Eosurcula

Eosurcula moorei, from Powell (1966).

Belongs within: Clavatulidae.

Eosurcula: a brief explosion
Published 24 April 2024

The Claiborne Formation is an extensive accumulation of clays and sandstones extending across the southeastern USA, with outcrops in Arkansas, Illinois, Kentucky and Texas. It formed during the middle Eocene, over a series of rapid marine transgressions followed by slower regressions (Eargle 1968). The climate at the time would have been relatively warm, and among the animals that could be found in the warm waters of the Claiborne were species of the conoid genus Eosurcula.

Eosurcula stena, from Jeffery & Tracey (1997).

Eosurcula is a genus of about ten species of medium-sized conoid. Most of these were described from the Claiborne Formation though representatives are also known from California (Powell 1966) and England (Jeffery & Tracey 1997). Shells are about two to four centimetres in length and elongate-fusiform in shape. They have a tall spire, with the whorls medially angulated, and a narrow body whorl tapering to a long, straight, unnotched anterior canal. The sculpture is dominated by spiral cords or carinae, particularly below the shoulder. The protoconch is narrowly conical and multispiral (Powell 1966).

The relationships of Eosurcula with modern conoids are not entirely certain. Powell (1966) included the genus in the Turriculinae (now Clavatulidae) and it does bear a distinct resemblance to the modern clavatulid genus Surcula. However, Powell also noted Eosurcula’s resemblance to another genus found in the Claiborne, Protosurcula, that he included in the Borsoniinae (now Borsoniidae). Despite its success while the Claiborne was forming, Eosurcula did not survive into later periods. The latter part of the Eocene was by a marked cooling of the Earth’s climate; perhaps Eosurcula could not withstand this downwards turn.

Systematics of Eosurcula
Eosurcula Casey 1904P66
|--*E. moorei (Gabb 1860) [=Pleurotoma moorei, Turris moorei]P66
|--E. beaumonti (Lea 1833) [=Pleurotoma beaumonti]P66
|--E. capayana Vokes 1939P66
|--E. concinna Casey 1904P66
|--E. helicoidea Casey 1904P66
|--E. lesueuri (Lea 1833) [=Pleurotoma lesueuri]P66
| |--E. l. lesueuriP66
| `--E. l. beta Harris 1937P66
|--E. pulcherrima (Heilprin 1879) [=Conus pulcherrima]P66
|--E. quadriplenta Harris 1937P66
|--E. sanctimauritii (Vaughan 1896) [=Pleurotoma sanctimauritii]P66
|--E. superpons Harris 1937P66
| |--E. s. superponsP66
| `--E. s. subpons Harris 1937P66
|--E. tardereperta Harris 1937P66
`--E. tuomeyi (Aldrich 1886) [=Pleurotoma tuomeyi]P66

*Type species of generic name indicated

References

Eargle, D. H. 1968. Nomenclature of formations of Claiborne Group, Middle Eocene coastal plain of Texas. Geological Survey Bulletin 1251-D: D1–D25.

Jeffery, P., & S. Tracey. 1997. The Early Eocene London Clay Formation mollusc fauna of the former Bursledon Brickworks, Lower Swanwick, Hampshire. Tertiary Research 17 (3–4): 75–137.

[P66] Powell, A. W. B. 1966. The molluscan families Speightiidae and Turridae: an evaluation of the valid taxa, both Recent and fossil, with lists of characteristic species. Bulletin of the Auckland Institute and Museum 5: 1–184, pls 1–23.

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