New osteological characteristics identify the first stem sleeper goby (Gobioidei, Odontobutidae) from the Upper Eocene

Amongst Gobioidei, one of the most diverse orders of teleost fishes, the families Odontobutidae (sleeper gobies) and Rhyacichthyidae are considered ‘basal gobioids’ and are therefore particularly interesting for understanding gobioid evolutionary history. However, their fossil record currently consists of only three extinct odontobutid species (two skeleton-based, one otolith-based). The aim of the study associated with this dataset (Bauer et al. 2025 in Papers in Palaeontology) was to investigate the possible relationship of one of the oldest known gobioid fossils, †Paralates chapelcornerensis from the Upper Eocene of southern England, to these families. To achieve this, osteological differences between Odontobutidae and Rhyacichthyidae were newly investigated using 19 extant species from museum collections. This taxon set included both genera of the Rhyacichthyidae, five genera of the Odontobutidae (including Terateleotris; representing 78% of all odontobutid genera), one genus of the Eleotridae (Eleotris Bloch & Schneider 1801), two of the Butidae (Oxyeleotris Bleeker 1874; Bostrychus Lacepède 1801), one of the Gobiidae (Gobius Linnaeus 1758), as well as Trichonotus Bloch & Schneider, 1801 (Trichonotidae) and Apogon Lacepède 1801 (Apogonidae). Both Trichonotus and Apogon were selected as outgroups because the Trichonotidae is the sister group of the Gobioidei, and Apogonidae is closely related to Trichonotidae + Gobioidei (Thacker et al. 2015). To ensure that characters were consistent between species and capable of distinguishing between genera, three species each of Odontobutis and Trichonotus were included. The analysis identified 17 osteological characters (15 newly described) that are different between Odontobutidae and Rhyacichthyidae. These characters were used for comparative morphology and phylogenetic analyses (maximum parsimony, Bayesian inference) to determine whether †Paralates can be assigned to either the Odontobutidae or Rhyacichthyidae.

This Dryad dataset provides an osteological atlas (Supplementary Data S1 in Bauer et al. 2025 in Papers in Palaeontology) illustrating the studied skeletal structures for †Paralates, Rhyacichthyidae and Odontobutidae, Eleotris (Eleotridae), Oxyeleotris and Bostrychus (both Butidae), Gobius (Gobiidae) and the used outgroups Trichonotus and Apogon.

Methods
Study of fossil material

The fossil material included 51 articulated fossils, > 100 isolated bones and four isolated otoliths of †Paralates from the Upper Eocene of the Chapelcorner Fish Bed (Chale, Isle of Wight, UK). The fossil material was studied and photographed under a Leica M165 C microscope equipped with a Jenoptik Gryphax Naos microscope camera. Photos were edited in Photoshop CS6 and Krita. The corresponding figures were prepared with Photoshop CS6 and Inkscape.

Study of recent comparative material

A total of 19 extant species were newly investigated. They encompass:

Apogonidae

Sphaeramia nematoptera (Bleeker 1856)

Apogon imberbis (Linnaeus 1758)

Trichonotidae

Trichonotus arabicus Randall & Tarr 1994

Trichonotus cf. setiger Bloch & Schneider 1801

Trichonotus nikii Clark & von Schmidt 1966

Rhyacichthyidae

Protogobius attiti Watson & Pöllabauer 1998

Rhyacichthys guilberti Dingerkus & Séret 1992

Rhyacichthys aspro (Valenciennes, 1837)

Odontobutidae

Microdous chalmersi Nichols & Pope 1927

Micropercops cinctus (Dabry de Thiersant 1872)

Odontobutis hikimius Iwata & Sakai 2002

Odontobutis obscurus Temminck & Schlegel 1845

Odontobutis platycephala Iwata & Jeon 1985

Perccottus glenii Dybowski 1877

Terateleotris aspro (Kottelat 1998)

Eleotridae

Eleotris pisonis (Gmelin 1789)

Butidae

Bostrychus africanus (Steindachner 1879)

Oxyeleotris marmorata (Bleeker 1852)

Gobiidae

Gobius incognitus Kovačić & Šanda 2016

For museum collection numbers of the species see Table 1 in Bauer et al. (2025) in Papers in Palaeontology.

New micro-CT (µCT) scans were obtained for species that were not available on Morphosource from previous works. Only for Sphaeramia nematoptera, Rhyacichthys aspro and Bostrychus africanus no µCt scan could be acquired and these species were examined based on X-ray imaging. The software Amira 3D 2021.2 (Stalling et al. 2005) was used for analysis and segmentation of individual bones as well as generation of 3D models. Images of the models were made with the in-program snapshot function.

A new MorphoSource project was created that contains µCT scans or X-ray images for all studied species as well as models of the studied bones. The reference is:

Bauer, E., Cerwenka, A. E., Schliewen, U. K. and Reichenbacher, B. 2025. Project: New osteological characteristics identify the first stem sleeper goby (Gobioidei, Odontobutidae) from the Upper Eocene [dataset]. MorphoSource. https://www.morphosource.org/projects/000642675

For MorphoSource DOI numbers of the µCT scans of the species see also Table 1 in Bauer et al. (2025) in Papers in Palaeontology.

Osteological study

The fossil material and µCt scans of the extant comparative specimens were used for osteological study. Six bony structures that were well-preserved in the fossil material of †Paralates were the focus: the first vertebra (atlas), the premaxilla, the palatine, the urohyal, the hyoid arch, and the pelvic girdle. These elements were segmented from the µCTs of the extant species and visualized in 3D.

The outcome served to identify structures and character states that differed between Odontobutidae and Rhyacichthyidae. The results are compiled in an osteological atlas comprising eight plates (Supplementary Data S1 in Bauer et al. 2025 in Papers in Palaeontology). The original TIFF files of this atlas are provided in the Dryad dataset.