Plesiocathartes insolitipes, sp. nov.; Waltonavis paraleptosomus, gen. et sp. nov.; W. danielsi, gen. et sp. nov; Lutavis platypelvis, gen. et sp. no. (1 Viewer)

[Fred Ruhe]

Gerald Mayr & Andrew C. Kitchener, 2022

New species from the early Eocene London Clay suggest an undetected early Eocene diversity of the Leptosomiformes, an avian clade that includes a living fossil from Madagascar

Palaeobiodiversity and Palaeoenvironments. doi:10.1007/s12549-022-00560-0

Abstract and free pdf: https://link.springer.com/content/pdf/10.1007/s12549-022-00560-0.pdf?pdf=button

We report the first records of the leptosomiform taxon Plesiocathartes from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK). Two species could be distinguished, one of which is described as Plesiocathartes insolitipes, sp. nov. (the other is classified as Plesiocathartes sp.). We furthermore describe two small leptosomiform-like birds from Walton-on-the-Naze, Waltonavis paraleptosomus, gen. et sp. nov. and W. danielsi, gen. et sp. nov., which are represented by partial skeletons of multiple individuals. In plesiomorphic characters of the coracoid and furcula, the new taxon Waltonavis is distinguished from Plesiocathartes and the extant Leptosomus; the skull of Waltonavis furthermore exhibits basipterygoid processes, which are absent in Leptosomus. Another new species, Lutavis platypelvis, gen. et sp. nov., is based on a partial skeleton from Walton-on-the-Naze and closely resembles Plesiocathartes in the overall shape of the tarsometatarsus. However, L. platypelvis is clearly distinguished from leptosomiform birds in the morphology of the hypotarsus, the more massive coracoid, and other skeletal features. Although we consider close affinities to Plesiocathartes and, hence, the Leptosomiformes to be possible, we could not identify derived characters that enable an unambiguous phylogenetic placement of L. platypelvis. In any case, the new species document an unexpected diversity of Plesiocathartes-like fossil in the London Clay and may open a new perspective for a reconstruction of the ancestral morphology of the avian clade Afroaves, which includes the Leptosomiformes and other arboreal landbirds.

Enjoy,

Fred

 

[Fred Ruhe]

Systematic palaeontology

Aves Linnaeus, 1758
Afroaves Ericson, 2012
Leptosomiformes Sharpe, 1891
Plesiocathartes Gaillard, 1908

Plesiocathartes sp.

Referred specimens: NMS.Z.2021.40.34 (Fig. 1a–f; both
tarsometatarsi and several pedal phalanges), collected in
1998 by Michael Daniels (original collector’s number WN
98977). NMS.Z.2021.40.35 (right tarsometatarsus), collected
in 1982 by M. Daniels (Fig. 1g–j; original collector’s number
WN 82409).

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).

Measurements (maximum length, in mm):
NMS.Z.2021.40.34: Tarsometatarsus, 25.6 (left), 26.8 (right).
NMS.Z.2021.40.35: Tarsometatarsus, 27.0 (right).

Comments: In size and morphology, the above tarsometatarsi from Walton-on-the-Naze correspond well to Plesiocathartes wyomingensis from the early Eocene North American Green River Formation (Weidig 2006). The exact stratigraphic age of the P. wyomingensis holotype is uncertain, but the fossil is likely to be a few million years younger than the specimens from Walton-on-the-Naze. The fossils furthermore are from different continents, so that it is unlikely that they belong to the same species.
NMS.Z.2021.40.34 is associated with a very poorly preserved
partial skeleton that may belong to the same species (in
this fossil, another left tarsometatarsus is preserved, which is
broken into two pieces). As in P. wyomingensis the
carpometacarpus of this latter fossil is longer than the tarsometatarsus, whereas both bones are of subequal length in P. kelleri (the carpometacarpus of other Plesiocathartes species is unknown).
As detailed in the descriptions below, the Plesiocathartes
fossils from Walton-on-the-Naze are distinguished in their
tarsometatarsus morphology from a specimen of an unnamed
Plesiocathartes species from the London Clay of the Isle of
Sheppey, which was described by Mayr (2002b). With the
strata exposed at Walton-on-the Naze being from division
A2 of the London Clay and the Plesiocathartes specimen
from the Isle of Sheppey stemming from the stratigraphically
younger divisions D–E (Rayner et al. 2009; Collinson et al.
2016), these differences possibly indicate temporal changes in
the composition of the London Clay faunas.

Plesiocathartes insolitipes, sp. nov.

Holotype: NMS.Z.2021.40.36 (Fig. 3a; partial skeleton, including right quadrate, several vertebrae, incomplete right coracoid, cranial extremity of left scapula, partial furcula, cranial portion of sternum, proximal end of right carpometacarpus, distal right tarsometatarsus and pedal phalanges of right foot), collected in 1993 by Michael Daniels (original collector’s number WN 93783).

Differential diagnosis: Distinguished from:
Plesiocathartes kelleri, P. major, and P. wyomingensis in the markedly widened distal ends of the phalanges third and fourth digits. Differs from Plesiocathartes sp. from Walton-on-the-Naze in its larger size (distal width of tarsometatarsus 6.2 mm in NMS.Z.2021.40.34 versus 7.2 mm in NMS.Z.2021.40.36) and in that the trochlea metatarsi IV has a better developed and medially inclined plantar flange; trochlea metatarsi II not forming a plantar projection. Also differs from P. kelleri in its larger size (Tab. 1) and from P. major in that the coracoid has the sternal margin oriented more obliquely to the longitudinal axis of the bone and the first phalanx of the hallux is proportionally longer (slightly longer than the first phalanx of the third toe, whereas it is shorter in P. major).
– P. geiselensis in its smaller size (mediolateral width of
trochlea metatarsi III 2.5 mm vs. 3.1 mm) and in that
the tarsometatarsus shaft is proportionally wider and the
dorsal articular surface of the trochlea metatarsi III is proportionally longer (reaching proximally to the level of the
distal rim of the foramen vasculare distale).
– P. europaeus in that the plantar articular surface of the trochlea metatarsi III is proportionally shorter and wider, with a straight proximal margin and a shallower trochlear furrow.

Etymology: From insolitus (Lat.): unusual and pes (Lat.): foot, in reference to the widened distal ends of some of the pedal phalanges.

Type locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation; early Eocene (early Ypresian).

Referred specimens: SMF Av 656 (Fig. 3b; partial skeleton, including several vertebrae, right coracoid, partial right scapula, partial furcula, distal portion of right ulna, proximal end of right carpometacarpus, right os carpi radiale, partial phalanx digiti majoris), collected in 1987 by Paul Bergdahl (original collector’s number BC 8712).

Measurements (maximum length , in mm):
NMS.Z.2021.40.36: Coracoid, maximum length as preserved,
30.7. SMF Av 656: Coracoid, maximum length as preserved,
30.3.

Fred

 

[Fred Ruhe]

?Leptosomiformes Sharpe, 1891
Waltonavis, gen. nov.

Type species: Waltonavis paraleptosomus, sp. nov.

Diagnosis: Characterised by (1) narrow and pointed upper beak; (2) pterygoid with prominent facies articularis basipterygoidea; (3) humerus with sharply delimited fossa musculi brachialis; (4) tarsometatarsus with very large medial
foramen vasculare proximale (Fig. 5c); (5) hypotarsus enclosing two canals; (6) trochlea metatarsi II reaching much farther distally than trochlea metatarsi IV; (7) trochlea metatarsi IV mediolaterally narrow and forming a plantar rim; (8) distal ends of first and second phalanges of third toe bearing distinct lateral bulges. Characters (3), (4), and (8) are here considered to be autapomorphies of the new taxon.

Differential diagnosis: Distinguished from the late early/
early middle Eocene Lapillavis from the Messel fossil site (Mayr 2016) in that the furcula has much narrower shafts (scapi clavicularum) and the foramen nervi supracoracoidei is situated closer to omal extremity of coracoid (Fig. 6). Apart from its much smaller size (Tab. 1), the new taxon differs from Plesiocathartes in that the coracoid has a narrower shaft (Fig. 2c, e), the medial foramen vasculare proximale of the tarsometatarsus is larger, the trochlea metatarsi II reaches much farther distally than the trochlea metatarsi IV and forms a more medially directed and much bulkier plantar projection, the trochlea metatarsi IV is proportionally narrower, and the hallux is proportionally longer (distinctly exceeding the other phalanges in length and measuring almost half the length of tarsometatarsus).

Etymology: The name is derived from the type locality of the
new taxon and avis (Lat.): bird.

Waltonavis paraleptosomus, sp. nov.

Holotype: NMS.Z.2021.40.16 (Fig. 5a; partial skeleton, including left quadrate and pterygoid, partial left scapula, left coracoid, cranial portion of sternum, left and proximal right tarsometatarsus), collected in 1988 by M. Daniels (original collector’s number WN 88599).

Diagnosis: As for genus; the coracoid is about as long as the tarsometatarsus (Fig. 12).

Etymology: The species epithet refers to the osteological similarity of the new species and the extant taxon Leptosomus.

Type locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation; early Eocene (early Ypresian).

Referred specimens: NMS.Z.2021.40.17 (Fig. 5c; partial skeleton, including left coracoid, proximal end of left carpometacarpus, left tarsometatarsus, and several pedal phalanges), collected in 1996 by M. Daniels (original collector’s number WN 96909). NMS.Z.2021.40.18 (Fig. 5d; right tarsometatarsus of a juvenile individual), collected in 1983 by M. Daniels (original collector’s number WN 83432). NMS.Z.2021.40.19 (Fig. 5b; partial skeleton, including right quadrate, partial mandible in a block of matrix, pygostyle, both coracoids, partial right scapula, partial left humerus, fragments of a radius, right carpometacarpus, partial right femur in a block of matrix, right tarsometatarsus, and several pedal phalanges), collected in 1988 by M. Daniels (original collector’s number WN 88585).

Waltonavis danielsi, sp. nov.

Holotype: NMS.Z.2021.40.20 (Fig. 5e; partial skeleton, including several vertebrae, right coracoid, partial right scapula, right humerus, proximal left humerus, right ulna, proximal and distal ends of left ulna, left carpometacarpus, both phalanges proximales digitorum majores, distal end of right femur, proximal end and fragmentary distal end of right tibiotarsus, partial left tarsometatarsus), collected in 1985 by M. Daniels (original collector’s number WN 85511).

Diagnosis: As for genus, the coracoid is distinctly shorter than the tarsometatarsus, which is more elongate and with a proportionally narrower proximal end than the tarsometatarsus of W. paraleptosomus (Fig. 12).

Etymology: The species is named in honour of Michael Daniels, who collected the fossils described in the present study.

Type locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation; early Eocene (early Ypresian).

Waltonavis sp.

Referred specimens: NMS.Z.2021.40.21 (Fig. 5f; partial
skeleton, including partial upper beak, right quadrate, several vertebrae, left coracoid, furcula, cranial extremities of both scapulae, right humerus, partial left humerus, right ulna lacking proximal end, fragmentary distal portions of both carpometacarpi, fragmentary right femur, distal end of left tarsometatarsus, and several pedal phalanges), collected in 1981 by M. Daniels (original collector’s number WN 81372). NMS.Z.2021.40.22 (Fig. 5g; both coracoids, cranial end of right scapula, proximal end of left humerus), collected in 1983 by M. Daniels (original collector’s number WN 83434). NMS.Z.2021.40.23 (Fig. 5h; proximal and distal ends of right tarsometatarsus, atlas, basiurohyal, and some pedal phalanges), collected in 1980 by M. Daniels (original collector’s number WN 80364).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation; early Eocene (early Ypresian).

Remarks: Because either the coracoid or the tarsometatarsus
are missing or incomplete, these specimens cannot be assigned to one of the two species above.

Fred


Fig. 1 Overview of selected skeletal elements of Waltonavis specimens from the early Eocene London Clay of Walton-on-the-Naze, Essex, UK; all fossils shown to scale. a Holotype of Waltonavis paraleptosomus, gen. et sp. nov. MS.Z.2021.40.16; pterygoid, quadrate, coracoid, scapula, sternum, and tarsometatarsi). b Specimen tentatively referred to W. paraleptosomus (NMS.Z.2021.40.19; quadrate, pygostyle, scapula, coracoids, humerus, carpometacarpus, tarsometatarsus, and pedal phalanges). c W. paraleptosomus (NMS.Z.2021.40.17; coracoid, proximal carpometacarpus, tarsometatarsus, and pedal phalanges). d W. paraleptosomus (NMS.Z.2021.40.18; right tarsometatarsus of a juvenile individual). e Holotype of Waltonavis danielsi, gen. et sp. nov. (NMS.Z.2021.40.20; atlas, pygostyle, coracoid, scapula, humeri, radius, right ulna, proximal and distal left ulna, carpometacarpus, os carpi ulnare proximal tibiotarsus, and tarsometatarsus). f–h Specimens identified as Waltonavis sp. (f NMS.Z.2021.40.21; partial, beak, quadrate, both humeri, coracoid, furcula, os carpi ulnare, partial ulna, tibiotarsus, and tarsometatarsus; g NMS.Z.2021.40.22; both coracoids, cranial end of right scapula, proximal end of left humerus; h NMS.Z.2021.40.23; proximal and distal ends of right tarsometatarsus, atlas, basiurohyal, and some pedal phalanges). mfv foramen vasculare proximale.
Scale bars: 10 mm

 

[Fred Ruhe]

?Afroaves Ericson, 2012
Lutavis, gen. nov.

Type species: Lutavis platypelvis, sp. nov.

Diagnosis: Characterised by a coracoid with a large extremitas omalis (measuring about one third of the entire length of the bone); pelvis wide and dorsoventrally shallow, with small tubercula praeacetabularia; tarsometatarsus with very large medial foramen vasculare proximale and hypotarsus forming two crests that delimit a distinct sulcus.

Differential diagnosis: Differs from Plesiocathartes europaeus, the type species of the taxon Plesiocathartes, in that the trochlea metatarsi IV forms a more pronounced plantar flange. Furthermore differs from Plesiocathartes spp. in the large medial foramen vasculare proximale (tarsometatarsus) and the morphology of the hypotarsus, which exhibits a distinct sulcus. Distinguished from Waltonavis, gen. nov. in that the coracoid is much more massive, the humerus has a concve proximodorsal margin and a dorsally prominent crista deltopectoralis, and the hypotarsus lacks tendinal canals (Fig. 2).

Etymology: The name is derived from lutum (Lat.): mud or clay and avis (Lat.): bird, in reference to the fact that the fossils were found in the London Clay

Lutavis platypelvis, gen. et sp. nov.

Holotype: NMS.Z.2021.40.37 (partial skeleton, including a few thoracic vertebrae and the pygostyle, substantial portions of both scapulae and coracoids, proximal and fragmentary distal end of the right humerus, proximal end of the left ulna, distal ends of both ulnae, partial carpometacarpi, carpal bones and wing phalanges, partial pelvis, right tarsometatarsus, and several pedal phalanges), collected in 1995 by Michael Daniels (original collector’s number WN 95848A).

Etymology: From πλᾰτῠ́ς (platús; Gr.): wide, in reference to the wide and shallow pelvis of the new species.

Type locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Formation; early Eocene (early Ypresian).

Diagnosis: As for genus.

Fred


Fig.1. d Lutavis platypelvis, gen. et sp. nov. (holotype,
NMS.Z.2021.40.37) reconstruction of coracoid.