Isorropodon Sturany, 1896

Kiel, Steffen, Amano, Kazutaka & Goedert, James L., 2023, New taxa, records, and data for vesicomyid bivalves from Cenozoic strata of the North Pacific region, Acta Palaeontologica Polonica 68 (2), pp. 297-320 : 301-303

Genus Isorropodon Sturany, 1896

Type species: Isorropodon perplexum Sturany, 1896 , by monotypy; Recent, eastern Mediterranean Sea.

Isorropodon humptulipsense sp. nov.

Fig. 3 .

1996 Vesicomya sp. ; Squires and Goedert 1996: 270.

1996 Vesicomya sp. ; Goedert and Kaler 1996: 67.

2007 Archivesica cf. tschudi ( Olsson, 1931) ; Amano and Kiel 2007: 282, figs. 24–29.

ZooBank LCID: urn:lsid:zoobank.org:act:A03AC3DA-5702-4AA3-8099-D69460782E1F .

Etymology: After the Humptulips Formation, the type horizon of the species.

Type material: Holotype: NRM Mo 204723 from CSUN loc. 1583, an internal mold showing muscle scars and pallial line . Paratypes: NRM Mo 204722 from CSUN loc. 1583; NRM Mo 204724, 25 from LACMIP loc. 12385; NRM Mo 204733, from as-yet undescribed boulders found near the East Fork Bridge site (reported here as site (iv) for the Humptulips Formation). Eocene, Washington State, USA .

Type locality: The seep deposit at CSUN loc. 1583, Washington State, USA .

Type horizon: Humptulips Formation, Eocene.

Material.— 39 specimens; 29 specimens from Humptulips Formation: two specimens from CSUN loc. 1583 ( NRM Mo 204722, 23) , 9 specimens from LACMIP loc. 12385 ( NRM Mo 204724–32) , 10 specimens from a block found near the East Fork Bridge site, ( NRM Mo 204733–42) , 8 specimens from the East Fork Bridge site ( NRM Mo 204753–60), and 10 specimens from Siltstone of unit B, Grays River mudstone ( NRM Mo 204743–52). Eocene, Washington State, USA .

Dimensions.—See Table 1.

Diagnosis.—Well-inflated shells reaching nearly 35 mm in length, umbones elevated, prosogyrate; lunule deeply incised; broad escutcheon bordered by blunt ridges; pallial line distinct, anteriorly narrow, posteriorly broadening, curving upward, either evenly or with slight indentation, no pallial sinus; RV hinge with elongate cardinal teeth 1 and 3a running parallel to shell margin, 3b pointing posteroventrally; LV hinge with cardinal teeth 2a and 2b elongate, parallel to shell margin, 2b with small posterior peg.

Description.—Shells reaching nearly 35 mm in length and 25 mm in height, inflated, umbones elevated, strongly prosogyrate, at anterior 16–18% of shell length; lunule deeply incised, elongate-heart shaped, escutcheon broad, demarcated by blunt ridges; anterior adductor muscle scar beanshaped, fused with pedal retractor scar; pallial line entire, distinct, anteriorly narrow, broadening posteriorly, starting at posterodorsal corner of anterior adductor muscle scar, running close and parallel to ventral shell margin, posteriorly curving upward, either evenly or with slight indentation, to join posterior adductor muscle scar at its base; no pallial sinus; distinct interior ridge running from umbones to ventral side of posterior adductor muscle scar; hinge with three cardinals in each valve; RV with elongate cardinal teeth 1 and 3a running parallel to shell margin, 3b bifid with raised edges, pointing posteroventrally; in LV cardinal teeth 2a and 2b elongate, parallel to shell margin, 2b with small posterior peg, cardinal tooth 4b thin, short.

Remarks.—For a more detailed description see Amano and Kiel (2007). The small, circular and deep subumbonal pit figured earlier ( Amano and Kiel 2007: figs. 24, 25) was likely an artifact because we have not seen it in additional specimens. Small-sized specimens reaching 2.5 mm in length were reported as “unidentified vesicomyid bivalve” from the middle Eocene Weatherwax seep deposit of the Humptulips Formation ( Hybertsen and Kiel 2018); these oval specimens have an almost Vesicomya -like hinge dentition and might be juveniles of Isorropodon humptulipsense sp. nov.

The pallial line of Isorropodon humptulipsense shows some variation in thickness and in its posterior-most part before reaching the posterior adductor muscle scar. In some specimens the pallial line curves evenly upward into the posterior adductor muscle scar ( Fig. 3B, G ), in others, it shows a slight indentation (but not a sinus) on its way upward ( Fig. 3F ). Other species of Isorropodon also show some variation of this feature. In I. megadesmum Oliver, Rodrigues, and Cunha, 2011 , the pallial line joins the posterior adductor muscle scar after a sinuous swing ( Oliver et al. 2011: fig. 9C); in I. bigoti Cosel and Salas, 2001 (their fig. 55) and I. curtum Cosel and Salas, 2001 (their fig. 59) both join after a straight section followed by an angulation of the pallial line, and this angulation can point either posteriorly ( I. bigoti ) or anteriorly ( I. curtum ). In I. nyeggaense Krylova, Gebruk, Portnova, Todt, and Haflidason, 2011 ( Krylova et al. 2011: figs. 6D, 7D), the pallial line just has a straight section before reaching the posterior adductor muscle scar.

Compared to other species of Isorropodon , I. humptulipsense is relatively large and has a distinct lunular incision. Isorropodon bigoti reaches only 21 mm in length, has a straighter ventral margin, and an indistinct lunular incision. The type species I. perplexum reaches a maximum of 15 mm in length, its hinge teeth are arranged mostly parallel to each other in contrast to the radiating teeth of I. humptulipsense , and it lacks a lunular incision. Isorropodon curtum is not only much smaller, it also has a shorter shell. Of similar size as I. humptulipsense is I. striatum , but its hinge teeth are arranged in a parallel fashion and its lunular incision is indistinct. Isorropodon megadesmum reaches only 12 mm in length and has an indistinct lunular incision, and I. nyeggaense : is up to 21 mm long, is less inflated than I. humptulipsense , and has an indistinct lunular incision. Isorropodon arguinense Hoffman, Cosel, and Freiwald, 2019 , is smaller and much more elongate than I. humptulipsense (i.e., Hoffman et al. 2019). The Miocene I. frankfortense Kiel and Amano, 2007 (originally introduced as I. frankfortensis ) from the Astoria Formation in southwestern Washington State is much smaller and has a more convex ventral margin than I. humptulipsense , and has the hinge teeth arranged in a parallel fashion ( Amano and Kiel 2007).

Somewhat similar is Cytherocardia cytheroides ( Mayer, 1868) , type of the little-known genus Cytherocardia Sacco, 1900 , from Miocene of Italy ( Mayer 1868; Sacco 1900), which has been suggested to be a member of Vesicomyidae ( Janssen and Krylova 2012) . The shells are oval with strong, elevated, prosogyrate umbones, thus somewhat have a similar shape to Isorropodon humptulipsense (i.e., Janssen and Krylova 2012: pl. 5: 19). However, the taxonomically important dentition of the right valve of C. cytheroides remains unknown, as are internal features such as pallial line and muscle scars. We find the allocation of Cytherocardia to the Vesicomyidae doubtful, and the lack of a lunular incision in C. cytheroides clearly distinguishes it from Isorropodon humptulipsense .

Stratigraphic and geographic range.—Seep carbonate deposits within the middle Eocene Humptulips Formation and upper Eocene Siltstone of unit B, Washington State, USA.

Genus Pliocardia Woodring, 1925

Type species: Anomalocardia bowdeniana Dall, 1903 , by original designation; Pliocene, Bowden Formation, Jamaica ( Dall 1903).

Figures

Fig. 3. The vesicomyid bivalve Isorropodon humptulipsense sp. nov., from middle to upper Eocene strata in western Washington State, USA. A. Rubber cast of the paratype, NRM Mo 204722, Humptulips Formation, CSUN loc. 1583, hinge area, RV (A1), LV (A2). B. Holotype, NRM Mo 204723, Humptulips Formation, CSUN loc. 1583, internal mold, view on LV.C. NRM Mo 204724, Humptulips Formation, LACMIP loc. 12385, articulated specimen, view on anterodorsal margin showing lunular incision. D. Paratype, NRM Mo 204733, Humptulips Formation, near East Fork Bridge site, view on anterodorsal margin and anterior adductor muscle scars. E. Paratype, NRM Mo 204744, Siltstone Unit B along Grays River, specimen with partially preserved shell. F. Paratype, NRM Mo 204743, Siltstone Unit B along Grays River, internal mold showing pallial line with slight indentation on RV. G. Internal mold, USNM 534952, Humptulips Formation, CSUN loc. 1583, view on RV with pallial line curving evenly into posterior adductor muscle scar (re-illustrated from Amano and Kiel 2007).