Thrissops kimmeridgensis sp. nov.

Figs 14 A – C, 15 A – D, 19 A, B, D, 20 C, 22 A, B

1895 Thrissops sp.; Woodward: p. 527–528.

1977 undetermined specimen of Thrissops; Patterson and Rosen: p. 100 (NHMUK PV P. 54597).

1984 Thrissops sp.; Schaeffer and Petterson: fig. 27 J.

2013 Thrissops sp.; Cavin, Forey and Giersch: p. 156, figs 32, 33.

2018 “ Thrissops ” from Kimmeridge; Yabumoto et al.: figs 7, 8.

2019 Thrissops from Kimmeridge; Cavin and Berrell: p. 12, fig. 9.

2020 Thrissops sp.; Martill and Brito: text fig. 3.26 C.

2024 Thrissops “Kimmeridgian”; Alvarado-Ortega: p. 27, fig. 16.

Holotype.

MJML K 2295.

Type locality.

Kimmeridge, Isle of Purbeck, Dorset, England.

Type horizon.

Upper Kimmeridgian.

Formation.

Kimmeridge Clay, Dorset, England.

Determination

(for comparison of taxa see Table 1; for measurements of specimens and counts of features see Suppl. material 1: table S 1). Maximum length 75.5 cm standard length (~ 90 cm total length); ~ 55–57 vertebrae (without ural centra); ~ 25–28 ribs anterior to anal fin; 29–30 anal pterygiophores; dentary with anterior hook armed with one large fang-like, posteriorly directed tooth; dentary teeth of irregular size, with the most anterior tooth and the teeth in the middle of the dentition being twice the size of the other dentary teeth (Figs 15 A – D, 19 A, B, D); curved maxilla with small teeth all in the same size (Figs 15 A – D, 19 A, B, 20 C).

Etymology.

The specific epithet in Thrissops kimmeridgensis refers to the village of Kimmeridge, Dorset, England where most of the specimens were found.

Additional material.

MJML K 26, 276, 306, 379, 452, 454, 518 A, 518 B, 525, 531 C, 535, 547, 574, 575, 944, 949, 969, 998, 1001, 1002, 1015, 1069, 1085, 1129, 1142, 1155, 1159, 1167, 1173, 1194, 1196, 1230, 1282, 1283, 1290, 1301, 1304, 1313, 1316, 1338, 1373, 1378, 1379, 1395, 1407, 1419, 1439, 1440, 1456, 1506, 1520, 1521, 1534, 1550, 1590, 1653, 1669, 1672, 1676, 1677, 1690, 1714, 1745, 1786, 1792, 1804, 1839, 1925, 1934, 1946, 1981, 2003, 2022, 2063, 2079, 2119, 2138, 2167, 2294, 2333, 2399, 2491, 2700, 2770; NHMUK PV P. 922, P. 3686, P. 3686 a, P. 54596, P. 54597, P. 54598, P. 54599, NHMUK PV OR 40336, 40720.

Features.

The features of Th. kimmeridgensis are very similar to the other Upper Jurassic Thrissops species described above (for the most distinguishing features see Table 1). The cranial features of Th. kimmeridgensis are described and figured in detail in Cavin et al. (2013), especially the ethmoid region.

Here, I add two drawings of the cranium of Th. kimmeridgensis; one of the holotype (MJML K 2295; Fig. 15 A, B), the other of the slightly disarticulated cranium of MJML K 1456 (Fig. 15 C, D). Additionally, I figure the jaws and dentition of three specimens (MJML K 1129, Fig. 19 A; MJML K 2119, Fig. 19 B; MJML K 1313, Fig. 19 D), and the shape of the maxilla (Fig. 20 C). In the following, I only describe features characteristic for Th. kimmeridgensis or which are not visible in the other above-described species.

Irregular size of dentary teeth.

The irregular size of the dentary teeth, the most striking feature to distinguish Th. kimmeridgensis from Th. formosus and Th. ettlingensis, was already recognised in Schaeffer and Petterson (1984, fig. 27 J) and Cavin et al. (2013, fig. 33 C) (see discussion in chapter discussion of features below).

Anterior ceratohyal.

The anterior ceratohyal is visible in some of the partially disarticulated craniums of Th. kimmeridgensis (MJML K 1173, 1205, 1283, 1313, 1456, 1786, 1946, 2119). It is a rectangular bone, nearly two times longer than high, and has an ovoid foramen near the center (Fig. 15 C, D). In MJML K 1456 a series of at least 13 elongated and dorsally bent branchiostegal rays are articulated proximally to the lateral surfaces of the anterior ceratohyal (Fig. 15 C, D).

Posterior ceratohyal.

The posterior ceratohyal or parts of it are visible in MJML K 1173, 1313, 1456 (Fig. 15 C, D), and 1946 (best visible in MJML K 1173, 1946). It is a nearly reniform bone, which is broader than the anterior ceratohyal. At least the posteriormost five branchiostegals, which are twice as broad as the branchiostegals anterior to it, are articulated to the lateral surface of this bone.

Gill arch.

Remains of the gill arch are visible in two specimens represented by rod-like elements of different length (MJML K 1313, Fig. 19 C; MJML K 1407) probably epibranchials, hypobranchials and ceratobranchials). In MJML 1407 the ceratobranchials bear gill filaments.

Preservation.

Of Th. kimmeridgensis only few are preserved as complete fish (Fig. 14), most of the specimens listed under additional material are preserved as isolated crania or caudal fins. It seems reasonable to assume that these are the remains left by larger predators, which are well known from the same sites in the Kimmeridge Clay (for example crocodiles, ichthyosaurs, plesiosaurs, pliosaurs and sharks; Martill and Etches 2000).

Thrissops specimens from further Upper Jurassic localities.

The following Thrissops specimens are probably independent species. However, since only a few specimens from these localities are known and mostly in poor preservation, they are still awaiting revision.

Thrissops cf. formosus (late Kimmeridgian, Wattendorf, Bavaria Germany): NKMB (Wattendorf specimen, see Mäuser 2015, fig. 1014).

Thrissops? curtus Woodward, 1919 (Tithonian, Purbeck and Portland, Dorset, England): NHMUK PV P. 417 a (holotype of Thrissops molossus Woodward, 1919; Fig. 16 D), P. 8381, P. 10612 (holotype; Fig. 16 A).

Thrissops portlandicus Woodward, 1895 (Tithonian, Isle of Portland, Dorset, England): NHMUK PV P. 5538 a (holotype; Figs 16 B, C).

Thrissops sp. (early Tithonian, Creys, France): OSUG (UJF-ID. 16053 *).

Thrissops sp. (early Tithonian, Daiting, Bavaria, Germany): coll. Tischlinger 76 / 72, 88 / 91; SNSB - BSPG 1964 XXIII 508.

Thrissops sp. (Tithonian, Purbeck, Dorset, England): NHMUK PV P. 12643.