Acesta, H. AND A. ADAMS, 1853

Hickman, Carole S., 2023, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs, PaleoBios 40 (5), pp. 1-51 : 37-38

ACESTA H. AND A. ADAMS, 1853

Type species — By monotypy, Ostrea excavata Fabricius (1779) . Holocene, Northeastern Atlantic continental

margin, Azores, Canaries; Pleistocene, widespread in Mediterranean.

The oldest reported occurrence is from Upper Jurassic rocks in India ( Kanjilal 1990). The Mesozoic record is largely confined to the margins of the southern conti- nents,with a continuous record across the Cretaceous-Paleogene boundary in Antarctica (Stilwell and Zinsmeister 1992) and Cretaceous records from Australia ( Darragh and Kendrick 1991, Stilwell and Mckenzie 1999) and New Zealand ( Beu 1973, Beu and Maxwell 1990). By the end of the Paleogene, the genus had achieved a cosmopolitan distribution, expanding its latitudinal and bathymetric range as global cooling became increasingly pronounced. The type species ( Fig. 24A, B ) represents an additional trend toward exploiting hard substrates at the shelf-slope break on continental margins in addition to occupying soft substrates at greater depths.

The shell of Acesta s.s. is typically thin and fragile despite its large size. Knowledge of the basic biology and life habits of the living species has been limited by their occurrence on vertical rock walls and overhangs in submarine canyons and on seamounts as prominent members of communities of attached suspension feeders. These communities have been difficult to sample and have gone undetected until the advent of ROV video observation. Observations of walls of submarine canyons, seamounts, and escarpments on the Eastern Pacific margin of North America have revealed dense populations (e.g., Clague et al. 2012) that show a remarkable degree of genetic connectivity in spite of their disjunct occurrences. The type species is slow-growing, long-lived (5–80 years), and one of the dominant species in deep, cold-water coral reef communities (Schleinkofer et al. 2021). The remarkable ability of these large bivalves to detach and swim (Kohl and Vokes 1994) may account for the preservation of their thin fragile shells in the massive siltstone facies of the Keasey Formation at localities where the hardground habitat is not exposed.

Because these in situ observations and collections of living Acesta have special significance for interpret- ing the fossil record (Upper Jurassic–Holocene) they merit additional discussion. The Mesozoic record is exclusively southern hemisphere, expanding in the late Eocene into the northern hemisphere during dramatic faunal turnover and transition from tropical Eocene fauna to the modern fauna ( Hickman 2003). The Acesta Community was first recognized and named from pecu- liar low-diversity, species-dominant fossil assemblages in Late Paleogene deep-water siltstones in Washington and Oregon, typified by the upper member of the Keasey Formation ( Hickman 1984). At the time the community was described it was not possible to provide an accurate habitat description, although association with brachiopods and basal root tufts of hexactinellid sponges and the parallelodontid genus Porterius (described above) indicated attachment to the substrate.

New habitat discoveries of Acesta have been identified as an important element in documenting historical underestimation of deep-sea biodiversity. For example, ROV exploration of a deep submarine canyon system on the Celtic margin has revealed a new biotope dominated by Acesta excavata and a species of large deep-sea oyster on vertical surfaces and overhangs ( Johnson et al. 2013). ROV mapping of vertical submarine canyon walls document its association with cold-water azooxanthellate corals ( Huvenne et al. 2011). It is a dominant and conspicuous member of bathyal cold-water coral bank facies on the northern European continental margin (Lopez Correa et al. 2005) and occurs with cold-water corals on carbonate mounds, pock marks, and craters associated with hydrocarbon seepage ( Jensen et al. 2010). The association with cold seeps and reducing environments extends to observations at cold seeps on the Louisiana slope in the Gulf of Mexico where another Acesta species lives attached to the tube openings of vestimentiferan worms and feeds on the lipid-rich eggs ( Järnegren et al. 2005). Although there is no clear evidence of chemosymbiosis, a novel intracellular bacterium has been isolated from the gills of A. excavata ( Jensen et al. 2010) . It seems likely that new interest in the deep-water file shells will continue to yield interesting new data.

Phylogeographic studies have likewise contributed useful data for understanding the fossil record. On the Eastern Pacific margin ROV images and samples of live animals from submarine canyons in the Gulf of California have extended the range of two northern species of Acesta and documented dense aggregations at greater depths than previously recorded for either species as well as an additional example of strong genetic connectivity with the high latitude populations (Walz et al. 2014). Range extensions have been subject to question and more detailed analyses. Initial collection by submersible of the European A. excavata from rock walls of a fjord on the south coast of Newfoundland was hailed as a dramatic range extension ( Gagnon and Haedrich 2003). However, a subsequent combination of molecular data and morphometric analysis of the shells of seven Acesta species have identified the northwestern Atlantic form as a new cryptic species ( Gagnon et al. 2015).

Thus far the Cenozoic fossil species from the Pacific Northwest and California are narrowly endemic, but the new habitat data and phylogeographic analyses of living taxa provide a new context for considering the fossil record.

Stratigraphic Range —Upper Jurassic–Holocene.

Figures

Figure 24. Living type species of Acesta, A. excavata (Fabricius, 1799), SBMNH 616346. Norway. A. Left valve exterior. B. Left valve interior. Scale bar=5 cm.