Tuesday, March 2, 2021

[Paleontology • 2021] Tamarro insperatus • A Fast-growing Basal Troodontid (Theropoda: Maniraptora) from the latest Cretaceous of Europe


Tamarro insperatus  
Sellés, Vila, Brusatte, Currie & Galobart, 2021

A characteristic fauna of dinosaurs and other vertebrates inhabited the end-Cretaceous European archipelago, some of which were dwarves or had other unusual features likely related to their insular habitats. Little is known, however, about the contemporary theropod dinosaurs, as they are represented mostly by teeth or other fragmentary fossils. A new isolated theropod metatarsal II, from the latest Maastrichtian of Spain (within 200,000 years of the mass extinction) may represent a jinfengopterygine troodontid, the first reported from Europe. Comparisons with other theropods and phylogenetic analyses reveal an autapomorphic foramen that distinguishes it from all other troodontids, supporting its identification as a new genus and species, Tamarro insperatus. Bone histology shows that it was an actively growing subadult when it died but may have had a growth pattern in which it grew rapidly in early ontogeny and attained a subadult size quickly. We hypothesize that it could have migrated from Asia to reach the Ibero-Armorican island no later than Cenomanian or during the Maastrichtian dispersal events.

Geographic and geological location of Sant Romà d’Abella site. Geographic location of the Tremp Syncline in the southern Pyrenean region (A). Geological map of the eastern part of the Tremp Syncline with the location of the Sant Romà d’Abella site (B). Landscape view of the fossil site (C) with a detailed picture showing the close spatial relationship between the troodontid metatarsal (MCD-7073) and axial elements of Pararhabdodon isonensis (D). Stratigraphic section at Sant Romà d’Abella area with the precise position of the fossiliferous Sant Romà d’Abella site (E).

The European troodontid Tamarro insperatus (MCD-7073) gen. et sp. nov. from the uppermost Maastrichtian of the Tremp Group, Tremp Basin, Southern Pyrenees. Photographic and interpretative ilustrations of the right second metatarsal in anterior (A-A’), lateral (B-B’), posterior (C–C’), medial (D-D’), and distal (F-F’) views. Scheme illustrating the gradual transition of the articular surface–grey arrow–for the accommodation of the metatarsal III (E). Detail of the distal end of the metatarsal II in anterior view showing the anterior contact surface of the metatarsal III (G). Detail of the articular surface of the metatarsal IV in the posterodistal part of the second metatarsal (H) ant anterior; f foramen; lat lateral; lc medial condyle; mc, lateral condyle; II–III contact between the second and third metatarsals; II–IV contact between the second and fourth metatarsals; plr plantar ridge; s sulcus.


Systematic palaeontology
Dinosauria Owen, 1842
Theropoda Marsh, 1881
Coelurosauria Huene, 1914
Maniraptora Gauthier, 1986
Troodontidae Gilmore 1924
? Jinfengopteryginae Turner, Makovicky, and Norell, 2012

Tamarro insperatus gen. et sp. nov.

Etymology: Tamarro, Catalan word referring to a small and elusive fantastic creature from the regional (Pallars county) folklore. Insperatus, from the Latin word for “unexpected”, in reference to the unexpected discovery of the specimen.

Holotype: MCD-7073, a partial right metatarsal II.

Diagnosis: Tamarro insperatus is a mid-sized basal troodontid distinguished by the following unique combination of characters (* marks potential autapomorphies): metatarsal II with marked plantar ridge; small foramen on the lateral surface of the plantar ridge of the metatarsal II*; sub-arctometatarsalian condition with the metatarsal III restricted to the plantar margin on its proximal part.

Type locality and age: Sant Romà d’Abella locality (Pallars Jussà, Catalonia); upper part of the Talarn Formation of the Tremp Group. The site is correlated with the C29r magnetochron14, latest Maastrichtian age.

The discovery of the new basal troodontid Tamarro insperatus at the very end of the Cretaceous in Europe represents the first evidence of bones of Troodontidae in Europe. Both phylogenetic and osteohistological results support the basal nature of the new taxon, most likely as a representative of the Asian clade Jinfengopteryginae. The histological analysis also has provided evidence for a mature individual with a high growth rate, which allowed the animal to attain subadult body size in a very short time. This growth strategy is similar to other basal troodontids, such as the Asian Mei long.

The fragmentary, yet controversial, record of troodontid-like teeth from Europe together with the likely Asian affinity of Tamarro, confirm that troodontids could have reached the ancient European Archipelago during the Late Cretaceous. Furthermore, the relatively large size of Tamarro insperatus compared with its closest phylogenetic relatives suggests it would be the largest among jinfengopterygines.

Albert G. Sellés, Bernat Vila, Stephen L. Brusatte, Philip J. Currie and Àngel Galobart. 2021. A Fast-growing Basal Troodontid (Dinosauria: Theropoda) from the latest Cretaceous of Europe. Scientific Reports. 11: 4855. DOI: 10.1038/s41598-021-83745-5