Gobiidae Cuvier, 1816

Family Gobiidae Cuvier, 1816 View in CoL vel Oxudercidae Günther, 1861

Genus and species indet.

Figures 5 View Fig & 6 View Fig

Material: Single specimen NMP-2235 ( Fig. 5 View Fig ) preserving semi-articulated fish remains mainly in the ventral view.

Description: Te small fish reaches a SL of 69 mm, with a large head representing slightly more than 30% of SL. Due to the preservation of the fossil exposing the ventral view and its partial disarticulation, other body proportions are not discernible. Te remains of the skull ( Fig. 6 View Fig ) allow the recognition of a post-orbitally enlarged neurocranium with partially preserved frontal, mesethmoid, parasphenoid, pterotic, and basioccipital bones. Te left frontal (exposing its internal side) is about two and a half times wider in the postorbital section than in its interorbital section. Te mesethmoid is a sub-squarish bone with no distinct details. Te parasphenoid is thin, narrow anteriorly, and enlarged posteriorly, with well-developed ascending processes. Te pterotic is manifested as a well-discernable postero-laterally oriented spike at the posterior margin of the neurocranium. Te basioccipital is partially preserved, exposing the articulation for the first abdominal vertebra.

Te remains of the jaws allow the discernment of the maxillae, partially preserved premaxilla, and dentary. Teeth in the premaxilla are arranged in at least two rows (similarly as in the dentary), with the outer row bearing about five enlarged teeth just aside from the symphyses. Te lower jaw joint was located at the level of the anterior orbital margin. Te palatine is “T”-shaped, with the ethmoid process larger than the maxillary one. Te quadrate is poorly preserved, with only the relatively robust preopercular process being well recognizable. Te symplectic and hyomandibula are mostly preserved as natural imprints in the sediment, but the “symplectic foramen” is determinable. Te preopercle and opercle are preserved mainly as imprints on the surface of the sediment, suggesting a slightly longer horizontal ramus of the preopercle and a triangular opercle with a convex dorsal margin.

Te vertebral column is too poorly preserved to provide details about its morphology and the number of vertebrae. Te caudal skeleton is partially preserved and allows the recognition of two large hypural plates, with each plate representing fused hypurals: the lower one hyp 1 + 2 and the upper one hyp 3 + 4. Te upper plate is fused with the ural vertebra. Te caudal fin is exhibits about 18 elongated fin rays, with its maximal length being slightly more than 20% of the SL. Other unpaired fins and their supportive skeletons are not sufficiently preserved.

Te pectoral fin is composed of approximately 13 elongated fin rays, with a maximal length of about 15% of the SL. Te pectoral girdle, the pelvic fins, and the pelvic girdle are not recognizable. Te body is covered by large, most probably cycloid, scales. No ctenii are observed on any scale. Te scales are ornamented with almost parallelly arranged radii in high numbers; a scale in the caudal section of the body shows 15 of them.

Notes: Te orientation of the gobioid specimen is interpreted as ventral based on the arrangement of key neurocranial elements. Te parasphenoid and basioccipital are exposed superficially within the same sedimentary layer, while the frontal bones are positioned beneath them. Tis configuration, along with other neurocranial elements penetrating deeper into the sediment, supports a ventral view. Although the topography of certain viscerocranial elements may suggest alternative interpretations, these discrepancies likely result from post-mortem disarticulation. Te absence of the pelvic girdle, fins, and pharyngeal elements is consistent with this pattern of disarticulation and incomplete preservation, rather than indicative of orientation.

Wiley and Johnson (2010) listed 14 synapomorphies diagnosing the order Gobiiformes ( sensu Günther, 1880) , and although only a single character is recognizable (fused hypurals 1 & 2 and 3 & 4, with the latter also fused with the urostyle), this character unambiguously places the fossil within the order Gobiiformes . Te specimen also shows the overall physiognomy of the skull and the presence of a symplectic foramen (i.e., suspensorial interspace), supporting its placement within the suborder Gobioidei . Te Gobioidei suborder is highly diverse, with approximately 2,200 extant species classified into more than 270 genera and eight families ( Nelson et al., 2016).

Although the fossil does not preserve usable morphological characters for precise classification, the “T”- shaped palatine is typically restricted to gobioid families with five branchiostegal rays, specifically Gobiidae and Oxudercidae ( Hoese, 1984; Regan, 1911; Reichenbacher et al., 2020). Both families are highly diversified, with many species inhabiting muddy and silty environments ( Nelson et al., 2016). Due to the missing number of osteological characters (especially otoliths, the hyoid arch with associated branchiostegal rays, and the endoskeleton of the unpaired fins), and thus a high degree of uncertainty, we refrain from attempting a classification to the lower level of the specimen.

However, the fossil record in the Pacific Ocean does indeed preserve several semi-articulated gobioid species (family Gobiidae ) from the Plio-Pleistocene and Pleistocene deposits of Japan, namely species of the genera Tridentiger Gill, 1859 , Chaenogobius Gill, 1859 , Amblychaeturichthys Bleeker, 1874 , and Rhinogobius Gill, 1859 (Uyeno & Iwato, 1975; Yabumoto, 1987; Yabumoto & Uyeno, 1994). Te latter genus, with scales presenting numerous parallel radii (e.g., Yabumoto, 1987: Fig. 5 View Fig ), especially resembles our Antipolo specimen. Until better-preserved specimens become available, the true systematic position remains unclear.