Channelaxinus antipoloensis, Kiel & Aguilar & Kase, 2020

Kiel, Steffen, Aguilar, Yolanda M. & Kase, Tomoki, 2020, Mollusks from Pliocene and Pleistocene seep deposits in Leyte, Philippines, Acta Palaeontologica Polonica 65 (3), pp. 589-627 : 594-595

Channelaxinus antipoloensis sp. nov.

Fig. 5 .

Zoobank LSID: urn:lsid:zoobank.org:act:E4A68DB5-CBE8-4083-B52D-3FB54D8B358D

Etymology: In reference to the type locality Antipolo Point.

Type material: Holotype NMNS PM 28177 , an articulated specimen; paratypes: NMNS PM 28178 , an isolated right valve, NMNS PM 28179 , an articulated specimen from the type locality.

Type locality: Antipolo Point, Leyte, Philippines .

Type horizon: Lower Pleistocene part of the Bata Formation.

Material.— The type material and one lot including eight specimens ( NMNS PM 28120 ) from the type locality .

Dimensions (in mm).—NMNS PM 28177, L = 43.0, H = 38.0, W = 25.3; NMNS PM 28178, L = 30.3, H = 24.0, W (single) = 8.5; NMNS PM 28179, L = 38.0, H = 30.0.

Diagnosis.—Shell of average size for genus; median ridges on shell surface developed as angulations rather than distinct ridges; lunule small for genus, comprising less than half of anterior shell margin; submarginal sulcus distinct but short.

Description.—Shell polygonal, umbones elevated, prosogyrate, rather blunt; short but distinct posterior submarginal sulcus; posterodorsal area of moderate height, rather short, bordered by distinct sulcus and sharp ridge; shell surface with two low angulations running from umbo to ventral shell margin; anterior margin straight to slightly concave; lunule deep, length slightly less than half of anterior margin; hinge plate narrow, edentulous; ligament nymph short.

Remarks.— Channelaxinus antipoloensis sp. nov. is here placed in Channelaxinus based on its deeply impressed lunule and flattened median slope that is bounded by low ridges. With these two characters, C. antipoloensis clearly differs from members of Conchocele , a genus of large thyasirids that is often found at fossil seep deposits(see above). Ascetoaxinus Oliver and Frey, 2014 , with its type species Ascetoaxinus quatsinoensis Oliver and Frey, 2014 , has a similar overall shell shape, but C. antipoloensis differs from Ascetoaxinus by lacking the rounded projections that scallop the lunule edge in Ascetoaxinus , and by having a small lunule. Thyasira Lamarck, 1818 differs by having shells that are less angular and thinner, and the posterior and submarginal sulci of C. antipoloensis are deeper and sharper than in Thyasira .

The most similar extant species is Channelaxinus adelaideanus ( Iredale, 1930) ; the original illustration is a sketch ( Iredale 1930: pl. 63: 6, 7) and photos of the holotype were provided by Oliver (2015: figs. 3E, F). They show a specimen with a more pronounced median ridge compared to the median angulation of C. antipoloensis ; also the less distinct radial ridge ventral to the median ridge is more pronounced in C. adelaideanus than in C. antipoloensis . Channelaxinus excavata (Dall, 1901) has a larger lunule and a more elongate submarginal sulcus than C. antipoloensis .

The only fossil so far assigned to Channelaxinus is Channelaxinus sp. from the middle Miocene (Langhian) Ca’ Cavalmagra seep deposit in northern Italy ( Kiel and Taviani 2017: 450, fig. 5); the illustrated specimens differs slightly from C. antipoloensis by having a longer posterodorsal area, which is particularly well-seen in Kiel and Taviani (2017: fig. 5.4). The early Miocene Thyasira minoensis Itoigawa, 1960 from the Mizunami Group in central Honshu, Japan is also similar and might belong to Channelaxinus but appears to have a more defined median ridge ( Itoigawa 1960: pl. 2: 1) compared to the median angulation C. antipoloensis . Another potential fossil Channelaxinus is the early Miocene (Altonian, ~late Burdigalian) Thyasira (Prothyasira) bartrumi Powell, 1935 from a deep-water faunule found near Auckland, New Zealand, which is smaller (L = 15 mm, H = 15 mm), and is more elongate and has a shorter anterior margin than C. antipoloensis ( Powell 1935: 332, pl. 76: 5, 6).

Three species of similar overall shell shape, especially regarding shell outline, the deep and sharp posterior and submarginal sulci, and the flattened posterior slope, are the extant Conchocele koyamai Habe, 1981 from the Pacific shelf of Honshu, Japan, Conchocele compacta minor Omori, 1954 from Miocene deposits in Honshu, Japan, and Thyasira compacta Ishizaki, 1942 from the late Pleistocene of southern Taiwan (cf. Huang et al. 2006). However, Oliver and Frey (2014) noted that C. koyamai bears fine, radiating, punctate sculpture, unknown from Conchocele and Channelaxinus . Similar granular radial sculpture appears to be present also in C. c. minor ( Omori 1954: pl. 7: 1–4) and T. compacta ( Ishizaki 1942: 346, 349, figs. 1a–f, 2).

Stratigraphic and geographic range.—Early Pleistocene, Leyte, Philippines.

Figures

Fig. 5. The thyasirid bivalve Channelaxinus antipoloensis sp. nov. from the Pleistocene Antipolo Point seep deposit in Leyte, Philippines. A. Paratype NMNS PM 28178, disarticulated right valve embedded in rock matrix; exterior of shell with the light coming from two different angles, emphasizing the posterodorsal sulcus (A1) and the median angulation (A2), and view on dorsal margin (A3). B. Holotype NMNS PM 28177, articulated specimen; views on right (B1) and left (B2) valves, the dorsal margin (B3) and the anterior margin showing the lunule (B4). C. Paratype NMNS PM 28179, articulated specimen with displaced valves; view on exterior of right valve (C1), close-up on edentulous hinge of left valve (C2). Note the frequent repaired shell injuries on all specimens.