pectoral girdle.[1] In 1996, while describing Copepteryx, Olson and Hasegawa tentatively assigned a 16.5 cm coracoid from the late Oligocene of Hikoshima, Japan to the genus, much larger than the coracoid of T. hildegardae and almost as long as that of C. hexeris. It was then the first appearance of the genus Tonsala in Japan.[3] In 2000, James L. Goedert and John Cornish referred to the type species two newly discovered specimens collected respectively in 1984 and 1986 in the lower part of the Pysht Formation, near the type locality of the holotype, and including two pelvis, fragmentary vertebral, forelimb and hindlimb remains, and portions of the dentary.[4] In 2011, Gareth J. Dyke, Xia Wang and Michael B. Habib proposed a new species based on Oligocene fossils found in the Pysht Formation and the underlying Makah Formation, Tonsala buchanani.[5]
In 2015, three new specimens, including two additional cranial remains, of Tonsala hildegardae, collected respectively in 1985, 1989 and 2008 by James L. Goedert in Oligocene sediments from the Pysht Formation, were described by Gerald Mayr, Goedert and Olaf Vogel.[6]
In 2016, Mayr and Goedert referred two additional specimens from the Late Eocene to Early Oligocene of the Jansen Creek Member of the Makah Formation to the genus Tonsala, including one partial pelvis collected in 2004 by Goedert, tentatively assigned to T. hildegardae, and a right coracoid collected in 2009 by D. W. Starr tentatively referred to the genus as ?Tonsala sp., and suggested that T. buchanani represented more than one species and had been incorrectly attributed to the genus Tonsala.[7]
In 2017, Mayr and Goedert used Tonsala as the type genus of the new clade
Tonsalinae, comprising all derived Plotopterids, aside from the primitive and fragmentary Phocavis and the small-sized Stemec and Plotopterum ; they additionally referred to the genus, as a new specimen of T. hildegardae, a partial skeleton including most notably the first known tarsometatarsus assigned to the genus.[8]
In 2019, the Japanese remains assigned tentatively to a new species of Tonsala by Olson and Hasegawa in 1996 were redescribed and assigned to a new genus and species, Stenornis kanmonensis[9]
In 2021, T. buchanani was moved to the genus Klallamornis by Mayr and Goedert, as K. buchanani, becoming the new type species of the genus. Additionally, criticism were drown towards Dyke et al, 2016, leading to a reevaluation of the specimens assigned to Tonsala ; one of the partial skeletons referred to the type species by Dyke et al. was only tentatively referred by Mayr and Goedert, as cf. Tonsala hildegardae.[10]
Etymology
The genus name, Tonsala, is constructed with the Latin prefix "Tonsa-", meaning "oar", and the suffix "-ala", meaning wing, referencing the adaptation of its forelimbs as a swimming apparatus ; the species name, hildegardae, was given to honour Hildegarde Howard, the American paleontologist who described Plotopterum.[1]
Description
Holotype of Tonsala hildegardae.
Tonsala hildegardae was a large and flightless seabird, comparable in size with a great penguin, and larger than its later relative Plotopterum. Its relatively slender femur tends to indicate that it was the sister taxa of a group including all plotopterids, aside from Phocavis and Plotopterum ; it has been suggested that Phocavis maritimus may be synonymous with Tonsala hildegardae.[7]
The genus was heavily adapted towards swimming and diving ; the wings were paddle-shaped, and the
boobies and gannets.[6] The caudal vertebrae were large, twice the size of those of the modern great cormorant. The tarsometatarsus was stouter than that of Phocavis and the hypotarsus had only two developed crests, like in other tonsaline plotopterids. The slanting of the distal articular surfaces of the second and fourth trochleae metatarsorum may be an indication that Tonsala had splayed toes, and possibly webbed feet.[8]
Fossilized skull bones of plotopterids are rare, but the cranium of Tonsala is known from three specimens, including a fragment of the beak associated with postcranial remains
suliform. The complete beak was presumably proportionally longer than those of its modern relatives, and was opened by elongated and very narrow nostrils, unlike modern Suloids for which the nostrils are greatly reduced or completely absents. The skull was devoid of vomer. The braincase, although badly preserved, was more reminiscent in proportion of those of sulids.[6] The lower mandible was deeply concave[4] and sharing its broadness with modern penguins.[5]
Another undescribed species referred by Mayr and Goedert to the genus as ?Tonsala sp., known from a single right coracoid from the Makah Formation, differs from T. hildegardae mostly by its lesser size.[7]
Classification
Plotopterids as a group were always, since the discovery of the holotype specimen of
Fregatidae, including the modern frigatebirds.[6][7]
In 2017, Mayr and Goedert proposed a new clade,
Tonsalinae, including Tonsala and all larger plotopterids known at the time from the Pacific Northwest and Japan, aside from the incomplete and primitive Phocavis and the smaller Plotopterum and Stemec, the latter of which presumably clading together. Due to the paucity of well-preserved remains, phylogenetic relationships was inferred based on the absence of a foramen vasculare distale on the tarsometatarsus of Tonsala, making it more similar by to those of larger forms like Copepteryx, Hokkaidornis, Olympidytes and Klallamornis than to those of the smaller forms Stemec and Plotopterum. The tarsometatarsus of Phocavis is not known, and relationships with other plotopterids can hardly been inferred.[8]
Within Tonsalinae, Tonsala was perceived as the most basal member,[8] with, according to Mayr in his 2016 article, two potential phylogenies ; the first of which, considering the eventuality of a single origin in the gigantism of Japanese and American plotopterids, would clade Olympidytes as the sister genus of a clade including the larger-sized Klallamornis of North America, from which the Japanese Copepteryx and Hokkaidornis would eventually descend. Another possibility envisioned by Mayr would be a distinction based on the presence or absence of a notch located on the dorsal surface of the tarsometatarsus by the disappearance of the foramen vasculare discale ; this distinction would leave two sister clades of derived tonsalines, one including the North American genera, for which the notch is still visible, and the other comprising the Japanese genera, in which the notch has completely disappeared.[7] Those considerations were corroborated in his 2017 article, for which he preferred the first hypothesis, based on the size of the tarsometatarsus ; the phylogenetic tree proposed in the article being as follows:[8]
Desmostylian Behemotops, and the plotopterids Klallamornis buchanani and K. abyssa.[4][7] In the Late Eocene to early Oligocene Makah Formation, at least two species of Tonsala coexisted with their relative Klallamornis abyssa, with another undescribed species of smaller plotopterid[7] and with the basal procellariiform Makahala mirae.[8]
The compressed wing elements and the extended scapular blade of Tonsala indicates that it was likely capable of producing wing upstroke movements to propulsate itself in water, with a force comparable to that of modern
pelagic foragers.[15] They may have nested in offshore volcanic islands, where they could raise their young sheltered from the diversifying mammalian predators.[10]
Based on two fragmentary skeletons of plotopterids, including one of Tonsala, it has been demonstrated that the bone-eating detritivorous worm
Cetacean corpses, used to have a more diverse diet and to also be able to feed on the remains of large marine birds.[16]Cow sharks teeth have also been recovered in association with Tonsala remains, suggesting they also fed on the carcasses of plotopterids.[6]
The extinction of Tonsala, alongside that of all of the plotopterids from the
Pacific marine environments of early pinnipeds like Enaliarctos, that may have competed with plotopterids for food, breeding sites, and preyed upon them.[4]
^ abcdefGoedert, J. L.; Cornish, J. (2000). "Preliminary Report on the Diversity and Stratigraphic Distribution of the Plotopteridae (Pelecaniformes) in Paleogene Rocks of Washington State, USA". In Zhou, Z.; Zhang, F. (eds.). Proceedings of the 5th symposium of the Society of Avian Paleontology and Evolution, Beijing, 1-4 June 2000. Beijing: Science Press. pp. 63–76.
^Ohashi, T. & Hasegawa Y. (2019). "New species of Plotopteridae (Aves) from the Oligocene Ashiya Group of northern Kyushu, Japan". Paleontological Research. 24 (4): 285–297.
