Archivesica pastori, 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 : 608-610

Archivesica pastori sp. nov.

Fig. 21.

Zoobank LSID: urn:lsid:zoobank.org:act:6D1578D2-432D-4FB2-9173-5BE9500E7DA3

Etymology: In honour of Douglas Pastor, owner of the property of the type locality at Liog-Liog Point, Leyte, Philippines.

Type material: Holotype NMP-2165a, a disarticulated right valve. Paratypes: NMNS PM 28194–28196 (a right valve and two left valves), NMP-2165b (left valve) , NMNS PM 28206 (right valve) from the type locality .

Type locality: Liog-Liog Point , Leyte, Philippines .

Fig. 21. The vesicomyid bivalve Archivesica pastori sp. nov. from the late Pliocene Liog-liog Point seep deposit. A. Left valve (paratype NMP-2165b) → and right valve (holotype NMP-2165a) nested into one another; inside view of the two shells (A 1), outside view on RW (A 2), close-up on hinge of RV A 3 ); close-up on hinge of LV (A 4). B. Paratype NMNS PM 28206 , large isolated right valve with broad posterodorsal area, outside view (B 1), close-up on hinge (B 2). C. Paratype NMNS PM 28194 , large isolated right valve with gently sloping posterodorsal margin; outside view (C 1), close-up on hinge C 2). D. Paratype NMNS PM 28195 , isolated left valve, posteriorly damaged, outside view (D 1), close-up on hinge (D 2). E. Paratype NMNS PM 28196 , disarticulated left valve, outside view (E 1) and close-up on hinge (E 2) .

Type horizon: Lower Pliocene part of the Bata Formation .

Material.— 22 specimens from the type locality: type material and NMP-2165a (representing 16 specimens) and 21 paratypes, NMP-2165b, NMNS PM 28194–28196 , 28206– 28222 .

Dimensions (in mm).—NMP-2165a, L = 143.2, H = 69.0.

Diagnosis.—Large, elongate Archivesica with three radiating teeth in each valve, cardinal 2b very thick, cardinal 3a occasionally reduced, cardinals 3b and 4b short; ligament nymph about half of total shell length.

Description.—Large elongate-oval shell, umbones blunt, elevated, slightly prosogyrate, situated anterior at c. 26–31% of total shell length; anterodorsal margin straight; anterior margin acutely rounded, anterior-most point somewhat above midline of shell; ventral margin straight or slightly concave; posterior margin broadly rounded; posterodorsal margin gently sloping. Hinge plate strong, broad, with three relatively thick cardinals in each valve, radiating outward from underneath umbo; right valve: cardinal 1 elongate, protruding, pointing anteroventrally, 3a elongate, subparallel to shell margin, reduced, 3b elongate-triangular, pointing posterodorsally; left valve: cardinal 2a moderately thin and of same length or longer as 2b, pointing anterorventrally, 2b triangular, thick, occasionally bifid, pointing to center of ventral margin, cardinal 4 short, narrow, pointing posterior, with shallow, elongate posterior depression; nymph plate elongate, about ½ of total shell length, starts tapering posteriorly after about half its length. Outer shell surface with faint, irregular growth increments.

Remarks.—Very similar regarding hinge dentition are the extant Japanese species Archivesica similaris (Okutani et al. 1997) and Archivesica tsubasa ( Okutani et al. 2000) , but both are more elongate than Archivesica pastori . As a result of being more elongate, their dorsal and ventral margins are more parallel than in A. pastori . However, this feature shows some variability in A. pastori , for example the specimen shown on Fig. 21B 1 has rather parallel dorsal and ventral margins, but is not as elongate as A. similaris and A. tsubasa . Another extant species with similar hinge dentition is Archivesica ochotica Scarlato, 1981 , which is less elongate compared to A. pastori and has a proportionally shorter ligament nymph ( Scarlato 1981). The early Miocene Japanese Archivesica sakoi Amano, Jenkins, Ohara, and Kiel, 2014 also shows a similar hinge dentition and overall shape, but its cardinal 3b in the right valve is more elongate and subparallel the shell margin unlike the shorter and oblique tooth of A. pastori , and A. sakoi also has more prominent and more prosogyrate umbones ( Amano et al. 2014), and is much smaller (L = 80 mm in A. sakoi compared to L = 143 mm in A. pastori ).

The hinge dentition of A. pastori is also somewhat similar to that of Archivesica magnocultellus ( Okutani et al. 2002) except that A. pastori lacks the large pit posterior to the cardinals seen in A. magnocultellus . Archivesica pastori differs from the type species of Archivesica , A. gigas ( Dall 1896) , by having a short cardinal 2a and a strong, thick cardinal 2b in the left valve, whereas A. gigas has a very elongate 2a and a narrow 2b. Furthermore, A. pastori has a long ligament nymph (c. ½ of shell length), whereas the ligament nymph is short in A. gigas .

Stratigraphic and geographic range.—Late Pliocene, Leyte, Philippines.