fossil decapod research. This study's detailed inventory and robust taphonomic observations are a substantial asset for paleontological research. The ability to examine taxa as populations rather than isolated individuals, especially for dominant species like *Orbitoplax weaveri*, offers unique avenues for investigating intraspecific variation, ontogeny, and paleoecological dynamics within an ancient ecosystem. The presence of novel and biogeographically restricted taxa underscores the distinctiveness of this Early Eocene fauna and its importance for tracing the origins and diversification of modern decapod lineages. This work serves as an essential reference point for future comparative studies on Cenozoic marine faunas and will undoubtedly stimulate further research into the paleoenvironmental conditions and evolutionary history of decapods in the Pacific Northwest.

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