Sororavis solitarius gen. et sp. nov.; Morsoravidae fam. nov. (1 Viewer)

[Fred Ruhe]

Gerald Mayr & Andrew C. Kitchener, 2023

A new fossil from the London Clay documents the convergent origin of a “mousebird-like” tarsometatarsus in an early Eocene near-passerine bird

Acta Palaeontol. Pol. 68 (X): xxx–xxx, 202
Abstract and free pdf: https://www.app.pan.pl/archive/published/app68/app010492022.pdf

We describe a partial skeleton of a small bird from the lower Eocene London Clay of Walton-on-the-Naze (Essex, UK), which shows close affinities to two phylogenetically controversial early Paleogene taxa, Morsoravis sedilis (lower Eocene of Denmark) and Pumiliornis tessellatus (lower/middle Eocene of Germany). Our phylogenetic analysis supports a clade including M. sedilis, P. tessellatus, and the new species, Sororavis solitarius gen. et sp. nov., and this clade is introduced as Morsoravidae fam. nov. Even though morsoravids resulted within crown group Psittacopasseres in our analysis, as the sister taxon of the Zygodactylidae and Passeriformes, they exhibit a presumably plesiomorphic tarsometatarsus morphology unlike that of zygodactyl psittacopasserines. The elongate tarsometatarsus of S. solitarius shows a resemblance to that of extant mousebirds (Coliiformes) and has a large trochlea for the second toe, which bears an extensive trochlear furrow. This distinctive foot morphology is likely to represent an adaptation for a grasping foot and may indicate a locomotory behaviour similar to that of mousebirds, which are arboreal acrobats that forage in the foliage of trees and shrubs. Since the Psittacopasseres and the Coliiformes are widely separated in current phylogenies, the “mousebird-like” tarsometatarsus of the new species represents a notable instance of convergent evolution in small arboreal birds.

Enjoy,

Fred

 

[Fred Ruhe]

Systematic palaeontology

Aves Linnaeus, 1758
Telluraves Yuri, Kimball, Harshman, Bowie, Braun, Chojnowski, Han, Hackett, Huddleston, Moore, Reddy, Sheldon, Steadman, Witt, and Braun, 2013
Australaves Ericson, 2012
Family Morsoravidae Mayr et Kitchener, 2023
LSID Zoobank: http://zoobank.org/urn:lsid:zoobank.org:act:B81B0E09-9CC3-44E9-9FCF-BB6667A90FE0.

Type genus: Morsoravis Bertelli, Lindow, Dyke, and Chiappe, 2010.

Included genera: Morsoravis Bertelli, Lindow, Dyke, and Chiappe, 2010; Pumiliornis Mayr, 1999; Sororavis Mayr et Kitchener, 2023

Diagnosis.—The new taxon is characterised by (1) a high number of 21 praesacral vertebrae (not ascertainable for Sororavis Mayr et Kitchener, 2023); (2) a long and slender tibiotarsus; (3) a very large tuberositas musculi tibialis cranialis that is situated at the medial margin of the tarsometatarsus on the level of the foramina vascularia proximalia; (4) a trochlea metatarsi II of cylindrical shape that is plantarly deflected and has a marked and extensive trochlear furrow; (5) a fossa on the dorsal tarsometatarsus surface immediately proximal to the trochlea metatarsi III; (6) a trochlea metatarsi IV that exhibits a plantarly directed, wing-like flange and has a laterally slanted distal margin, which forms an angle of about 45° relative to the longitudinal axis of the tarsometatarsus (the presence of the flange is not ascertainable for Morsoravis); (7) a very wide first phalanx of the fourth toe (not ascertainable
for Sororavis Mayr et Kitchener, 2023). Characters (4) and (7) are here considered diagnostic apomorphies of the new taxon.

Genus Sororavis Mayr et Kitchener, 2023
LSID Zoobank: http://zoobank.org/urn:lsid:zoobank.org:act:E26943A7-0011-4266-A5C3-1F706C9EAD18.

Type species: Sororavis solitarius Mayr et Kitchener, 2023; see below.

Etymology: From Latin soror, sister and avis, bird; in reference to the phylogenetic closeness of the new taxon to Morsoravis, to which the new name is intended to be phonetically similar.

Diagnosis.—The new taxon differs from Morsoravis Bertelli, Lindow, Dyke, and Chiappe, 2010, in that the tarsometatarsus is more elongated (ratio length of bone to minimum width of shaft 12.9 vs. 8.9 in M. sedilis). It is distinguished from Pumiliornis Mayr, 1999 in that the tibiotarsus has a pons supratendineus and the tarsometatarsus is more elongated (ratio length of bone to minimum width of shaft 12.9 vs. 8.3 in P. tessellatus).

Sororavis solitarius Mayr et Kitchener, 2023
LSID Zoobank: http://zoobank.org/urn:lsid:zoobank.org:act:A979CC23-3409-4CDD-B28E-2FEA52691A86

Etymology: From Latin solitaries, single, solitary, in reference to the fact that only one specimen of this species has been identified among the numerous birds from Walton-on-the Naze in the Daniels collection.

Holotype: NMS.Z.2021.40.75 (Fig. 1; tip of upper beak; fragments of mandibular rami; both coracoids; partial furcula; partial sternum; right humerus lacking distal end; proximal and distal ends of left humerus; proximal end of right ulna; left tibiotarsus lacking proximal end; proximal end of left tarsometatarsus; right tarsometatarsus; four pedal phalanges), collected in 1986 by M. Daniels (original collector’s number WN 86530).

Type locality: Walton-on-the-Naze, Essex, UK.

Type horizon: Walton Member of the London Clay Formation (previously Division A2; Rayner et al. 2009; Aldiss 2012); lower Eocene (lower Ypresian, 54.6‒55 million years old; Collinson et al. 2016).

Diagnosis.—As for genus. The new species is of similar size to Morsoravis sedilis (tarsometatarsus length 14.3 mm vs. ~13.0 mm in M. sedilis; Bertelli et al. 2010) and distinctly larger than Pumiliornis tessellatus (tarsometatarsus length 8.9‒9.9 mm; Mayr 1999).

Measurements (maximum length, in mm).—Right coracoid, 11.0; right humerus, length as preserved, 14.0, estimated total length, ~15‒16; left tibiotarsus, length as preserved, 16.5, estimated total length, ~23‒25; right tarsometatarsus, 14.3.

Fred


Fig. 1. The bones preserved in the holotype of the morsoravid bird Sororavis solitarius gen. et sp. nov. (NMS.Z.2021.40.75), from the lower Eocene London Clay of Walton-on-the-Naze, UK. A1, tip of upper beak in dorsal view; A2, fragments of mandible; A3, A4, left coracoid in dorsal (A3) and ventral (A4) views; A5, A6, right coracoid in dorsal (A5) and ventral (A6) views; A7, partial furcula; A8, A9, cranial portion of sternum in ventral (A8) and lateral (A9) views; A10, A11, partial right humerus in cranial (A10) and caudal (A11) views; A12‒A15, proximal (A12, A13) and distal (A14, A15) portions of left humerus in caudal (A12, A14) and cranial (A13, A15) views; A16, proximal end of right ulna in cranioventral view; A17, A18, partial left tibiotarsus in caudal (A17) and cranial (A18) views; A19‒A24, right tarsometatarsus in dorsal (A19), medial (A20), plantar (A21), lateral (A22), proximal (A23), and distal (A24) views; A25, A26, proximal end of left tarsometatarsus in plantar (A25) and dorsolateral (A26) views; A27, first phalanx of third toe in dorsal and plantar view; A28, second to fourth phalanges of fourth toe in different views (plantar, dorsal, and lateral, respectively).