Soroavisaurus

Soroavisaurus Temporal range: Maastrichtian ~70–66 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Illustration of the tarsometatarsus
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Clade:	Avialae
Clade:	†Enantiornithes
Family:	†Avisauridae
Genus:	†Soroavisaurus Chiappe 1993
Species:	†S. australis
Binomial name
†Soroavisaurus australis Chiappe 1993

Soroavisaurus is an extinct genus of enantiornithean birds related to Avisaurus. It lived during the Late Cretaceous of Argentina. The only known species, S. australis, is known from fossils collected from the Lecho Formation (Maastrichtian age) of Estancia El Brete, in the southern tip of the province of Salta, Argentina. A binominal name of this animal means "Southern sister Avisaur".^[1]

Discovery and naming

In 1975, fossil-bearing deposits of the Lecho Formation were discovered at the locality of El Brete, Salta Province, Argentina by a team from the Fundación Miguel Lillo.^[2] In the years after this, Argentine paleontologist José Bonaparte carried out field surveys at this site. Among the discoveries made were about 60 fossilised bird bones, which were added to the collection of the National University of Tucumán. In 1981, British paleontologist Cyril Walker published a study in which he illustrated some of these bones and determined that while they represent multiple species, all of them would have been members of a group which he named the Enantiornithes. In particular, he points out that three types of tarsometatarsi (one of the bones in a bird leg) are present in the collection.^[3]

Twelve years after Walker's study was published, Argentine paleontologist Luis M. Chiappe studied the bird tarsometatarsi from El Brete and named each of the three types as new genera and species, one of which was given the name Soroavisaurus australis. The generic name combines the Latin word soror (meaning "sister") with Avisaurus (another fossil bird), as Chiappe determined these two types of birds to be close relatives, while the specific name means "southern". He designated a left tarsometatarsus with the specimen number PVL-4690 as the holotype of this species, in addition to referring another specimen (PVL-4048) to it.^[4] Both of these specimens were formerly thought to be remains of a species of Avisaurus.^[5][6]

More of the enantiornithine specimens from El Brete have been referred to Soroavisaurus after the genus was erected, though some of these referrals have been questioned. In 2002, Walker and Chiappe worked together on a study which considered the specimens PVL-4030 and PVL-4033 to be fossilised tibiotarsi (another bone in a bird leg) of Soroavisaurus.^[7] Walker had previously considered both of these specimens to be comparable to Martinavis, referring to both as cf. Martinavis in an unpublished manuscript. In 2009, he authored another study with Irish paleontologist Gareth J. Dyke which once again finds PVL-4030 to be a Martinavis specimen, assigning it to an unnamed species of the genus. However, this study still considers PVL-4033 to be a Soroavisaurus tibiotarsus.^[8]

Description

All known specimens of Soroavisaurus preserve only bones from the hind limbs. The holotype, PVL-4690, is a 46.9 mm (1.85 in)-long left tarsometatarsus. A larger specimen, PVL-4048, includes another left tarsometatarsus, 51.5 mm (2.03 in)-long and associated with the whole hallux (digit I) and four intermediate phalanges.^[4]

The tarsometatarsus is slender in shape and formed from the fusion of the tarsal and metatarsal bones, though the metatarsals of Soroavisaurus are fused only in the proximal portion (the end closer to the torso). The metatarsals are shaped such that they form a depression in the hind part of the tarsometatarsus starting near the top and stretching down about two-thirds the length of the bone. When seen from above, the articular surface at the proximal end of the tarsometatarsus (where it would have joined to another bone) therefore resembles a kidney in shape, as the hind surface curves slightly inwards. Of the three metatarsals (II, III and IV) that form the tarsometatarsus, metatarsal III is the largest. It is located between the other two, and is spaced more widely apart from metatarsal II than from metatarsal IV. Metatarsal IV is weaker than the other two as it is particularly flattened, and is especially slender in its proximal half such that a gap is formed between it and metatarsal III in the upper part of the tarsometatarsus. This gap is a defining feature of Soroavisaurus and is not present in any other known enantiornithine bird.^[4]

Phylogeny

The cladogram below is from Wang et al., 2022:^[9]

Enantiornithes

Cruralispennia Protopteryx Elsornis Flexomornis Longipterygidae Iberomesornis bh bh Longirostravis Rapaxavis Euenantiornithes c e Sinornis e Enantiornis Halimornis Concornis Neuquenornis c b Eoalulavis Liaoningornis Gobipteryx Hebeiornis Eocathayornis Qiliania Avisauridae Intiornis Soroavisaurus Avisaurus Gettyia n Mirarce e Feitianius fn Longusunguis fn n Dunhuangia fn Musivavis Pengornithidae Pengornis m Yuanchuavis Eopengornis Parapengornis m Grabauornis Parvavis Cratoavis Gretcheniao Huoshanornis Shengjingornis Zhouornis Bohaiornithidae Bohaiornis ij Eoenantiornis Fortunguavis Mystiornis Shenqiornis Sulcavis

l

Key to letters:

b = Boluochia
c = Cathayornis
e = Enantiophoenix
f = Houornis
h = Longipteryx
i = Parabohaiornis
j = Pterygornis
l = Vorona
m = Yuanjiawaornis
n = Yungavolucris

Paleobiology

Due to their large size and strong talons, Soroavisaurus could occupy the same ecological niche as extanct birds of prey, noticing prey from afar on the plains or in water.^[1]

References

1. Matthew P. Martyniuk (2012). A Field Guide to Mesozoic Birds and Other Winged Dinosaurs. Pan Aves. p. 142. ISBN 9780988596504. Retrieved 29 August 2022.
2. Bonaparte, J. F.; Salfity, J. A.; Bossi, G.; Powell, J. E. (1977-12-07). "Hallazgo de dinosaurios y aves cretácicas en la Formación Lecho de El Brete (Salta), próximo al límite con Tucumán". Acta Geológica Lilloana (in Spanish): 5–17. ISSN 1852-6217.
3. Walker, Cyril A. (1981). "New subclass of birds from the Cretaceous of South America". Nature. 292 (5818): 51–53. Bibcode:1981Natur.292...51W. doi:10.1038/292051a0. ISSN 0028-0836.
4. Chiappe, Luis (1993). "Enantiornithine (Aves) tarsometatarsi from the Cretaceous Lecho Formation of northwestern Argentina". American Museum Novitates (3083): 1–27. S2CID 39613970.
5. Brett-Surman, M. K.; Paul, Gregory S. (1985-06-01). "A new family of bird-like dinosaurs linking Laurasia and Gondwanaland". Journal of Vertebrate Paleontology. 5 (2): 133–138. Bibcode:1985JVPal...5..133B. doi:10.1080/02724634.1985.10011851. ISSN 0272-4634.
6. Chiappe, Luis M. (1992-09-03). "Enantiornithine (Aves) tarsometatarsi and the avian affinities of the Late Cretaceous Avisauridae". Journal of Vertebrate Paleontology. 12 (3): 344–350. Bibcode:1992JVPal..12..344C. doi:10.1080/02724634.1992.10011464. ISSN 0272-4634.
7. Chiappe, Luis M.; Walker, Cyril A. (2002). "11. Skeletal Morphology and Systematics of the Cretaceous Euenantiornithes (Ornithothoraces: Enantiornithes)". In Chiappe, Luis M.; Witmer, Lawrence M. (eds.). Mesozoic Birds: Above the Heads of Dinosaurs. University of California Press. pp. 240–267. ISBN 978-0-520-20094-4.
8. Cyril A. Walker; Gareth J. Dyke (2009). "Euenantiornithine birds from the Late Cretaceous of El Brete (Argentina)" (PDF). Irish Journal of Earth Sciences. 27: 15–62. doi:10.3318/IJES.2010.27.15. S2CID 129573066. Archived from the original (PDF) on 2012-03-20.
9. Wang, Xuri; Cau, Andrea; Luo, Xiaoling; Kundrát, Martin; Wu, Wensheng; Ju, Shubin; Guo, Zhen; Liu, Yichuan; Ji, Qiang (2022-02-11). "A new bohaiornithid-like bird from the Lower Cretaceous of China fills a gap in enantiornithine disparity". Journal of Paleontology. 96 (4): 961–976. Bibcode:2022JPal...96..961W. doi:10.1017/jpa.2022.12. ISSN 0022-3360. S2CID 247432530.