Early Pliocene otolith assemblages from the outer-shelf environment reveal the establishment of mesopelagic fish fauna over 3 million years ago in southwestern Taiwan

Understanding the diversity of deep-sea fish fauna based on otoliths in the tropical and subtropical West Pacific has been limited, creating a significant knowledge gap regarding regional and temporal variations in deep-sea fish fauna. To address this gap, we collected a total of 122 bulk sediment samples from the Lower Pliocene Gutingkeng Formation in southwestern Taiwan to reconstruct the otolith-based fish fauna. Using planktonic foraminiferal biostratigraphy, we determined the age of the samples to be 5.6 to 3.1 Ma. A total of 8314 otoliths were assigned to 64 different taxa from 33 families, including the discovery of one new genus, Gutingichthys gen. nov., and three new species: Benthosema duanformis sp. nov., Benthosema parafibulatum sp. nov., and Gutingichthys changi sp. nov. Comparisons with other regional otolith-based assemblages highlighted the exceptional diversity of our collection, making it the most diverse fossil fish fauna reported from Taiwan to date. Otolith diversity analysis revealed very few taxa were dominant in the assemblage, particularly the mesopelagic Myctophidae, with a wide variety of minor taxa. The co-occurrence of shallow-water elements suggests episodic storm events as a potential source. The predominance of deep-sea and oceanic fishes indicated an outer-shelf to upper slope environment, resembling the modern outer-shelf and upper slope fish fauna in the region. Our findings suggest an early establishment and persistent presence of the mesopelagic fish community since the Early Pliocene. Further investigations of the Upper Miocene and Pleistocene sections of the Gutingkeng Formation would provide valuable insights into the evolution of deep-sea fish fauna in the area.


Introduction
Marine fish fossils in Taiwan have primarily come from shallow-water deposits, representing nearshore, coastal, and shallow-water communities (Lin et al., 2018;Lin & Chien, 2022;Lin et al., 2022aLin et al., , 2022bLin et al., , 2022c)).Fish otoliths have emerged as the richest source of fish fossils in Taiwan, surpassing skeletal records due to better preservation potential (Lin et al., 2021).However, our understanding of paleo-deep-water and pelagic fish communities in Taiwan remains limited.One possible deep-water fish fossil is a large otolith of Coelorinchus discovered in the Plio-Pleistocene Maanshan Mudstone of the Hengchun Peninsula, as depicted by Hu (1990, pl. 8, figs. 2, 8, mistakenly cited as Hu, 1989in Lin et al., 2021).
Existing records of relevant otoliths from the northwest Pacific are primarily confined to temperate Japan with significant studies conducted by Ohe (1977Ohe ( , 1981Ohe ( , 1983)).Comprehensive reviews of otolith-based fossils from mesopelagic lanternfish (Myctophidae) in Japan have been provided by Schwarzhans and Ohe (2019) and Schwarzhans et al. (2022).Notably, van Hinsbergh and Helwerda ( 2019) conducted an extensive study on a deepsea Plio-Pleistocene otolith assemblage in the northwest Philippines.In southern Taiwan, the Gutingkeng Formation, a thick mudstone series, is well-known for its wellpreserved foraminifera and nannofossils (Horng & Shea, 1994;Hu, 1989;Shieh et al., 2002), as well as ostracods and solitary corals (Hu, 1989;Hu & Tao, 1982).This formation is believed to have been deposited in a relatively deeper-water setting compared to other shallow-water layers in the Western Foothills of Taiwan (Chen & Yu, 2016) and the formation has the potential to yield deepsea fossil assemblages.
In this study, we conducted quantitative sampling from the lower part of the Gutingkeng Formation in the Tainan and Kaohsiung areas (Fig. 1) between 2020 and 2022.The collection, which was dated using planktonic foraminiferal biostratigraphy, provides one of the most comprehensive and richest records of outer-shelf and pelagic fish communities from the Zanclean (Early Pliocene) subtropical West Pacific.

Geological setting
The Gutingkeng Formation is exposed vastly in the southwestern region of Taiwan (Torii, 1932), spanning an area of approximately 750 square kilometers in the Tainan and Kaohsiung areas (Keng, 1981;Chen & Yu, 2016;Fig.1; Additional file 1: Figs.S1, S2).This formation displays considerable thickness, reaching 2280 m in Tainan and over 3600 m in the Kaohsiung area (Chen & Yu, 2016).It primarily consists of massive dark gray mudstone and shale, occasionally intercalated with thick beds of muddy sandstone (Keng, 1981;Lin, 1990;Luo, 1988).Within the formation, several notable thick-bedded sandstone lenses can be observed.The extensive barren badlands formed by the massive dark gray mudstone have earned the local nickname "Moon World".
During the Late Miocene to Early Pliocene, the southwestern basin of Taiwan was predominantly filled with deep-water deposits.However, as the Late Pliocene and Pleistocene progressed, the basin gradually filled and became shallower (Chou, 1971;Covey, 1984;Luo, 1988).The continuous deposition of the Gutingkeng Formation is overlain by the Chiting Formation (Fig. 2).Nannofossil dating has provided evidence of the formation's age spanning from the Late Miocene to the Pleistocene, although exposures of the Miocene layers are comparatively limited in comparison to the more extensive Plio-Pleistocene deposits (Chi, 1979).Notably, exposures of the latest Miocene and the Miocene-Pliocene boundary can be found along the Erhjen River in Kaohsiung, but investigations at this site have focused solely on nannofossils (Horng & Shea, 1994).
The overall sedimentary environment of the lower Gutingkeng Formation is characterized as an outershelf to slope setting (Keng, 1981;Lin, 1990;Luo, 1988).Studies of benthic foraminiferal assemblages have indicated the dominance of taxa such as Bolivina, Siphogenerina, Quinqueloculina, and Rotalia, suggesting deposition in a moderately deep, outer-shelf and opensea environment (Oinomikado, 1955).
Sediment Dark gray mudstone.
Foraminifera Presence of Pulleniatina primalis indicates an age of approximately 3.7 Ma (last appearance datum, LAD).The composition of benthic foraminiferal assemblage suggests a mid-shelf environment, slightly shallower than Site 2 (see below).
Available material Five bulk samples weighing a total of approximately 25 kg.
Sediment Dark gray mudstone, with abundant fossils.
Foraminifera Presence of P. primalis indicates an age of approximately 3.7 Ma (LAD).The site represents a midshelf to outer-shelf environment with depths ranging from 50-200 m.
Available material 53 bulk samples weighing a total of approximately 266 kg.
Sediment Dark gray mudstone, rich in foraminifera and pyrites, but no otoliths were found.
Foraminifera Presence of Globoconella inflata indicates an age of approximately 3.13 Ma (first appearance datum, FAD).The site represents an outer-shelf environment with depths approximating 200 m.
Available material Three bulk samples weighing a total of approximately 13.7 kg.
Sediment Dark gray mudstone, containing foraminifera, but no otoliths were found.
Foraminifera Presence of Globoturborotalita nepenthes indicates an age of approximately 4.4 Ma (LAD).The site suggests a possible outer-shelf environment with depths approximating 200 m.
Available material Three bulk samples weighing a total of approximately 14.5 kg.
Sediment The exposure at the site is approximately 3 m in height (Fig. 3; Additional file 1: Fig. S2).The top portion is covered, while the lower part consists of thick mudstone interbedded with thin sand layers.The mudstone can be roughly divided into an upper layer of about 1 m in thickness and a lower layer of about 1.8 m in thickness.The upper mud layer is sandwiched by two very thin sand layers, each about 2 cm thick.The upper and lower mud layers are separated by another sand layer, which is about 20 cm thick.The mud is bluish-gray in color and devoid of sedimentary structures.Shell fragments and otoliths can be found on the surface of exposures of the sediments of the lower mud layer, and bulk samples were taken.The sand layers, which are yellow to brownish in color, are unconsolidated.The lowest sand layer includes argillaceous sand, interbedded mud and argillaceous sand, and bioclastic sand.The bioclastic sands are light brown and contain a large amount of biological debris, including shells, fish otoliths, and shark teeth.Both mud and sand layers were sampled for otolith analysis in this study.
Foraminifera mud layer: Co-occurrence of P. primalis and Sphaeroidinella dehiscens indicates an age range of 5.6 to 3.7 Ma.The site suggests a mid-shelf environment.Bioclastic sand layers: Co-occurrence of Globorotalia menardii and P. primalis indicates an Early Pliocene age.The environment is interpreted as mid-shelf to outer-shelf.
Available material Six bulk samples weighing a total of approximately 27.2 kg were collected from the bioclastic sand layers, while 57 bulk samples weighing a total of approximately 255 kg were collected from the mud layer.
Sediment Dark gray mudstone.Available material Seven bulk samples weighing a total of approximately 35.5 kg.

Otolith preparation and identification
A comprehensive collection of 122 bulk sediment samples, weighing over 600 kg in total, was obtained from various locations in Tainan (n = 58) and Kaohsiung (n = 64).The initial step involved drying and weighing the bulk sediments, followed by the allocation of five kg of sediment for each sample to facilitate subsequent diversity analyses.The sediment was then immersed in water and sieved through a 500-μm mesh screen.Otoliths larger than the 500-μm screen were meticulously hand-picked under a stereomicroscope.The images of the otoliths were captured using a digital camera adapted to a Nikon SMZ1270 stereomicroscope.For each otolith, a stacked image was created using the Helicon Focus software to account for varying depths of field.Subsequently, these images were digitally retouched using Adobe Photoshop.
For otolith identification, various pertinent references were consulted.These included Ohe (1985), Rivaton and Bourret (1999), Schwarzhans (1999Schwarzhans ( , 2013)), Schwarzhans and Aguilera (2013), Lin andChang (2012), andNolf (2013).In addition, a direct comparison was made with our laboratory's modern otolith collection, registered under the code CHLOL.Whenever possible, otoliths were identified to the species level.All collected materials are currently stored in the Biodiversity Research Museum, Academia Sinica, Taiwan, and the imaged specimens are archived under the registration code ASIZF.

Diversity analysis
To assess the abundance and distribution of otoliths in the sampled sediments, we calculated the abundance (density) of otoliths in the sediments by dividing the otolith count by the dry sediment weight (kg) for each sample.The number of identified taxa was also compared with respect to sediment weight.We conducted separate analyses for samples from Tainan and Kaohsiung, and the six sand layer samples from Site 5 were analyzed separately from the rest of the mud layer samples, due to their difference in the abundance of otoliths.The results of these analyses were visualized using box plots.
Otolith diversity was evaluated using Hill numbers, which are widely employed in ecology and paleoecology to quantify biological diversity (Hill, 1973).The Hill numbers were calculated for three orders: q = 0 ( 0 D), q = 1 ( 1 D), and q = 2 ( 2 D).The Hill number of 0 D represents species richness (number of species), the 1 D Hill number represents Shannon diversity (number of abundant species), and the 2 D Hill number represents Simpson diversity (number of dominant species) (Chao et al., 2014).As the observed Hill numbers depend on sample coverage, otolith diversity was further estimated using a specimenbased rarefaction method, which provides reliable diversity indices independent of sample sizes (Chao & Jost, 2012; see also Lin et al., 2023).Confidence intervals (95%) were calculated based on 300 bootstrap replicates following the approach described by Hsieh et al. (2016).Rarefaction curves were analyzed and visualized using the iNEXT package in R software (Chao et al., 2014;Hsieh et al., 2016).Additionally, the rank abundance of identified taxa is presented to illustrate the taxonomic composition and relative abundance of the fossil assemblages.

Systematic paleontology
A list of taxa and their abundance from the collection are presented in Table 1.The classification scheme generally follows that of Nelson et al. (2016), while following Davis et al. (2016) for Acropomatiformes (see also Ghedotti et al., 2018;Smith et al., 2022).In addition, the family Microdesmidae is distinguished from the Gobiidae sensu Nelson et al. (2016) due to the distinct small otoliths they possess, which differ from the typical gobiid otoliths.The authorship of higher taxa was determined using the method described by van der Laan et al. ( 2014), while for genus and species, Fricke et al. (2023) were followed.In the descriptions, the term "otolith" specifically refers to the saccular otolith (sagitta), unless stated otherwise.The general otolith terminology followed the conventions outlined by Nolf (1985Nolf ( , 2013) ) and Smale et al. (1995).Descriptions and diagnoses for new species and remarks on other taxa are concise, and the synonymy list is limited to nominal species and relevant records from Taiwan and neighboring areas based on published images.

Figure 4a-b
Remarks All specimens in the here described collection represent juvenile individuals.These otoliths exhibit typical Bathycongrus characteristics, which include a prominent dorsal depression, a unified sulcus that narrows and extends anteriorly with its colliculum, and an angular ventral rim (Schwarzhans, 2019b).

Order Clupeiformes Bleeker, 1859
Family Engraulidae Gill, 1861 Early Pliocene mesopelagic fish Fauna in Taiwan Remarks The otoliths of Engraulis japonicus can be easily distinguished by their extremely elongated shape (OL/ OH = 2.1-2.5),slender profile, funnel-like sulcus, and protruding rostrum (Lin & Chang, 2012: pl. 71).While this species is not uncommon in the collection, the anterior tip and ventral denticles are often not well-preserved.

Figure 4e
Remarks The otoliths of Stolephorus exhibit notable similarities to those of E. japonicus (as described above).However, they can be differentiated from the latter by their relatively more compact outline (less elongated, with OL/OH < 1.9) and greater otolith height (Lin & Chang, 2012: pl. 71).

Figure 4i
Remarks The sulcus type of the otolith bears a striking resemblance to that of Gonostoma (Nolf, 2013: pl. 48).However, due to the lack of preservation in the anterior portion of the otolith, a definitive identification cannot be made with certainty.

Figure 4f, g
Remarks The otoliths of the studied specimens show a distinctive tall shape and thick posterior margin as depicted in Lin et al (2017: Fig. 4C-E).However, identifying them below the genus level may pose challenges due to overlapping interspecific variations and the absence of comprehensive modern otolith reference material.Nevertheless, within our collection, at least two types of otoliths with varying heights were observed (Fig. 4f with OL/OH > 0.7 vs Fig. 4g with OL/OH < 0.6), suggesting the presence of two distinct taxa.

Figure 4h
Remarks A large and thin otolith, characterized by a posterior angle and a vertical posterior rim, is attributed to Neoscopelus macrolepidotus.Comparative modern otoliths of all three species of Neoscopelus from Taiwan are presented in Fig. 5.The otoliths of N. macrolepidotus can be differentiated from other congeners by their more extended rostral part and large ventral area inclining towards anterior, especially evident in larger specimens (Fig. 5a).In addition, the postero-dorsal angle is rounded and less pronounced than that of other congeners.Although the anterior part of the rostrum in our fossil otolith is not preserved, the general outline of the fossil specimen agrees well with those of N. macrolepidotus.

Family Myctophidae Gill, 1893 Genus Benthosema Goode & Bean, 1896
Remarks Benthosema otoliths typically exhibit certain characteristics, including a robust antirostrum and a specific arrangement of ventral denticles.This arrangement consists of 1-3 anterior denticles separated from a single, strong posterior denticle by a smooth gap along the ventral margin (Schwarzhans & Ohe, 2019;Schwarzhans, 2019a).However, we found that the presence of a consistent smooth gap may vary within species such as Benthosema pterotum (Alcock, 1890).These intra-specific variations are mainly attributed to ontogeny (Fig. 7).(Gilbert, 1913), are known in Taiwanese waters today (Shao, 2023).Among them, the otoliths of B. suborbitale exhibit distinct characteristics, including a salient depression on the postero-dorsal rim and two consistently spaced ventral denticles separated by a smooth gap (Schwarzhans, 2019a: Fig. 49-4).The otoliths of B. fibulatum feature a more pronounced posterior rim that extends further backward and a gently curved ventral rim (Fig. 7a-c).The otoliths of B. pterotum are characterized by equal dorsal and ventral areas with both rostrum and antirostrum similar in size (Fig. 7d-h).Juvenile otoliths of B. pterotum are more rounded (Fig. 7h), while in adults, the otoliths are semirectangular and elongated (Fig. 7d).The presence of a smooth gap on the ventral margin may become more evident in larger specimens.Description The otoliths are generally characterized by their large, regularly rounded shape, with a vertical posterior rim that results in a truncated appearance of the otoliths.The dorsal rim is gently curved to flat, while the ventral rim is curved.There is a prominent antero-ventral denticle located just before the midline of the otolith, and a much stronger posterior denticle positioned at the postero-ventral corner.These two denticles are separated by a smooth gap, and there may be some minute denticles between the rostrum and the antero-ventral denticle (Fig. 6h, j).The antirostrum is slightly smaller than the rostrum, and both are pointed.An angle is present at the postero-dorsal corner.The sulcus is well-divided, very wide, and centrally positioned on the inner face of the otolith.The ostium is slightly greater in height and longer than the cauda.The dorsal and ventral areas of the otolith are similar in size.Occurrence Taiwan: Lower Pliocene Gutingkeng Formation in Chochen (Tainan) and Tianliao (Kaohsiung), southwest Taiwan.Philippines: Upper Pliocene-Lower  Description The otoliths are oval-shaped with curved dorsal and ventral rims.The posterior rim is also curved and extends backward.There is a noticeable anteroventral denticle positioned just before the midline of the otolith, and a stronger, sometimes protruding posterior denticle located at the postero-ventral corner.These two denticles are often separated by a smooth gap, and there may be 2 or 3 min denticles between the rostrum and the antero-ventral denticle.The rostrum is broader than the antirostrum, and both are pointed.An angle is present at the postero-dorsal corner.The sulcus is well-divided and wide in height, while the ostium is longer than the cauda.The dorsal area is similar in size to the ventral one.

Remarks
Remarks Benthosema parafibulatum exhibits otoliths that closely resemble those of modern species B. fibulatum (Fig. 7a-c  Remarks See remarks under the genus Benthosema.

Figure 8c
Remarks Four otoliths characterized by oblique anterior and posterior rims and a straight dorsal rim exhibit the closest resemblance to those of Bolinichthys supralateralis (Smale et al., 1995: pl. 17, fig.B1-3).This represents the first recorded fossil occurrence of the genus in the region.

Genus Ceratoscopelus Günther, 1864
Ceratoscopelus warmingii (Lütken, 1892) Remarks The otoliths of Ceratoscopelus can be readily distinguished from all other myctophid genera by their large and protruding rostrum, the presence of a tiny or nearly absent antirostrum, and a curved posterior rim (Smale et al., 1995: pl. 17 9J-AC).These similarities include a marked elevation in the anterior part of the dorsal rim and a wide in height sulcus.However, our fossil otoliths possess a more elongated shape with a more extended posterior rim and are most similar to those of D. chrysorhynchus.

Figure 8f
Remarks Among the otoliths, a single large and tall specimen closely resembles those of D. effulgens (Schwarzhans, 2013: pl.13, Fig. 1-7).It has a curved ventral rim and a relatively flat dorsal rim with a large dorsal area and a somewhat short posterior rim.Although the species is not recorded in the Fish Database of Taiwan (Shao, 2023), it has been reported to have a distribution ranging from 0° and 29°N in the West Pacific (Froese & Pauly, 2023).

Figure 9a
Remarks A single specimen with a high-bodied outline (OL/OH = 1.03) and curved dorsal and ventral rims is assigned to the genus Diogenichthys.Remarks The otoliths are rounded and semi-square in shape with equal dorsal and ventral areas.Among our specimens, their outline closely resembles the otoliths of Lampanyctus alatus as depicted by Smale et al., (1995: pl. 22, fig. D) and Rivaton and Bourret (1999: pl. 120, Fig. 1-7).
Our single specimen falls within this range with an OL/ OH ratio of 1.32.

Figure 9f-h
Remarks The small to medium-sized otoliths of Coelorinchus display similar outline to those of Coelorinchus formosanus Okamura, 1963, characterized by having a notable elevation in the anterior part of the dorsal rim (Fig. 10a-e).However, it is important to note that other closely related Coelorinchus species exist.Therefore, when it comes to species identification based on juvenile or subadult individuals, we adopt a conservative approach.Remarks The twelve specimens of Hymenocephalus aff.papyraceus exhibit distinct features such as a salient antero-dorsal lobe, rounded ventral and posterior rims, well-separated but closely aligned collicula, and a welldelineated collicular crest located in the middle of the otolith and just above the crista inferior.These characteristics bear a strong resemblance to those of H. papyraceus (Schwarzhans, 2014: Fig. 14I), a species currently known only from the southeastern coast of Japan (Nakayama, 2020; Schwarzhans, 2014).The presence of these otoliths suggests a possible southward extension of the species during the Early Pliocene.However, we note that our specimens represent younger individuals and display a much shorter, less pointed posterior rim and a less oblique ventral area compared to the otolith depicted by Schwarzhans (2014: Fig. 14I), which belongs to a larger individual.Extant otoliths of other species of Hymenocephalus found in Taiwan are presented in Fig. 10f-l for comparison.

Figure 11a
Remarks The otolith of Hymenocephalus striatissimus, represented by a single specimen, features a high-bodied shape with a fused colliculum and a prominent anterodorsal lobe that extends to approximately one third of the otolith's height (OH).These characteristics align well with the otoliths of the extant H. striatissimus (Schwarzhans, 2014: Fig. 32; Fig. 10f-g (1983: pl. 12, Fig. 9, pl. 13, Fig. 4) are not clear enough for species identification, but they certainly do not belong to H. striatissimus, as the otoliths possess a well-divided sulcus.

Family Bregmacerotidae Gill, 1872
Genus Bregmaceros Thompson, 1840 Remarks Bregmaceros otoliths are relatively common in the collection, although many of them are represented by juveniles and/or poorly preserved specimens.The taxonomy of extant Bregmaceros species in the region has been subject to ongoing revision (Ho et al., 2011;Koeda & Ho, 2019), with new species still being named (Ho et al., 2020).It has been suggested that Bregmaceros mcclellandi Thompson, 1840 may not exist outside of the northern Indian Ocean (Froese & Pauly, 2023) and that previous records of B. mcclellandi in Taiwan is a misidentified species of Bregmaceros pseudolanceolatus Torii, Javonillo & Ozawa, 2004(Ho et al., 2011).Consequently, the identification of corresponding Bregmaceros otoliths also requires updating.For example, the otoliths of B. mcclellandi in Lin and Chang (2012) contain misidentifications, and those in Rivaton and Bourret (1999) may represent a mixture of several Bregmaceros species.
Images of extant Bregmaceros otoliths from Taiwan are provided in Fig. 12 for comparative purposes.
In the present collection, otoliths of Bregmaceros japonicus Tanaka, 1908 and Bregmaceros nectabanus Whitley, 1941 are recorded (see below).Both species are characterized by a robust and wide dorsal lobe, distinguishing them from other Bregmaceros species in the region that possess a narrower dorsal lobe or a pointed, antenna-like dorsal denticle (Fig. 12d-g, j-k).The distinction between B. japonicus and B. nectabanus is subtle, but B. japonicus typically exhibits a broader and more extended posterior lobe positioned just behind the caudal depression, resulting in a stockier otolith outline.In contrast, B. nectabanus displays a less pronounced posterior lobe located above the caudal depression, giving the otolith a slender outline that inclines towards the anterior.Remarks Two triangular, massive otoliths with a fused colliculum, a flat inner face, and a predominantly convex and thickened outer face are confidently classified as belonging to the carapid family.The outline of the otoliths and the proportion of the sulcus to the otolith closely resemble those of the genus Encheliophis (Smale et al., 1999: pl. 33, fig. E), suggesting their affiliation with this genus.

Genus Onuxodon Smith, 1955
Onuxodon sp.Remarks These otoliths share many characteristics with those of Encheliophis (see above), particularly in terms of the sulcus and overall outline.However, these specimens are generally smaller in size, displaying a much shorter posterior part and a less convex outer face, which aligns more closely with the otoliths of the genus Onuxodon.

Figure 13a-b
Remarks Two otoliths belonging to the genus Neobythites are present in the collection.While they bear similarities to Neobythites sivicola (Jordan & Snyder, 1901) (see Lin & Chang, 2012: pl. 81), a common modern species found in the area, the poor preservation of the specimens prevents a definitive identification at the species level.

Etymology
The genus is named after the type horizon, the Gutingkeng Formation.
Diagnosis OL/OH = 1.79-2.75,OL/OT = 4.11-4.55.Elongate, rectangular otoliths with vertical or obtuse posterior and anterior rims and nearly horizontal, straight dorsal and ventral rims.Sulcus filled with a single oblong colliculum, entirely surrounded by elevated cristae forming a broad oval bulge around the sulcus.
Remarks The distinct characteristics of the elongated otoliths, including the vertical posterior and anterior rims, oblong sulcus that is not inclined, and the presence of thick and swollen cristae, are highly characteristic.Such swollen cristae are rarely observed in extant bythitids, with an example being Tuamotuichthys bispinosus Møller et al., 2004(see Møller et al., 2004 for figures).These otoliths clearly represent a fossil species belonging to an extinct genus (Schwarzhans, pers. comm. 2022).
Etymology The name is derived from the name of Dr. Chih-Wei Chang (National Academy of Marine Research, Ocean Affairs Council, Kaohsiung, Taiwan), who has made significant contributions to the field of ichthyology, ocean science, and fish otolith studies in Taiwan.The name is a tribute to Dr. Chang's expertise and his role as a mentor to the author (CHL).

Diagnosis See diagnosis under the genus.
Description The otoliths of this species exhibit an elongated to rectangular shape.In juveniles, the shape is more rounded and obtuse with an aspect ratio (OL/ OH) of approximately 1.8.However, in larger individuals, the otoliths become more elongated with OL/OH ranging from 2.3-2.7.The posterior and anterior rims of the otoliths are variable, with some specimens bearing two angles, as seen in the holotype (Fig. 13d2), while others have nearly straight and almost vertical rims (Fig. 13f2, i2).The ventral and dorsal rims are flat and straight, contributing to the overall rectangular shape.The otoliths are characterized by their thickness.The sulcus of these otoliths is oblong, centrally positioned.It is completely filled with a single, large colliculum, which is entirely surrounded by swelling and highly elevated cristae.This distinctive feature sets them apart from other species.

Remarks See remarks under the genus.
Occurrence Currently only known from the Lower Pliocene Gutingkeng Formation in Chochen (Tainan) and Tianliao (Kaohsiung), southwest Taiwan.Remarks This single, moderately thick otolith has a regular oblong shape with tapering ends, an oval sulcus, and similar-sized dorsal and ventral areas.The thickness, outline, and sulcus type are typical characteristics shared by several genera, including Cataetyx and Grammonus.However, the single specimen available is not sufficient to make a definitive identification beyond the level of family.Further specimens and additional diagnostic features would be necessary for a more specific classification.

Order Kurtiformes Jordan, 1923
Family Apogonidae Günther, 1859 Genus Jaydia Smith, 1961 Jaydia sp.Remarks A single massive otolith with its typical apogonid-type outline and sulcus is assigned to the genus Jaydia.Its distinctive features include a prominent antero-dorsal lobe, a smaller lobe in the dorsal rim, a truncated posterior rim, and a curved ventral rim.The ostium is wider in height than the oblong cauda (OsL/ CaL = 1.5).It closely resembles the Jaydia otoliths depicted in Lin and Chang (2012: pl.99, as Apogon).

Family Gobiidae Cuvier, 1816
Remarks Gobiid otoliths are commonly found from the studied sites and rank as the second most abundant (Table 1).Taiwan is home to a considerable number of extant gobiid species (Shao, 2023), which share a similar otolith morphology.However, the lack of knowledge about the otoliths of extant gobiid species in the region poses a significant challenge and complicates the identification of relatively young fossils, as described in this study.Therefore, it can be challenging to assign otoliths to specific species or even genus.In this study, a conservative approach was taken, and only otoliths with distinct characteristics were identified to lower taxonomic levels, while the remaining otoliths were assigned to the family level only (e.g., 14 h-k).Remarks The otoliths of Myersina filifer exhibit distinct characteristics that allow for their easy identification.They are characterized by their tall and high-bodied outline (OL/OH = 0.76-0.86),which sets them apart from other gobiids.Additionally, the otoliths typically feature crenulated dorsal rims, concave anterior and posterior rims, and a centrally located sulcus (Lin & Chang, 2012: pl. 125).The antero-ventral corner of the otoliths shows variation in shape, which can be either rounded or, less frequently, angular.

Figure 14a-d
Remarks The 16 thickset gobiid otoliths, characterized by their rectangular shape, massive swelling cristae, and oblong sulcus, are tentatively assigned to the genus Priolepis.The identification is based on comparison with similar otoliths depicted by van Hinsbergh and Helwerda (2019: Fig. 91) and several Priolepis otoliths by Schwarzhans et al., (2020: pl. 8, Fig. 2-9).It is important to note that these otoliths demonstrate varied outlines, particularly the variations in the shape of the dorsal rim, and it is likely that our fossil otoliths represent multiple taxa within the genus.Further taxonomic investigation and comparison with additional specimens and reference materials will be necessary to provide a more precise identification and potentially discern the presence of different species within the collected Priolepis otoliths.

Figure 14e-f
Remarks The otoliths of Parachaeturichthys are characterized by a large dorsal lobe that extends in the posterodorsal direction.While similar enlarged lobes can also be observed in the otoliths of Amblychaeturichthys and Chaeturichthys, the size of the specimens in question more closely resembles those of Parachaeturichthys.This similarity can be observed in previous references such as Ohe (1985: p. 132-133), Iizuka andKatayama (2008: p. 145), andOhe et al., (2020: Fig. 7).Remarks This single specimen displays a squarish shape and a deeply incised sulcus, which bears a strong resemblance to those depicted by Ohe (1983: pl. 10, Fig. 1-4) and van Hinsbergh and Helwerda (2019: figs. 84-88), although they did not provide any otolith images of this extant temperate species.It is highly likely that the specimen belongs to the same taxon as those depicted in the references.However, due to the limited availability of only one otolith, identification to the species level was only tentative.

Figure 13k
Remarks The very small otolith described (< 1 mm), with its tall outline (OL/OH < 0.63) and distinct features, can be confidently assigned to the family Microdesmidae (e.g., Nolf, 2013: pl. 325).The otolith exhibits a very straight and vertical posterior rim, as well as a parallel lower half of the anterior rim.In addition, the otolith displays elevated and thickset cristae.

Figure 14m
Remarks The otolith is tentatively assigned to the family Exocoetidae based on its distinctive features.Although the anterior half of the otolith is not preserved, the presence of a thick fringe ventral rim that curves towards the outer face and a wide cauda that is only slightly bent at the tip are characteristic traits observed in otoliths of exocoetids.However, without further details or additional better-preserved specimens, it is not possible to make a more specific identification.

Figure 14l
Remarks The otoliths exhibit thinness, a pronounced concavity in the outer face, and typical percomorph sulcus type, and show similarities to otoliths of the family Carangidae.However, due to the limited preservation of the otoliths, it is not possible to assign them to a particular species or genus within the Carangidae.

Figure 15a-b
Remarks The otoliths exhibit characteristic features, including a thick and convex structure (biconvex) with both faces of the otolith being convex, especially the outer face.They also have a well-divided sulcus that is deeply incised and surrounded by very thick cristae.
These features comply with those of Arnoglossus (Lin & Chang, 2012;Schwarzhans, 1999).It is worth noting that Arnoglossus is the most abundant flatfish genus in the collection.However, it is also important to acknowledge that a more comprehensive reference collection for this genus in Taiwan is still under development.

Figure 15d
Remarks Two oblong otoliths exhibit a vertical posterior rim and an obtuse anterior rim.They are also characterized by a flat dorsal rim, a slightly elevated antero-dorsal part, and an oblique antero-ventral rim.The sulcus is elongated and divided into a straight ostium and a short, squarish cauda.The described otoliths align with the otoliths of Heteromycteris as described by Schwarzhans (1999: figs. 876-880).Although an extant species, Heteromycteris matsubarai Ochiai, 1963, is known from Taiwan according to Shao (2023), but the otoliths of this species are not known.
The key features include a slightly enlarged antero-dorsal part, a deep ventral area, and a sulcus that is only slightly divided into an elongate ostium and an oval cauda (Schwarzhans, 1999, figs. 872-875), although this is not clearly seen in our fossil.Given the similarities in morphology and the presence of characteristic features, it is plausible that the otoliths belong to Pardachirus.

Figure 15e
Remarks The triangular shape, flat dorsal rim, curved anterior and ventral rims, and vertical posterior rim are Early Pliocene mesopelagic fish Fauna in Taiwan typical features observed in Cynoglossus otoliths (Schwarzhans, 1999).Additionally, the narrow and channel-like ostium and the wide, triangular cauda further support the assignment to the genus.However, due to the limited preservation of the specimens, it is not possible to provide a more specific identification at the species level.

Figure 15g
Remarks The single otolith, despite being small (< 2 mm) and showing some abrasion, exhibits diagnostic characters that align with the family Callionymidae.These include a triangular outline, marked middorsal angle, and the slightly inclined sulcus opening antero-ventrally.Furthermore, the presence of a short posterior rim without an extended posterior part suggests a closer resemblance to the genus Repomucenus (Lin & Chang, 2012: pl. 124).

Figure 15i
Remarks The otolith, with its distinct sulcus widening towards the caudal tip and a thick dorsal rim, suggests an affiliation with the family Trichiuridae.Specifically, it shows similarities to the otoliths of the genus Trichiurus as depicted in Lin and Chang (2012: pl. 129).However, due to the limited preservation and the lack of the anterior part of the otolith, further taxonomic assignment is not possible at this time.Nelson et al., 2016Family Champsodontidae Jordan & Snyder, 1902 Genus Champsodon Günther, 1867 Champsodon sp.

Figure 15h
Remarks The two otoliths are well-recognized as Champsodon by a pentagonal outline that is pointed at both anterior and posterior ends, a high angled mediodorsal rim, two angles on the ventral rim, a deeply incised and oblong ostium, and a narrow, shallow, and downward oriented, but rather indistinct, cauda (Schwarzhans, 2019c: pl. 2, figs. 1-8).It should be noted that identifying Champsodon species based on otoliths alone can be challenging as the differences between species are often subtle (Schwarzhans, 2019c).

Figure 15f
Remarks Paraperci otoliths are typically triangular with pointed anterior and posterior ends and a marked dorsal angle in the middle of the dorsal rim.The well-delineated sulcus is characterized by a short widening ostium that opens anteriorly and is accompanied by a short rostrum.The long cauda is slightly bent downward and widened posteriorly.Additionally, a deep and oval dorsal depression and an upward curved ventral furrow are observed (Lin & Chang, 2012: pl. 122;Schwarzhans, 2019c: pl. 1, figs. 1-10).While these otoliths are easily recognized as Parapercis, there can be overlapping inter-and intra-specific variations within this species-rich genus.Thus, no species determination was possible.

Figure 15j
Remarks A small and triangular otolith displays characteristics consistent with the family Labridae.It has a distinctly elevated collum that separates the ostium and cauda, both of which are deeply incised.This sulcus type is typical of Labridae.Lin and Chang (2012) have provided otolith images of various genera and species of Labridae from Taiwan; however, it is important to note that the specimen appears to be from a juvenile individual and lacks specific features necessary for identification at the generic or specific level.Davis et al., 2016 Family Acropomatidae Gill, 1893

Figure 16a-b
Remarks Acropoma otoliths are not uncommon and quite recognizable by their notable postero-dorsal and mediodorsal angles, nearly straight sulcus, and large ventral area.The sulcus follows a typical percomorph configuration with well-developed cristae, an oval and short ostium that is mostly filled with a colliculum, and an elongate cauda that bends only slightly ventrally at the tip.Two species, Acropoma hanedai Matsubara, 1953 and Acropoma japonicum Günther, 1859, are known from Taiwan (Shao, 2023).The otoliths of these two species can be distinguished based on the shape of the posterior rim in larger specimens.In A. japonicum, the posterior rim is gently curved and exhibits a less pronounced postero-dorsal angle, whereas A. hanedai has a straighter and more vertical posterior rim with a more salient angle, resulting in a more compact otolith outline appearance (Fig. 17 for comparative figures).Both species have been found in our Gutingkeng collection, but A. hanedai is much rarer compared to A. japonicum.Remarks The otoliths of Malakichthys are characterized by their massive and thickset structure as well as their high-bodied and pentagonal outline.They share some similarities of the sulcus with the otoliths of Acropoma, particularly in juveniles (see above).However, one distinguishing feature of Malakichthys otoliths is that the ostium appears to bend upwards more strongly.In Taiwan, there are four known species of Malakichthys (Shao, 2023), but their otoliths exhibit subtle or no distinct differences that can be used for species-level identification (Lin & Chang, 2012: pl.319).Therefore, it is challenging to assign these otoliths to a specific species within the genus based on their morphology alone.

Family Synagropidae Smith, 1961
Genus Synagrops Günther, 1887 Remarks Otoliths of Synagrops japonicus (Döderlein, 1883) are very distinctive by having a protruding and oval ostium that forms the anterior part of the otolith, and a long and narrow cauda that flexes strongly towards the antero-ventral direction (Lin & Chang, 2012: pl. 94).In the case of our two fossil otoliths, the overall shape and surface are heavily abraded and not well-preserved.However, despite the poor condition, the peculiar ostium characteristic of S. japonicus otoliths is still visible.Nelson et al., 2016 Family Mullidae Rafinesque, 1815 Genus Upeneus Cuvier, 1829 Upeneus sp.

Figure 16j
Remarks Two juvenile otoliths are reasonably assigned to Upeneus based on the sulcus.The presence of a triangular ostium, which opens widely towards the anterior, and a cauda with a long straight part and a downward-bent posterior tip forming a deep expansion are indicative of Upeneus otoliths (Lin & Chang, 2012: pl. 111).However, the expansion of the caudal tip (bulb) is difficult to see clearly because of preservation.

Figure 18b
Remarks This small (< 1.5 mm) and tall otolith is characterized by an oblique posterior rim, lobated rim margins, and a deep excisure.The sulcus is well-divided into a deep, short ostium with a salient ventral lobe and a straight, elongate cauda.These combined features comply well with those of Antigonia (Lin & Chang, 2012: pl. 131), but it is worth noting that our identified otolith is from a juvenile specimen.

Figure 18f
Remarks Utricular otoliths, represented by 76 specimens in the Gutingkeng Formation collection, are relatively common.However, it is important to note that these otoliths do not possess significant taxonomic characteristics that can be used for identification.As a result, the utricular otoliths in question were not subjected to an identification attempt.

Abundance, diversity, and sample coverage
In this study, the extensive collection of 122 bulk sediment samples from the study sites provided a unique opportunity to conduct a comprehensive evaluation of otolith-based fish abundance and diversity.Unfortunately, no fish otoliths were found in the samples from sites 3 and 4. Nevertheless, our biostratigraphic analysis allowed for age determination for both sites, which is valuable for documentation purposes (Material and methods).It is worth noting that the distribution of otoliths in sediments is likely to be uneven (Agiadi et al., 2022), and to capture a more representative faunal assemblage, multiple replicates with appropriate sampling coverage are necessary (Lin & Chien, 2022).While previous studies may not have routinely incorporated diversity assessment and sample coverage analysis, our adoption of this robust sampling strategy has significantly improved the reliability and accuracy of our findings.
The assessment of otolith abundance in the sediments, or otolith density, provided a general view of fossil distribution in the present study (Fig. 21).The abundance and diversity of otoliths were found to be low in sites 1 and 6 (Table 1), and the preservation of otolith specimens was also less favorable compared to sites 2 and 5. Notably, the mud samples exhibited much lower otolith density and diversity compared to the bioclastic sand layers.The sand layers showed over one order of magnitude higher otolith density and two times greater diversity (Fig. 21).
The results from the rarefaction curves revealed interesting patterns in three different diversity indices (Fig. 22).While a total of 64 taxa were observed in all assemblages (Table 1), the rarefaction curve for species richness ( 0 D) indicated a higher diversity of approximately 75 taxa with increased sampling effort.These additional taxa would likely be represented by a wide variety of marginal species with considerable low individual numbers.In contrast, the rarefaction curves for Shannon ( 1 D) and Simpson ( 2 D) diversity indices reached asymptotes suggesting that our sample coverage for abundant and dominant taxa was sufficient.It is likely that there may be up to five abundant taxa and only three dominant taxa present in the entire Lower Gutingkeng Formation.Overall, these findings suggest a fish community with very few highly dominant fish species and numerous minor species during the Early Pliocene in southwestern Taiwan.

Taxonomic composition and paleoecological significance
The otolith assemblages from the Lower Pliocene Gutingkeng Formation exhibit a taxonomic composition comprising 8,314 otoliths assigned to 64 different taxa from 33 families (Table 1).Remarkably, it contains one new fossil genus and three new fossil species, highlighting the significance of the otolith collection in expanding our understanding of the fossil fish fauna in Taiwan.In terms of taxonomic diversity, this collection represents the most diverse fossil fish fauna documented from Taiwan so far (Lin et al., 2021).
Rank abundance analysis of the otolith assemblages from the Gutingkeng Formation indicates strong dominance of Benthosema (Myctophidae), Gobiidae, and Polyipnus (Sternoptychidae) (Fig. 23).Among these, the mesopelagic fish family Myctophidae alone constitutes over 85% of the otolith count across all samples.Additionally, there are several marginal taxa with lower individual counts, including Gonostomatidae, Macrouridae, Champsodon (Champsodontidae), and Acropomatiformes, which are characteristic of typical deep-sea inhabitants (Nelson et al., 2016).The presence of Atrobucca nibe (Sciaenidae) also suggests slightly deeper water depths compared to other sciaenids known as shallow-water inhabitants (Shao, 2023).Overall, the habitat preferences of these deep-sea and oceanic fishes align well with the paleoenvironmental interpretation of an outer-shelf to upper slope environment, as supported by previous studies (Keng, 1981;Lin, 1990;Luo, 1988;Oinomikado, 1955).
However, a notable difference in taxonomic composition is observed between samples from the mud and bioclastic sand layers.In the sand layer samples, in addition to outer-shelf fishes, otoliths of shallow-water Early Pliocene mesopelagic fish Fauna in Taiwan taxa derived from various depth settings are frequently found (Table 1).These include gobies (Gobiidae), flounders (Bothidae), cardinalfishes (Apogonidae), and seabreams (Sparidae), which are typically associated with shallow-water and shelf environments (Froese & Pauly, 2023;Nelson et al., 2016).The sand layer samples contain a considerable abundance of gobiid otoliths indicating a diverse gobiid fish fauna (Table 1).However, further research integrating otolith morphology with molecular data may be helpful to gain a comprehensive understanding of ancient goby fauna in the area (Gierl et al., 2022).The co-occurrence of various mollusks and even shark teeth in the sand layers is also noteworthy as these are not commonly observed or present in the mud samples (Fig. 3).The sand layers can potentially originate from either turbidity currents or storm events.Distinguishing between episodic storm deposits and turbidite deposits can be challenging, as both processes involve sediment transport from shallower-water to deeper settings and deposition by underwater currents.However, certain characteristics commonly found in turbidite deposits, such as alternating layers of coarser and finer sediments, well-defined graded bedding, and distinctive sedimentary structures like cross-bedding and ripple marks, are not evident within these sand layers.While turbidite deposits typically exhibit relatively thicker beds compared to episodic storm deposits, it's worth noting that the observed sand layers are relatively thin in our case (Fig. 3; Additional file 1: Fig. S2).Based on these observations, it can be inferred that the sand layers likely represent episodes of storm events (distal tempestites) or periods of strong water currents, resulting in the transportation and deposition of sediments from shallower depths.

Establishment of mesopelagic fish fauna over 3 million years in southwestern Taiwan
Based on our biostratigraphic analysis, the bulk sediment samples containing otoliths are dated to 5.6 to 3.1 Ma, indicating an Early Pliocene Zanclean age.While a comprehensive comparison with adjacent otolith-based assemblages is not yet feasible due to data gaps in time and space and inconsistent sampling methods, our findings indicate a higher species diversity compared to the deep-sea Plio-Pleistocene fish otolith assemblage from the Cabarruyan region in northwest Philippines, where 53 taxa have been described (Van Hinsbergh & Helwerda, 2019).In contrast, the well-known Late Pliocene Kakegawa assemblages from central Japan exhibit rich and diverse tropical to subtropical fauna with more than 100 otolith taxa being reported (Ohe, 1977(Ohe, , 1981(Ohe, , 1983)).However, aside from the revision of myctophid otoliths by Schwarzhans and Ohe (2019), extensive revision are needed for these reports.
Among the identified species in our collection, the myctophid Benthosema pterotum stands out as the most abundant fish (Table 1), mirroring its status as one of the most abundant species observed in modern mesopelagic fish communities in the area (Wang & Chen, 2001).The annual bycatch from deep-sea trawling fisheries alone is estimated to exceed 3,000 tons for this species in waters off southwestern Taiwan (Chiou et al., 2020).Other common mesopelagic fish taxa found in our fossil collection include Diaphus, Bregmaceros, Polyipnus, and Acropoma (Fig. 23, Table 1), which are also frequently encountered in the discards of mesopelagic trawlers (Koeda & Ho, 2019;Wu et al., 2004).
When excluding the shelf and nearshore elements derived from the bioclastic sand layers, the overall composition of the Gutingkeng fish assemblages at the family level already resembles the modern outer-shelf and upper slope fish fauna in southwestern Taiwan (Koeda & Ho, 2019;Shao et al., 2008Shao et al., , 2012)).The dominant mesopelagic taxa and their relative abundance further suggest a close affinity and a potential early establishment and persistence of modern mesopelagic fish fauna in southwestern Taiwan and the northern part of the South China Sea since at least the Early Pliocene.However, it is also important to note that many otoliths described here are labeled under open nomenclature and have not being identified to species.Thus, they may or may not be directly comparable to the modern fish fauna at the genus or species level.Future works focusing on the Late Miocene and Pleistocene portions of the lower and upper Gutingkeng Formation would provide a more comprehensive understanding of the succession of deep-sea fish fauna in the area.

Conclusions
Our study provides valuable insights into the diversity and composition of otolith-based deep-sea fish fauna in the Pliocene West Pacific, specifically in Taiwan.Through the analysis of 122 bulk sediment samples from the Lower Pliocene Gutingkeng Formation, we reconstructed a previously overlooked yet highly diverse fish fauna consisting of 64 taxa, including one new genus and three new species.The dominance of mesopelagic Myctophidae and deeper-water taxa suggest an outer-shelf to upper slope environment, while the co-occurrence of shallow-water elements indicates episodes of storm events.Our findings highlight the early establishment and persistence of the mesopelagic fish community in southwestern Taiwan since the Early Pliocene.This unique deep-sea fish fauna, exclusively documented through fossil otolith data due to preservation conditions, contributes to filling the knowledge gap in the paleoichthyological fauna in the West Pacific.
The baseline data provided by our study will facilitate future research comparing coeval assemblages across the entire West Pacific.

Fig. 1
Fig. 1 Maps showing the sampling sites.a Overview of geological map of Taiwan (modified after Chen, 2016).b Detailed geological map of study area (map extracted from National Geological Data Warehouse, Central Geological Survey, Ministry of Economic Affairs, Taiwan).c-d Detail maps of the sampling sites in Chochen, Tainan (c) and Tianliao, Kaohsiung (d)

Fig. 2
Fig. 2 Stratigraphic correlation of the southern part of the Western Foothills in Taiwan (modified after Chen, 2016).The studied formation, the Gutingkeng Formation, is indicated in yellow

Foraminifera
Co-occurrence of S. dehiscens and Sphaeroidinellopsis paenedehiscens indicates an age range of 5.6 to 3.1 Ma.The site suggests a possibly outer-shelf environment with depths approximating 200 m.

Fig. 3
Fig. 3 Measured section of the outcrops in Site 5 (see text for description).Sampling horizons are indicated

Fig. 21
Fig. 21 Differences in density of otoliths (a-b) and species richness (c-d) for the sampled sites.Box plots with lower (25th percentile), median, and upper (75th percentile) boundaries, and outliers (solid circle) outside of 10th and 90th percentiles are presented.Note change of scale for samples from the sand layers in Kaohsiung (c-d)

Fig. 22
Fig. 22 Rarefaction curves of otolith diversity (Hill numbers) among three sampling sites and all sites combined in terms of species richness ( 0 D), Shannon diversity ( 1 D), and Simpson ( 2 D) diversity.Shaded areas represent 95% confidence interval for their respective sites based on 300 bootstrap replicates

Table 1
List of otolith-based fish taxa from the Lower Pliocene Gutingkeng Formation, southern Taiwan
Genus Diaphus Eigenmann & Eigenmann, 1890Remarks Diaphus otoliths represent the second most abundant myctophid in the collection.While most of the otoliths belong to juvenile individuals and were not