In situ preservation of “Pholas tzayi Hu, 1992” (Mollusca, Pholadidae) from Chinshui Shale (Pliocene) in western Taiwan

(Mollusca, Pholadidae) from Chinshui Shale (Pliocene) in western Taiwan Yen-Ju Lin , Jia-Yu Lin, Leh-Chyun Wu Department of Science Education, National Taipei University of Education, No. 134, Sec. 2, Heping E. Rd., Da-an District, Taipei 106, Taiwan Department of Geosciences, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan Department of Geology, Chinese Culture University, 55, Hwa-Kang Road, Yang-MingShan, Taipei, 11114, Taiwan


INTRODUCTION
The Chinshui Shale is one of the petroleum target units in NW Taiwan (Pan, 2015).
Because the Chinshui Shale is deposited relatively close to the PlioPleistocene boundary, this contribution helps us to understand the invertebrate faunal evolution across the < A c c e p t e d M a n u s c r i p t > boundary. We focused on the wellpreserved "P. tzayi" and its implications for understanding the faunal succession and environmental settings of Chinshui Shale. The goals of this study are: 1) to investigate the living conditions of fossil assemblage from Chinshui Shale; 2) to speculate about its possible cause of death, and 3) to infer whether the studied assemblage is allochthonous or autochthonous.

Geologic settings
The studied area is exposed along the Laotianliao Creek in the Touwu Township, Miaoli County. It is located approximately 1 km west of Mingde Dam (Fig. 1). Exposure is on the Laotianliao Creek bed in the downstream direction of Mingde Dam. The studied section of the Chinshui Shale is located on the axis of Chinshui Anticline. The thickness of the unit cropping out varies from 4 m to 70 m.
Studied section is approximately 6 m thick, dominated by intensely bioturbated muddy sandstones with rare sedimentary structures (Fig. 2a). The section is composed of three parts (Fig. 2b). The lower part is approximately 2 m thick and is dominated by finegrained sandstone with hummocky cross-stratification (HCS), intense bioturbation, and rare fossil shells. The middle part is approximately 1.8 to 2 m thick and is dominated by < A c c e p t e d M a n u s c r i p t > Page 5 of 33 pages bioclasts and foraminifers which are filled in burrows. The size of burrows is larger than the one in the lower part and some sedimentary structures (e.g., HCS) are present. The upper part is 2 to 2.2 m thick and contains well-preserved fossils and transported bioclasts, and bioturbation is low compared to other parts. Fossils are dominated by pholadid bivalve "P. tzayi" and they tend to be concentrated within one layer with one preferred orientation (Fig. 2c). There are some burrows above the bivalve layer made either by bivalves or by arthropods. The boundary between Chinshui and the overlying Cholan Formation is approximately 20 m above the section based on the regional geologic map.
Thus, the studied section represents the upper portion of Chinshui Shale.

Fossils
A total of 30 specimens of "Pholas tzayi Hu, 1992", including incomplete valves, were collected from the unit. At least 18 of them have complete outlines suitable for length and width measurements. Eight specimens are preserved in very good conditions.
Valves are unbroken and shell fragments are easily separated from molds and crumble when touched. The body cavity is filled with mud and no internal structures can be observed without further preparation. Terminology in Kennedy (1974) and Carter et al.

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Page 6 of 33 pages Illustrated specimens are deposited at the National Museum of Natural Science, Taichung (NMNS) and the Department of Geosciences, National Taiwan University, Taipei (NTUG).

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Remarks: Several fossil species of Pholadidae have been reported from Taiwan (Lin and Liew, 1978;Wang, 1983Wang, , 1984 (Table 1), studied bivalves are identified as "Pholas tzayi Hu, 1992". Hu (1992) described it in a monograph and it was named after Professor Dong-Jiang Tzay who collected the specimens. However, pholadid bivalves were not reported from the Zhudong section of the same unit reported in Hu (1993). Masuda and Huang (1994) reported two species Barnea (Anchomosa) manilensis (Philippi, 1847) and Barnea (Umitakea) dilatata (Souleyet, 1843) from the Chinshui Shale of Houlong section, Miaoli.
A detailed systematic study of fossil specimens from Taiwan is a subject of a separate publication. For this study collected specimens are referred to as "P. tzayi Hu, 1992".

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Abundant trace fossils are observed in the outcrop (Fig. 9). Although a more detailed investigation is needed in order to understand fully the trace fossil ichnoassemblage in the studied area, they are common members in Scolicia ichnofacies, which represents a neritic zone in middle offshore settings below the fair-weather wave base and above storm wave base (5 m to 15 m of water depth). This is in agreement with the ichnofacies reported for the Chinshui Shale in Chen (1994).

Taphonomy
The influence of rapid burial on infaunal bivalve taphonomy has been recently discussed by Hryniewicz et al. (2020). Pholadids similar to the ones described here are generally unable to extricate once they are buried and die; thus, they usually develop mass accumulations in areas where there is little sedimentation (Carmona et al., 2007;Hryniewicz and Gaździcki, 2016). Although incomplete bivalve shells occur in the section throughout, there is at least one layer containing shells of "P. tzayi" with the preferred orientation perpendicular to bedding surface (Figs. 2c, d). Thus, specimens of "P. tzayi" were not redeposited nor were disturbed by other infaunal organisms based on the associated trace fossils (Fig. 9). In addition, there is no sign of escaping structures associated with body fossils of "P. tzayi". Therefore, the cause of death, leaving the

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Page 9 of 33 pages preservation of bivalves in situ could be an obrution event with rapid burial due to a sudden increase in water depth and sedimentation rate (Shanmugam, 2018; Jelby et al., 2020).

Depositional environments
Based on previous studies (Yu and Tseng, 1995), the depositional settings of Chinshui Shale range from delta to barrier island transitional-offshore marine.
Transitional to offshore settings are between fair weather wave base and storm wave base (roughly 5 m -15 m water depth), in where muddy sands accumulated with some bioturbation and rich shelly bioclasts and microfossils (Walker, 1984;Walker and Plint, 1992;Yu and Tseng, 1995) (Fig. 10).

Boring strategy and implication for field geology
Boring strategies of Pholadidae have been reported and reviewed recently (Nair and Ansell, 1968;Kennedy, 1974;Röder, 1977;Kennedy, 1993;Carmona et al., 2006). A few studies (e.g., Nair and Ansell, 1968;Röder, 1977;Hryniewicz and Gaździcki, 2016) pointed out that there are two main types of boring strategies adapted by pholadoid bivalves depending on the substrate types.

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Page 10 of 33 pages Anterior boring is associated with species inhabiting rigid and semi-rigid substrates such as hardening resin of amber-producing trees (Smith and Ross, 2017;Bolotov et al., 2021), wood, or rock (Nair andAnsell, 1968;Kennedy, 1974;Röder, 1977;Hari et al., 2009;Monari, 2009). On the other hand, ventral boring is associated with species inhabiting less rigid substrates, such as firmgrounds (Hryniewicz and Gaździcki, 2016; and references therein). The absence of umbonal-ventral groove in studied specimens  indicates the latter type for "Pholas tzayi".

CONCLUSIONS
Although "P. tzayi" has been reported in Hu (1992), this is the first documentation of the in situ preservation of pholadid bivalves in the field. Its preferred orientation, vertical to bedding (with posterior pointing upward), and completeness of intact shells allow us to understand seafloor conditions at the time of deposition. "P. tzayi" adapted to tranquil muddy substrate and firmground below the fair weather wave base. It is absent in the overlying unit Chuolan Formation that deposited in shallower marine above the fair weather wave base and shoreface settings under the influence of orbital forcing (e.g., Vaucher et al., 2021). "P. tzayi" occurs only in the upper Pliocene Chinshui Shale, close to the PlioPleistocene boundary. Thus, it is a key fossil, perhaps an index fossil, in the region.
Due to the fact that "P. tzayi" was found buried in situ (Fig. 2c) an obrution event with rapid sedimentation is the most probable cause of death.