Melanoides aff. tuberculata

(Fig. 6 A-G)

MATERIAL EXAMINED. — Taiwan • 8 specimens; Shuiniukeng (= Water Buffalo Valley), New Taipei City; Tananwan Formation; Early Pleistocene, Calabrian; ASIZF0100913 - ASIZF0100919, ASIZF0100940.

DESCRIPTION

Shell turriculate, thick, elongated, with HSh being greater than twice of WSh. HBW and HSp of adult shell about 60% and 40% of HSh respectively. HA longer than WA. Surface smooth on last two whorls, but adapical whorls covered with widely spaced, prosocline collabral undulations. Spiral threads on abapical part of the base. Spire tall, comprising 2/3 of the shell. Suture shallow, but impressed in adapical whorls, last two whorls with slight shoulder at shallow suture. Whorls 7-8 in number (early whorls missing), increasing in diameter slowly, profile flattened. Aperture ovate, but not well preserved in all specimens.

REMARKS

Thiaridae species exhibit considerable polymorphism in shell ornamentation, which allows them to adapt to diverse environments across their range. In some lineages, predominantly parthenogenetic reproduction results in minimal intrapopulation variability in key morphological traits, allowing for regional morphological comparisons and identification (Samadi et al. 2000).

The studied specimens closely resemble extant Melanoides tuberculata (Müller, 1774) populations in Taiwan (Chiu 2006; Chen 2011). These shells are characterized by the general shell outline and the presence of wavy longitudinal ribs on each whorl, which become less distinct towards the body whorl. They also differ from other Taiwanese thiarids. Specifically, Stenomelania costellaris (Lea & Lea, 1851) has more distinct sutures and smaller shells; S. plicaria (Born, 1778) exhibits smoother and higher shells; S. torulosa (Bruguière, 1789) displays thick longitudinal ribs with spiral grooves. However, due to the strong morphological variations within Thiaridae species, fragmentation of the specimens, and the lack of criteria for observing adult shell characteristics, we identified the specimens as Melanoides aff. tuberculata.

Melanoides tuberculata has been reported from various localities across East Asia. In Taiwan, it was first described M. formosensis as a new species alongside M. tuberculata, while it has been suggested that M. formosensis may be a morphotype or a subspecies of M. tuberculata (Pace 1973). Chen (2011) documented possible shell morphological variations throughout Taiwan, demonstrating that such variability can be observed even among individuals from the same locality.

Fossil occurrences of M. tuberculata from Neogene and Quaternary deposits are rare in Asia. Nomura (1935) reported this species from the Pleistocene Toukoshan Formation (referred to as Byoritu Beds in his report) in Houlong Township, Miaoli County (= Wangwa, Shinchiku-shu), northern Taiwan. Although the figure in the original publication is difficult to interpret, the accompanying description serves as evidence of its occurrence in Taiwan. Furthermore, Nomura (1935) also indicated its presence in the Miocene-Pliocene deposits of Java. Additionally, M. tuberculata has been reported from the Neogene of Nepal (Gurung et al. 1997).