First discovery of actinopterygian cutting-edged teeth from the middle Norian (Late Triassic) at the Tulong section, southern Tibet, China

Abstract

Actinopterygians (ray-finned fishes) successfully passed through the Permian-Triassic Mass Extinction (PTME) and flourished in the Triassic with diverse feeding specializations and occupation of various trophic levels. Birgeria, one of the largest actinopterygian fish of the Triassic, was characterized by a strong, blunt rostrum and three rows of sharp cutting-edged teeth, making them the top predators in the Early Mesozoic oceanic ecosystem. These fishes rapidly radiated and diversified globally during the Early and Middle Triassic, but the fossil record is rare for the Neo-Tethys in the Late Triassic. Here, we report new actinopterygian teeth with cutting edges from Norian-age strata in the Tulong section, which was located on the northern margin of the Indian Plate at that time. The tooth features, such as the polished acrodin cap, the ratio of the acrodin cap in length, and the tiny vertical striae at the tooth base, suggest an affinity with Birgeria, which is reported in this region for the first time. Furthermore, we infer that the carnivorous Birgeria, which co-occurred with the enigmatic ichthyosaur Himalayasaurus tibetensis, played the role of predator in this part of the Neo-Tethys marine realm during the Late Triassic. These new findings increase the known diversity of actinopterygians during the Late Triassic and provide further insight into the marine fauna of this epoch.

Keywords: Actinopterygii; Birgeria; Himalayasaurus tibetensis; Qulonggongba formation; Tooth morphology.

Figure 1. The geologic and paleogeographic map of the Tulong section.

(A) Geologic map based on An et al. (2020) and the location of the Tulong section; (B) reconstructed paleogeographic position of the Tulong section, modified after Cai et al. (2016) with permission from Elsevier.

Figure 2. Stratigraphic profile of the Qulonggongba Formation at the Tulong section.

(A) The stratigraphic column showing lithologies and fossil horizons in this study; (B) pictures of conodont Epigondolella triangularis; (C) pictures of conodont Epigondolella tozeri; (D) photo of the layer yielding the actinopterygian teeth fossils. In the conodont images, (1) upper view; (2) lateral view; (3) lower view; and the scale bar equals 0.5 mm.

Figure 3. Tooth fossils from the Tulong section.

(A) Tooth CUG-TLV-2 in lateral view; (B) CUG-TLV-2 in lingual view; (C) CUG-TLV-3; (D) CUG-TLV-4; (E) CUG-TLV-13; (F) CUG-TLV-1; (G) CUG-TLV-6 in labial view; (H) CUG-TLV-6 in lateral view; (I) CUG-TLV-5; (J) CUG-TLV-12; (K) CUG-TLV-7; (L) close-up of CUG-TLV-7 from apical view; (M) CUG-TLV-8; (N) close-up of CUG-TLV-8 from apical view; (O) CUGW-TLV-9; (P) CUG-TLV-10; (Q) CUG-TLV-11. Scale bar equals 5 mm.

Figure 4. CT images and histological structure of the CUG-TLV-12.

(A) The sagittal section along the labiolingual side, with the dotted lines indicating the location of the cross-sections (A1, A2, and A3); (A1) cross-section of the acrodin cap; (A2) cross-section near the cap-base junction; (A3) cross-section of the tooth base. (B) The histology of the vertical plane in normal transmitted light. Scale bar equals 2 mm.

Figure 5. Transverse histological section taken from tooth CUG-TLV-13.

(A) Transverse cross-section in normal transmitted light, showing the apical part of the tooth root with jaw fragment in CUG-TLV-13, and the enamel and the jaw area highlighted by the red-outlined boxes, (B) and (C), respectively; (B1) close up of the enamel area from the box B in normal transmitted light; (B2) close up of the enameloid area from the box B in polarized light; (C1) close up of the jaw area from the box C in normal transmitted light. Abbreviations: d, dentine; e, enameloid; edj, enameloid -dentine junction; p, pulp; pfb, parallel fibred bone; so, secondary osteon.

Figure 6. Paleogeographical distribution of Birgeria in the Triassic.

Paleogeographic map adopted from Scotese (2014) © PALEOMAP Project (http://www.scotese.com/), records of Birgeria mostly adopted from Ni et al. (2019) with modification. Red dots mean the fossil records with valid species, and yellow dots mean that with bony fragments. Early Triassic period: 1. Birgeria aldingeri, Spitzbergen; 2. B. groenlandica, Greenland; 3. B. nielseni, Madagascar; 4. B. americana, Nevada 5. fragments, British Columbia (Schaeffer & Mangus, 1976); 6. fragments, Siberia (Berg, Kazantseva & Obruchev, 1967); 7. fragments, Bolivia (Beltan et al., 1987). Middle Triassic period: 8. B. liui, South China; 9. B. Stensiöi, Switzerland; 10. B. mougeoti, Germany; 11. B. velox, California; 12. fragments, Poland (Chrząstek, 2008); 13. fragments, Belgium (Duffin & Delsate, 1993); 14. fragments, Austria (Krainer, Lucas & Strasser, 2011); 15. fragments, South China (Jiang et al., 2016); 16. fragments, Saudi Arabia (Vickers-Rich et al., 1999); 17, fragments, India (Chhabra & Mishra, 2002). Late Triassic: 18. B. guizhouensis, South China; 19. B. acuminata, Europe; 20. fragments, “Rhaetian Bone Beds” and other European fossil occurrences (Tintori & Lombardo, 2018); 21. fragments, Canada (Orchard et al., 2001); 22. fragments, Nevada (Tackett, Zierer & Clement, 2023); 23. fragments, Bolivia (Beltan et al., 1987); 24. fragments, Tulong section, Tibet, in this study.