Porterius gabbi
Creators: name: Hickman, Carole S.
Publisher: name: Zenodo
Year: 2023
Subjects: Biodiversity
Taxonomy
Animalia
Mollusca
Bivalvia
Arcida
Parallelodontidae
Porterius
Porterius gabbi

description: PORTERIUS GABBI (DICKERSON, 1917)
FIGS. 5A–H, 6A–G
Barbatia gabbi Dickerson (1917). p. 167; pl. 29, fig. 4.
Barbatia andersoni Van Winkle (1918). pp. 81–82; pl.
6, fig. 2.
Parallelodon (Porterius) andersoni (Van Winkle), Clark (1925). pp. 79–80; pl. 8, fig. 33; pl. 9, figs. 1, 6.
“ Barbatia” andersoni Van Winkle, Reinhart (1935). p. 7.
Porterius gabbi (Dickerson), Schenck (1936). p. 7.
Cucullaria (Porterius) gabbi (Dickerson), Effinger (1938). p. 366; pl. 46, fig. 6.
Porterius gabbi (Dickerson), Weaver (1943). pp. 53–54; pl. 9, figs. 7, 10, 11, 14; pl. 11, fig. 3.
Porterius gabbi (Dickerson), Vokes in Warren, Norbisrath and Grivetti (1945) (checklist).
Porterius gabbi (Dickerson), Warren and Norbisrath (1946). p. 227.
Porterius gabbi (Dickerson), R.C. Moore and Vokes (1953). pp. 115, 118, 119.
Porterius gabbi (Dickerson), Hickman (1984), pp. 1223, 1226; fig. 7c.
Amended description —Shell small (length <20 mm), thin; shape ovate-elongate, moderately inflated, strongly inequilateral, with small prosogyrous umbones far anterior to midline; anterior margin rounded and narrow relative to obliquely expanded posterior margin; dorsal margin straight, ventral margin with faint byssal flexure but no indication of gape; exterior sculpture of well-developed radial ribs separated by deep narrow grooves; posterior ribs becoming broader and flat-topped during growth, anterior ribs remaining narrow and increasing in number, either by splitting or intercalation; commarginal sculpture absent except for irregular growth checks; shell interior with well-developed adductor scars and simple pallial line; inner shell surface with slightlyupraised radial threads of subequal prominence, most distinct where they terminate at pallial line; hinge plate slightly arched, divided into anterior and posterior sets of teeth interrupted by edentulous central gap resulting from overgrowth of duplivincular ligament; two or three well-developed posterior teeth, elongate and distinctly parallel to hinge; variable number of small anterior teeth, crowded and chevron shaped with short, vertical posterior segment and longer anterior segment subparallel to hinge; both sets of hinge teeth finely striate.
Discussion —The elongate posterior hinge teeth parallel to the hinge plate (Figs. 5B–D, F, G; 6B, D, G) separated by a broad edentulous central gap (Figs. 5B, C; 6B, G) from the complex of small anterior teeth with anterior segments subparallel to the hinge (Figs. 5B, C) distinguish Porterius. Asymmetric growth of the ligament increasingly displaces the anterior and posterior cardinal margins and is less evident in juvenile shells (Fig. 6D). The edentulous gap becomes increasingly large during ontogeny. The faint flexure in the ventral margin of the valves (Figs. 5A, B; 6C) suggests that a weak complex of fine byssal threads provided semi-infaunal attachment, in contrast to the byssal gape characteristic of epifaunally attached arcoids. The ovate-elongate shell shape, lack of a posterior umbonal carina, fine exterior radial sculpture, and fine micro-striation of the hinge teeth provide a superficial similarity to Paris Basin Eocene species of Barbatia (Hickman 2021, Figs. 5, 6)
Porterius gabbi is part of the Acesta Community of Hickman (1984), where it occurs in patchy epifaunal assemblages in association with giant limids, a terebratulid brachiopod, a vermetid gastropod, basal root tufts of hexactiellid sponges, and occasionally with hexactinellid spicules and fragments of spicular meshworks.
Reliance on poor literature illustrations of fossil material, coupled with failure to examine actual specimens of this species as the type of Porterius, has led to questionable allocation of species that extend its geographic range to Europe and the Western Pacific and its temporal range from Eocene to Holocene. Unfortunately, systematic treatment of parallelodontids in the Treatise on Invertebrate Paleontology (Newell 1969) illustrates the hinge of Porterius with a drawing of an Eocene arcoid from the Paris Basin. The clarification presented here and by Hickman (2021) is therefore followed by treatment of an older Paleocene–middle Eocene species of Porterius from California.
Material examined — 18 specimens.
Figured Keasey hypotypes — USNM 561838 (right valve),length 22.6 mm, height 10.5 mm, Loc. USGS 15518; UCMP 110736 (left valve in matrix), length 13.0 mm, height 9.0 mm, Loc. UCMP IP2549; UCMP 110737 (left valve), length 17.3 mm,height 10.6 mm, Loc. UCMP 2540.
Figured Gries Ranch hypotypes — UCMP 32429 (left valve), length11.1 mm, height 7.2 mm, Loc. UCMP 3607; UCMP 110738 (right valve, adult), length 22.9 mm,height 11.0 mm), Loc. UCMP IP2289; UCMP 110739 (left valve, juvenile), length 10.0 mm, height 6.5 mm, Loc. UCMP IP2289.
Localities —Keasey Formation: Upper Member: UCMP IP2540, 2549; UGSG 15269, 15315, 15518, 15581, 15601. Gries Ranch Formation: CAS 181 (Dickerson’s holotype); UCMP 3607 (Effinger’s hypotype), UCMP IP2289. Lincoln Creek Formation: UW 126 (old log dam on Porter Creek).
Stratigraphic range and age —This species is narrowly restricted to upper member of the Keasey Formation, Gries Ranch Formation, and coeval beds in the Lincoln Creek Formation. The age of these beds has been treated alternatively as late Eocene or early Oligocene. The peculiar community assemblages of this “turnover fauna” (Hickman 2003) postdate a dramatic regional extinction of the “tropical Eocene Fauna” and predate establishment of the Oligocene recovery fauna that subsequently led to the temperate “modern fauna.” The transitional turnover assemblages are correlated with the dramatic Oi-1 climate cooling and glaciation at 33.5 MA.
description: Published as part of Hickman, Carole S., 2023, Paleogene marine bivalves of the deep-water Keasey Formation in Oregon, Part II: The pteriomorphs, pp. 1-51 in PaleoBios 40 (5) on pages 10-13, DOI: 10.5070/P940561331, http://zenodo.org/record/10913477