ONTOGENETIC CHANGE IN THE CRAB ORBITOPLAX WEAVERI (DECAPODA, BRACHYURA, EURYPLACIDAE) FROM THE EOCENE LOOKINGGLASS FORMATION, OREGON, WASHINGTON

Orbitoplax Tucker & Feldmann, 1990, is a well-documented eubrachyuran crab from the Paleogene of west coastal North America. Extant members of the family to which it is referred, Euryplacidae, are typical of the Caribbean and western North Pacific, and they are not found on the Pacific Coast of North America today (OBIS, www.obis.org, data accessed 1/30/2023). The fine preservation of specimens of species in the genus has been noted, and unusually well-preserved eyestalks are common. Here, a series of specimens, probably corpses, of O. weaveri (Rathbun, 1926), represents a possible growth series. Such a series is unusual among fossil decapods, as typically only larger adults are preserved (Schweitzer et al., 2014). All of the fourteen studied specimens were collected from the Eocene Lookingglass Formation in Oregon. The eleven smallest specimens were recovered from the Agnes locality. The remaining three specimens are larger and from the Dora locality. Initial findings demonstrate a continuous growth series among all fourteen specimens and allometric growth in the carapace length and fronto-orbital width as compared to carapace width. Other differences based upon size include increased regional definition in smaller specimens, in which the regions are more inflated but less well-ornamented with granules. Anterolateral spines are longer and sharper in larger specimens. Such changes in increasingly larger specimens suggest ontogenetic processes versus environmental conditions as the cause of the differences based upon carapace size. Because the smaller specimens seem to be concentrated at one locality, juveniles and adults may have inhabited different environments, commonly observed in extant brachyurans. Further investigation of lithology and paleoenvironment will permit testing this hypothesis. Specimens were donated for study by Robert Manley, Mount Angel, Oregon, USA.