An articulated Late Triassic (Norian) thalattosauroid from Alaska and ecomorphology and extinction of Thalattosauria

Abstract

Thalattosaurians are a cosmopolitan clade of secondarily aquatic tetrapods that inhabited low-latitude, nearshore environments during the Triassic. Despite their low taxic diversity, thalattosaurians exhibit remarkable morphological disparity, particularly with respect to rostral and dental morphology. However, a paucity of well-preserved material, especially leading up to their extinction, has hampered efforts to develop a robust picture of their evolutionary trajectories during a time of profound marine ecological change. Here, we describe a new taxon based on an articulated and nearly complete skeleton from Norian sediments of southeastern Alaska, USA. The holotype is the most complete North American thalattosaurian yet described and one of the youngest occurrences of the clade worldwide. We present a new hypothesis of interrelationships for Thalattosauria and investigate potential feeding modes in the Alaskan taxon. An integrated view suggests that the absence of pelagic lifestyles and restricted ecological roles may have contributed to thalattosaurs' eventual extinction.

Figure 1

Location and collection site. (a) Map of southeastern Alaska showing Keku Islands (modified from Riehle et al.). (b) Photograph of intertidal outcrop where UAMES 23258 was discovered (arrow). (Image courtesy Thomas Adams; used with permission).

Figure 2

Gunakadeit joseeae gen. et sp. nov. Holotype specimen UAMES 23258 in right lateral view.

Figure 3

Skull of Gunakadeit joseeae. (a) Photograph. (b) Interpretation of the holotype specimen UAMES 23258 in oblique right lateral view. Abbreviations: an, angular; ar, articular; atc, atlas centrum; atna, atlas neural arch; axc, axis centrum; axns, axis neural spine; bo, basioccipital; cbr, ceratobranchial; cp, coronoid process; d, dentary; en, external naris; f, frontal; j, jugal; L, left; mx, maxilla; n, nasal; op, opisthotic; or, orbit; p, parietal; pfor, pineal foramen; pof, postorbitofrontal; pmx, premaxilla; prf, prefrontal; pt, pterygoid; q, quadrate; rap, retroarticular process; scl, scleral element; sq, squamosal; st, suptratemporal; stf, supratemporal fenestra; sur, surangular; sym, symphysis.

Figure 4

Jaw and teeth of Gunakadeit joseeae. (a) Photograph of right lower jaw. (b) Detail of single dentary tooth of the holotype specimen UAMES 23258 in right lateral view. Abbreviations: an, angular; cr, crown; d, dentary; er, enameled ridge; mx, maxilla.

Figure 5

Cervical region and hyoid elements of Gunakadeit joseeae. (a) Photograph. (b) Interpretation of the holotype specimen UAMES 23258 in right lateral view. Colour scheme: light orange, ceratobranchial; dark orange, basihyal; reddish orange, epibranchials. Abbreviations: atc, atlas centrum; atna, atlas neural arch; axc, axis centrum; axns, axis neural spine; bhy, basihyoid; cbr, ceratobranchial; cl, clavicle; dia, diapophysis; dr, dorsal rib; epibr, epibranchial elements; icl, interclavicle; ns, neural spine; p, parietal; q, quadrate; rap, retroarticular process; v, vertebra.

Figure 6

Dorsal region and pectoral girdle of Gunakadeit joseeae. (a) Photograph. (b) Interpretation of the holotype specimen UAMES 23258 in right lateral view. Abbreviations: cl, clavicle; cor, coracoid; dc, dorsal centrum; dia, diapophysis; dr, dorsal rib; gast, gastralia; hum, humerus; int, intermedium; icl, interclavicle; mc, metacarpal; ph, phalanx; ns, neural spine; rad, radius; sc, scapula; ul, ulna; ule, ulnare.

Figure 7

Anterior caudal region and pelvic girdle of Gunakadeit joseeae. (a) Photograph. (b) Interpretation of the holotype specimen UAMES 23258 in right lateral view. Abbreviations: brom, bromalite; cc, caudal centrum; ch, chevron; fem, femur; fib, fibula; gast, gastralia; il, ilium; isch, ischium; L, left; mt, metatarsal; naf, neural arch facet; ns, neural spine; ph, phalanx; pub, pubis; R, right; sc, sacral centrum; sr, sacral rib; tar, tarsal; tib, tibia.

Figure 8

Time-scaled strict consensus topology of Thalattosauria. Thickened lines represent age range of each taxon; dashed line represents range of uncertainty. Colour scheme: red, Eastern Tethys (modern China); blue, Western Tethys (modern Europe); black, eastern Panthalassa (modern western North America).