Ophisternon sp.
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publication ID |
https://doi.org/10.1186/s13358-024-00347-0 |
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persistent identifier |
https://treatment.plazi.org/id/4D2687FF-FFA0-FFB9-FF4E-8C70F46AFACA |
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treatment provided by |
Felipe |
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scientific name |
Ophisternon sp. |
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Figure 7 View Fig
Material: Single specimen COG-001 ( Fig. 7 View Fig ) preserving the semi-articulated remains of the anterior part of the body, including the skull.
Description: Te remains of this small fish have a measurable length of about 48 mm, with a large head measuring ca 20 mm. Te skull is preserved in a dorsolateral view, exposing its left side and part of the dorsicranium. However, the margins among individual bones are not discernable due to insufficient preservation. Te neurocranium is antero-posteriorly elongated. Two protrusions are recognizable in the anterior part of the skull, interpreted to be remains of the palatine and lateral ethmoid. Both frontals are recognizable, and although their precise shape is indeterminable, they were strongly elongated. Two additional protrusions are preserved in the otic part of the skull and interpreted as remains of the sphenotic and pterotic. In the posterior midline of the skull, a short supraoccipital bone is recognizable with a short and low occipital crest.
Te remains of the lower jaw preserve the anguloarticular, a small and rounded retroarticular, and partially the dentary. Te remains of the dentary preserve an elongated lower limb reaching caudally the level of articulation with quadrate bone (see arrow in Fig. 7B View Fig ). Te anguloarticular seems to have a more or less developed coronoid process. Te articulation of the lower jaw is at the midpoint between the lateral ethmoid and sphenotic protrusions of the skull roof. Five short branchiostegal rays are recognizable, with the third one being the longest. Te posterior-most tips of the branchiostegal rays reach the level of the ventral tip of the cleithrum. Te cleithrum is crescent-shaped, anteriorly elongated, and relatively close to the skull. Te posttemporal connects the skull with the dorsal section of the cleithrum (via supracleithrum), and preserves a more robust and longer dorsal limb than the ventral one, which is about half as long and significantly thinner.
Other remains of the skull and the attached pectoral girdle are not recognizable. A fragment of the vertebral column preserves the remains of the anterior most 16 vertebral centra. While the first six vertebrae are preserved more or less in a dorso-ventral view, the more posterior ones show lateral aspects. Skull fragments partly cover the anterior-most vertebra, but the socketlike articulation for the occipital part of the skull is determinable. Te vertebrae bear well-developed lateral apophyses, particularly well discernable in the 9th and 10th vertebrae. Tese lateral apophyses served for articulation with pleural ribs. From the fifth vertebra onwards, the dorsal prezygapophyses and ventral postzygapophyses are recognizable—these structures would likely be present in the more anteriorly located vertebrae, but the preservation does not allow clear determination. Te pleural ribs are thin, set sub-horizontally, and recognizable in association with several vertebrae, including the second one (the condition of the first vertebra is not clear).
Notes: Swamp eels ( Synbranchiformes ) are highly specialized percomorph fish classified within three families: Synbranchidae , Chaudhuriidae , and Mastacembelidae ( Nelson et al., 2016) . Wiley and Johnson (2010) listed six synapomorphies diagnosing this order. Although the specimen is relatively poorly preserved, the extension of the dentary postero-ventrally along the ventral margin of the anguloarticular supports its classification within this order.
Te order is divided into two suborders: Mastacembeloidei and Synbranchoidei . Based on the partially recognizable articular plug of the first vertebra and the absence of the pectoral fin and scapulocoracoid, it is reasonable to assume that the specimen belongs to the suborder Synbranchoidei , which includes a single family, Synbranchidae (Wiley & Johnson, 2010) . Te entire family was revised by Rosen and Greenwood (1976), who recognized four separate genera: Macrotrema Regan, 1912 , Synbranchus Bloch, 1795 , Ophisternon McClelland, 1844 , and Monopterus Lacepède, 1800 , but research of this peculiar group has progressed significantly in recent years, leading to a deeper understanding to its diversity and anatomy (see Kottelat, 2013; Britz et al., 2016, 2020a, 2020b, 2020c, 2023). Due to the presence of the posttemporal, which connects the pectoral girdle with the skull, the probable presence of the triangular coronoid process at the anguloarticular, and the distal tips of the branchiostegal rays reaching the level of the ventral tip of the cleithrum, it is possible to classify the specimen as a member of the genus Ophisternon (see Rosen & Greenwood, 1976). Although Rosen and Greenwood (1976) listed six branchiostegal rays for the genus, only five are recognizable in the fossil. Tis condition is interpreted as an artifact of preservation rather than a natural state.
According to Jamandre (2023), the only synbranchiform family native to the Philippines is Synbranchidae , which includes two species of the genus Monopterus ( M. albus (Zuiew, 1793) and M. javanensis ( Lacepède, 1800)) and a single species of the genus Ophisternon ( O. bengalense McClelland, 1844 ). Te genus Ophisternon is remarkably distributed from Middle and South America, Africa, India, Southeast Asia, and Australia (Rosen, 1975) and includes six species ( Nelson et al., 2016). Due to insufficient and incomplete preservation, species determination is not possible at this moment. On the other hand, the specimen represents the first fossil record of swamp eels worldwide—until now, only Holocene remains of synbranchids ( Synbranchus ) were reported from southwestern Amazonia (Prestes-Carneiro et al., 2018, 2020) and Argentina (Bogan et al., 2012).
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