Rhynchaeites litoralis, sp. nov. (1 Viewer)

[Fred Ruhe]

Gerald Mayr & Andrew C. Kitchener, 2023

Multiple skeletons of Rhynchaeites from the London Clay reveal
the osteology of early Eocene ibises (Aves, Threskiornithidae)

Vol.0123456789)1 3PalZ

https://doi.org/10.1007/s12542-022-00647-1

Abstract and free pdf: Multiple skeletons of Rhynchaeites from the London Clay reveal the osteology of early Eocene ibises (Aves, Threskiornithidae) - PalZ

We describe a new species of Rhynchaeites from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK), which is represented by a partial skeleton comprising a skull and most major postcranial bones. Multiple further partial skeletons are assigned to Rhynchaeites sp. and include skeletal elements that are rarely preserved in Paleogene birds. Rhynchaeites is for the first time included in a phylogenetic analysis, which did not unambiguously resolve its affinities, but provided weak support for a sister group relationship to the Threskiornithidae. If Rhynchaeites is a stem group representative of ibises, its skeletal morphology indicates significant homoplasy in the evolution of Aequornithes, the higher level clade including the Threskiornithidae and most other aquatic or semi-aquatic birds. In particular, Rhynchaeites has much shorter legs than extant ibises and the new fossils show that its palate was schizognathous. Current phylogenies suggest that a desmognathous palate as well as long legs evolved multiple times independently within Aequornithes. Unlike in extant ibises, the tip of the beak of Rhynchaeites lacks a densely pitted surface. We hypothesize that vision played a greater role in the foraging strategy of Rhynchaeites, whereas extant ibises are tactile probers and use their beaks for “remote sensing”

The paper describes Rhynchaeites litoralis, sp. nov.

Another new species from the Daniels collection.

Enjoy,

Fred

 

[Fred Ruhe]

Systematic paleontology

Class Aves Linnaeus, 1758
Order Pelecaniformes Sharpe, 1891 (sensu Sangster et al. 2022)
Family Threskiornithidae Richmond, 1917
Subfamily Rhynchaeitinae Mayr, 2002
Genus Rhynchaeites Wittich, 1898

Emended diagnosis. (1) Beak long, slender, and schizorhi-
nal, tip slightly decurved, and proximal portion of maxilla
dorsoventrally deep; (2) palate schizognathous; (3) at least
three thoracic vertebrae co-ossified to form a notarium; (4)
unfused thoracic vertebra with large pneumatic openings on
the lateral surfaces of its corpus; (5) coracoid with deeply
excavated, cup-like cotyla scapularis and short processus
acrocoracoideus; (6) sternum with deep incisura medialis
and wide trabecula mediana; (7) carpometacarpus with long
symphysis metacarpalis distalis; (8) ilium not co-ossified
with crista spinosa synsacri; (9) tarsometatarsus shorter than
femur.

Rhynchaeites litoralis, sp. nov.

Holotype. NMS.Z.2021.40.28 (Fig. 2; partial skeleton including partial skull with mandible, os basihyale, both quadrates several vertebrae, wing phalanges and carpal bones, furcula,
partial left scapula, both coracoids, sternum, proximal end of
right humerus, distal end of left humerus, partial left ulna, left femur, distal end of right tibiotarsus, proximal end of left tibiotarsus, proximal and distal ends of both tarsometatarsi,
and some pedal phalanges); collected in 1991 by Michael Daniels (original collector’s number WN 91677).

Differential diagnosis. Somewhat larger han Rhynchaeites messelensis (Tab. 1); os carpi radiale proximodistally narrower and with better defined notch for tendon of musculus ulnometacarpalis ventralis; tarsometatarsus with proportionally wider distal end, less tapering plantar articular surface of trochlea metatarsi III, and trochlea metatarsi IV with more laterally slanting distal margin. Differs from Mopsitta tanta in that crista bicipitalis of humerus does not form a ventral projection.

Etymology. The species epithet is derived from litoralis (Lat.), littoral, in reference to the coastal location of the type locality.

Type locality and horizon. Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation (previously Division A2; Rayner et al. 2009; Aldiss 2012); early Eocene (early Ypresian, 54.6‒55 million years ago; Collinson et al. 2016).

Taxonomic remarks. The fossil material from Walton-on-the-Naze in the Daniels collection includes various other specimens that belong to Rhynchaeites (Fig. 3), and some of these are distinctly smaller than the holotype of Rhynchaeites litoralis. In extant Threskiornithidae, female individuals can be considerably smaller than male ones and R. litoralis may also have been sexually dimorphic in size, with the smaller specimens representing females of the species. However, the holotype of R. litoralis also differs from other Rhynchaeites specimens from Walton-on-the-Naze, in which a humerus is preserved, in that the crista bicipitalis does not form a ventral projection. Some differences in the shape of the coracoid (see below) may also indicate the presence of more than one species of Rhynchaeites in the material from Walton-on-the-Naze. Therefore, w prefer to only refer the holotype to the new species and classify all other specimens as Rhynchaeites sp.
Superficially, Rhynchaeites litralis resembles Paraortygoides
radagasti from the London Clay of Walton-on-the-Naze, which was described as a stem group galliform by Dyke and Gulas (2002). P. radagasti is only known from the fragmentary bones preserved in the holotype, and these differ from Rhynchaeites in, e.g., the presence of large pleurocoels on the corpus of the thoracic vertebrae (in Rhynchaeites there are large pneumatic openings; see Mayr 2021a for the difference between these structures), the less well-delimited tuberculum coracoideum of the scapula, the narrower processus extensorius of the carpometacarpus, the narrower crus longum of the os carpi ulnare, and the further distally reaching trochlea metatarsi II.

Fred


Fig. 2 Holotype of Rhynchaeites litoralis, sp. nov. from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK (NMS.Z.2021.40.28). a Skull; b caudal end of mandible; c os basihyale; d quadrates; e vertebrae; f wing phalanges and carpal bones; g furcula; h partial left scapula; i right coracoid; j left coracoid; k sternum; l proximal end of right humerus; m distal end of left humerus; n distal end of left ulna; o proximal end of left ulna; p left femur; q distal end of right tibiotarsus; r proximal end of left tibiotarsus; s proximal end of left tarsometatarsus; t distal end of right tarsometatarsus; u distal end of right tarsometatarsus; v pedal phalanges.
Scale bar equals 10 mm.