Leptochiton terryiverseni, Hybertsen & Kiel, 2018

Hybertsen, Frida & Kiel, Steffen, 2018, A middle Eocene seep deposit with silicified fauna from the Humptulips Formation in western Washington State, USA, Acta Palaeontologica Polonica 63 (4), pp. 751-768 : 763-764

Leptochiton terryiverseni sp. nov.

Fig. 16 .

Etymology: For Terry Iversen, who helped in collecting this material.

Type material: Holotype: NRM Mo 185036 consisting of three articulated intermediate valves . Paratypes: NRM Mo 185035 , NRM Mo 185037, UWBM 108862 , three isolated intermediate valves .

Type locality: The Satsop Weatherwax seep deposit, Washington State, USA .

Type horizon: Basal Humptulips Formation, middle Eocene.

Material.—The type material only.

Dimensions.—The largest specimen is 6.7 mm wide and 3 mm long.

Diagnosis.— Leptochiton having moderately elevated, round-backed valves with angulation, lacking jugal area of the tegmentum; pleural area with fine, subparallel rows of granules; lateral areas with irregular but distinct commarginal ridges.

Description.—Elongate-oval outline, valves of moderate elevation (h/w ~0.33), round-backed with an angulation, side slopes slightly convex; jugal area of tegmentum absent, pleural area sculptured by fine granules arranged in subparallel, longitudinal rows; sculpture changes at abruptly at diagonal ridges, granules on lateral areas stronger, arranged somewhat irregularly near the middle, but forming distinct commarginal ridges toward the lateral margins. The apophyses are separated by a broad jugal sinus.

Remarks.— Squires and Goedert (1995) reported the extant Leptochiton alveolus ( Lovén, 1846) from Eocene– Oligocene seep deposits in western Washington. Their illustrated specimens are from the Oligocene Lincoln Creek Formation and show a quincunx arrangement of granules on the tegmentum, which is different from the longitudinal arrangement of granules in L. terryiverseni n. sp. Also mentioned by Squires and Goedert (1995) were two valves of L. alveolus from seep deposit LACMIP loc. 12385 in the Humptulips Formation, but they did not illustrate them. Hence it remains unclear whether they belong to the species from the Lincoln Creek Formation or to L. terryiverseni . Extant species similar to L. terryiverseni include L. micropustulosus Kaas, 1984 from ca. 1135 to 1236 m depth on the Barbados Prism ( Kaas 1984) and Leptochiton sarsi Kaas, 1981 from Scandinavian waters ( Kaas 1981; Dell’Angelo et al. 2009), both of which differ from L. terryiverseni by the granulae on the tegmentum gently curving around the diagonal ridges, in contrast to the sharp change in granulae arrangement in L. terryiverseni . Leptochiton tenuidontus Saito and Okutani, 1990 from a hydrothermal vent site in 1395 m depth in the Okinawa Trough ( Saito and Okutani 1990) shows a similar arrangement of the granulae, but its valves are less elevated than those of L. terryiverseni .

Among fossil species of Leptochiton , an Eocene specimen from Seymour Island, Antarctic Peninsula ( López Cabrera and Olivero 2011), has the longitudinally arranged granule distributed over a much broader area on the tegmentum than in L. terryiverseni , and laterally the granulae tend to radiate outward. In the late Eocene Leptochiton sp. reported from the Lincoln Creek Formation in western Washington ( Dell’Angelo et al. 2011), the granulae on the anterior-lateral sides tend to radiate outward rather than being longitudinally arranged, as in L. terryiverseni . Recently, that specimen was hesitantly assigned to the extant L. cascadiensis ( Sigwart and Chen 2017) . The wood-associated Leptochiton lignatilis Bertolaso, Garilli, Parrinello, Sosso, and Dell’Angelo, 2015 from the middle to late Miocene of northern Italy has irregularly arranged granulae ( Bertolaso et al. 2015) in contrast to the longitudinal arrangement in L. terryiverseni . Early Oligocene specimens from France assigned to Leptochiton cf. algesirensis ( Capellini, 1859) have outwardly directed beaded riblets rather than longitudinally arranged granules ( Cherns and Schwabe 2017). Finally, the Paleocene Leptochiton faxensis Sigwart, Andersen, and Schnetler 2007 has a quincunx arrangement of granules.

Stratigraphic and geographic range.— Type locality and horizon only.

Figures

Fig. 16. The lepidopleurid polyplacophoran Leptochiton terryiverseni sp. nov. from the middle Eocene Satsop Weatherwax seep deposit, Washington State, USA; all specimens represented by intermediate valves. A. NRM Mo 185035 (paratype); isolated valve with preserved apophyses in dorsal (A1) and anterior (A2) views; close-up of sculpture (A3). B. NRM Mo 185036 (holotype), three articulated valves in dorsal (B1), posterior (B2), and lateral (B3) views. C. NRM Mo 185037 (paratype); isolated valve showing the broad jugal sinus in lateral (C1), dorsal (C2), and ventral (C3) views.