The Halcyornithidae from the early Eocene London Clay of Walton-on-the-Naze (1 Viewer)

[Fred Ruhe]

Gerald Mayr & Andrew C. Kitchener, 2024

The Halcyornithidae from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK): A species complex of Paleogene arboreal birds

Geobios, Volume 83, April 2024, Pages 45-60

Abstract: https://www.sciencedirect.com/science/article/abs/pii/S0016699523000633

We report fossils of the Halcyornithidae from the early Eocene London Clay of Walton-on-the-Naze (Essex, UK). The material includes remains of nine individuals of these small zygodactyl birds, which belong to at least seven different species, two of which are newly described. This documents a high species richness of halcyornithids in the London Clay, and these birds apparently underwent a significant radiation in the early Paleogene. One of the specimens includes a skull with a proportionally much smaller neurocranium than in Pseudasturides macrocephalus from Messel, suggesting an increased encephalization of halcyornithids towards the middle Eocene. A previously undescribed variation in sternum morphology of halcyornithids distinguishes the fossils we assign to the taxa Pulchrapollia and Cyrilavis and indicates that halcyornithids occupied disparate ecological niches. Furthermore, sternum morphology supports a sister group relationship between the halcyornithid taxa Pulchrapollia and Pseudasturides. The closest extant relatives of the Halcyornithidae remain elusive, but our new osteological data substantiate the hypothesis of a sister group relationship to the Messelasturidae, another group of Eocene birds with zygodactyl feet.


Enjoy,

Fred

 

[Fred Ruhe]

Systematic palaeontology

Aves Linnaeus, 1758
Halcyornithidae Harrison and Walker, 1972
Genus Pulchrapollia Dyke and Cooper, 2000

Remarks: The holotype of Pulchrapollia gracilis is a partial skeleton
from Walton-on-the-Naze, which was described by Dyke and
Cooper (2000). The specimen was revisited by Mayr (2002a),
who noted that the coracoid associated with the type material of
P. gracilis (Fig. 3(A)), which lacks a foramen nervi supracoracoidei
and has a hook-shaped extremitas omalis, does not belong to a species of the Halcyornithidae (contra Dyke and Cooper, 2000). The
true affinities of this bone still have to be established, but overall
it resembles the coracoid of some charadriiform birds.
The fossils that we tentatively refer to Pulchrapollia exhibit a
similar tarsometatarsus morphology to the holotype of P. gracilis,
with a large medial foramen vasculare proximale and a small lateral
one. Unlike in Cyrilavis olsoni, the sternum is craniocaudally
short, with a deep carina sterni, the apex carinae of which does
not strongly project cranially. The femur is more elongate than that
of Cyrilavis. The tarsometatarsus is slenderer than in Pseudasturides
and the tibiotarsus is proportionally longer than in the latter taxon.

cf. Pulchrapollia gracilis Dyke and Cooper, 2000

Referred specimen: NMS.Z.2021.40.63 (Fig. 2(A)), a partial
skeleton including left coracoid, omal portion of right coracoid, cranial portion of right scapula, partial furcula, sternum, left humerus, distal portion of right humerus, partial pelvis, right femur, left femur lacking distal end, left tibiotarsus lacking proximal end,
proximal portion of right tibiotarsus, partial left and right tarsometatarsi, os metatarsale I; collected in 1984 by M. Daniels (original collector’s number WN 84482A).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Member of the London Clay Fm. (previously Division A2; Rayner et al., 2009; Aldiss, 2012); early Eocene (early Ypresian 54.6–55 Ma; Collinson et al., 2016).

Measurements (maximum length, in mm): Left coracoid, 17.7;
left humerus, 27.9; right femur, 19.5 [20.6 in the holotype of P. gracilis; Dyke and Cooper, 2000]; left tibiotarsus, >29.2; left arsometatarsus, length as preserved, 14.3. See Table 1.

Remarks: Of the halcyornithids from the Daniels collection, this
specimen is most similar to the holotype of Pulchrapollia gracilis.
However, and as detailed below, some bones are distinguishable
from the corresponding elements of the P. gracilis holotype in minor features. These differences may indicate a distinctness at a
the species level, but we prefer a cautious taxonomic approach
and tentatively assign the fossil to P. gracilis.

Fred


Fig. 1. Overview of halcyornithid specimens from the early Eocene London Clay of Walton-on-the-Naze described in the present study. A. cf. Pulchrapollia gracilis (NMS. Z.2021.40.63). B. ?Pulchrapollia eximia nov. sp. (holotype, NMS.Z.2021.40.64). C, Undetermined species (NMS.Z.2021.40.71). D. ?Pulchrapollia tenuipes nov. sp. (holotype, NMS. Z.2021.40.65). E. ?Pulchrapollia sp. (NMS.Z.2021.40.66). F. Cyrilavis cf. colburnorum (NMS.Z.2021.40.67). G. Undetermined species (gen. et sp. indet. A; NMS.Z.2021.40.68). H. Right tarsometatarsus of an undetermined species (gen. et sp. indet. B; NMS.Z.2021.40.69). I. Left tarsometatarsus of another undetermined species (gen. et sp. indet. C; NMS. Z.2021.40.70). Scale bar: 5 mm.

 

[Fred Ruhe]

?Pulchrapollia eximia nov. sp.

Derivation of the name: The species epithet is derived from
eximius (Lat.), extraordinary; in reference to the exceptional
preservation of the holotype skeleton.

Holotype: NMS.Z.2021.40.64 (Fig. 2(B)), a partial skeleton
including skull, right quadrate, both coracoids, right scapula, furcula, sternum, proximal portion of right humerus, distal portions
of both humeri, partial carpometacarpi, left ulna, proximal portion
of right ulna, proximal portions of both radii, os carpi radiale, phalanx proximalis digiti majoris, partial pelvis, left femur, right tibiotarsus lacking proximal end, left tarsometatarsus, partial right tarsometatarsus; collected in 1985 by M. Daniels (original ollector’s
number WN 85508).

Type locality and horizon: Walton-on-the-Naze, Essex, United
Kingdom; Walton Mb. of the London Clay Fm.; early Eocene.

Measurements (maximum length, in mm): Left coracoid, 17.6;
right coracoid, 17.7; left ulna, 31.3; left carpometacarpus, 14.7;
right carpometacarpus, 14.7; left femur, 20.7; right tibiotarsus,

29.0; left tarsometatarsus, 15.7. See Table 1.

Differential diagnosis: Differs from Pulchrapollia gracilis Dyke
and Cooper, 2000 in that the fossa metatarsi I of the tarsometatarsus is located farther proximally (Fig. 5(L, N)). Furthermore differs from NMS.Z.2021.40.63, a specimen tentatively referred to P. gracilis, in that the sternum has a mediolaterally narrow spina externa and in that the trochlea metatarsi II has a less bulging medial margin (in distal view of the tarsometatarsus). It is differentiated from Cyrilavis olsoni (Feduccia and Martin, 1976) in that the sternum has a craniocaudally shorter and dorsoventrally deeper carinae sterni; tarsometatarsus with well-developed lateral foramen vasculare proximale. Distinguished from Cyrilavis colburnorum Ksepka et al., 2011 in that the femur is more elongated and slender. It differs from Pseudasturides macrocephalus (Mayr, 1998) in that the neurocranium is proportionally smaller, the tibiotarsus longer than the ulna (distinctly shorter in P. macrocephalus), and the pelvis proportionally longer (ratio humerus length : synsacrum length 1.40 vs. 1.57 in P. macrocephalus). It is differentiated from Serudaptu pohli Mayr, 2000 in its distinctly smaller size (ulna length
31.3 mm vs. 46.5 mm in S. pohli), sternum with dorsoventrally deeper and craniocaudally shorter carina sterni, pelvis proportionally longer (ratio humerus length : synsacrum length 1.40 vs. 1.68 in S. pohli), and ungual phalanges with laterally open sulcus neurovascularis. It is distinguished from Halcyornis toliapicus Koenig, 1825 in that the neurocranium is proportionally smaller.

Fred

 

[Fred Ruhe]

?Pulchrapollia tenuipes nov. sp.

Derivation of the name: The species epithet is derived from
tenuis (Lat.), slender, and pes (Lat.), foot, in reference to the slender
tarsometatarsus of the new species.

Holotype: NMS.Z.2021.40.65 (Fig. 2(D)), a partial skeleton including two vertebrae, both coracoids, left and partial right scapula, right carpometacarpus, left tibiotarsus, left tarsometatarsus, a few pedal phalanges; collected in 1991 by M. Daniels (original collector’s number WN 91700).

Type locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Mb. of the London Clay Fm.; early Eocene

Measurements (maximum length, in mm): Right coracoid, 17.3; left carpometacarpus, 15.2; left tibiotarsus, 34.0; left rsometatarsus, 16.5. See Table 1.

Differential diagnosis: Distinguished from all other halycornithids
except Halcyornis toliapicus (of which postcranial bones are unknown) in the slenderer tarsometatarsus, with the ratio of tarsometatarsus length to width of distal end being 4.3 (less than 4.0 in other species); unlike in other halcyornithids from Waltonon- the-Naze, the extremitas sternalis of the coracoid forms a marked medial bulge and the hypotarsal sulcus for musculus flexor hallucis longus is more pronounced. Differs from Pulchrapollia gracilis Dyke and Cooper, 2000 in that the coracoid has a shallow facies articularis scapularis (as per NMS.Z.2021.40.63, a specimen tentatively referred to P. gracilis), the carpometacarpus has a portionally shorter os metacarpale alulare, and in that the fossa
metatarsi I is situated farther proximally. It is differentiated from
?Pulchrapollia eximia in that the tarsometatarsus is only slightly
shorter than the coracoid (distinctly shorter in ?P. eximia). Distinguished from Pseudasturides macrocephalus Mayr, 1998 in that the tibiotarsus is proportionally longer, with the ratio of tibiotarsus to coracoid being 1.96 vs. 1.41 in the P. macrocephalus holotype.Cannot be differentiated from Halcyornis toliapicus Koenig, 1825, owing to the fact that the skull is unknown.

Fred

 

[Fred Ruhe]

?Pulchrapollia sp.

Referred specimen: NMS.Z.2021.40.66 (Fig. 2(E)), a partial skeleton including right quadrate, left and right coracoid (isolated
and in block of matrix, respectively), partial sternum in small blo of matrix, partial left and right humeri (isolated and in block of matrix, respectively), right ulna, proximal and distal portions of left ulna, partial left and right carpometacarpi, both phalanges proximales digitorum majores, partial pelvis in small block of matrix, right femur, right tibiotarsus, distal portion of left tibiotarsus, partial
left tarsometatarsus, collected in 1997 by M. Daniels (original
collector’s number WN 97969).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Mb. of the London Clay Fm.; early Eocene.

Measurements (maximum length, in mm): Right ulna, 30.5; left
carpometacarpus, 15.1; right femur, 19.2; right tibiotarsus, 32.5;
left tarsometatarsus, 15.8.

Remarks: This specimen cannot be referred to any of the above described species. Unlike in the ?Pulchrapollia eximia nov. sp. holotype, the caudal surface of the quadrate (Fig. 8(I-L); NMS.
Z.2021.40.65) exhibits a small pneumatic foramen (Fig. 8(K)).
Unlike in ?P tenuipes nov. sp., the coracoid has a cup-like cotyla
scapularis (Fig. 3(E)). As in the latter species, but unlike in other
halcyornithids from Walton-on-the-Naze, the extremitas sternalis
forms a marked medial bulge. Furthermore as in ?P. tenuipes nov.
sp., the os metacarpale alulare of the carpometacarpus is shorter
than in Pulchrapollia gracilis . The femur is stouter than that of the P. gracilis holotype. As in ?P. tenuipes nov. sp., the tarsometatarsus
(Figs. 5(P), 6(E)) is only slightly shorter than the coracoid. The unusually splayed trochlea metatarsi II is broken and was incorrectly reattached during preparation.

Fred

 

[Fred Ruhe]

Genus Cyrilavis Martin, 2010

Remarks: When erecting the taxon Cyrilavis, Martin (2010) neither
gave a diagnosis of the new taxon nor did he perform any comparisons with Pseudasturides or any other halcyornithid.
Ksepka et al. (2011) listed the absence of a lateral foramen vasculare proximale of the tarsometatarsus as an autapomorphic feature of Cyrilavis. This foramen is very small in many halcyornithid specimens from Walton-on-the-Naze, including the one described below, and its remains to be determined whether its absence is of taxonomic significance (in which case it would preclude an assignment of the London Clay fossil to C. colburnorum). Some of the characters listed by Ksepka et al. (2011) to differentiate Cyrilavis from other halcyornithid taxa are only known from C. colburnorum and not the type species (e.g., a cup-shaped cotyla scapularis of the coracoid, the shape of the inentia intercotylaris of the tarsometatarsus, and the extent of the fossae temporales). If our assignment of the above fossils to the taxon Pulchrapollia is correct, Cyrilavis is distinguished from Pulchrapollia and Pseudasturides in the proportionally longer carina sterni, which has a more cranially projected apex carinae. Another feature that is here considered apomorphic for Cyrilavis is a proximally projected tuberculum dorsale of the humerus, which is present in the C. colburnorum holotype and the London Clay fossil described below, but absent in the fossils assigned to Pulchrapollia, Pseudasturides, and Serudaptus.

Cyrilavis cf. colburnorum Ksepka et al., 2011

Referred specimen: NMS.Z.2021.40.67 (Fig. 2(F)), including tip
of upper beak, both processus supraorbitales, partial right quadrate, right ramus mandibulae, several vertebrae, both coracoids and scapulae, partial furcula, sternum, proximal portion of right humerus, distal end of left humerus, distal portion of right ulna, left radius, partial right carpometacarpus, phalanx proximalis digit majoris, partial pelvis, distal portion of left femur, proximal and distal portions of right tibiotarsus, fibula, right tarsometatarsus, several pedal phalanges; collected in 1986 by M. Daniels (original collector’s number WN 86547).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Mb. of the London Clay Fm.; early Eocene.

Measurements (maximum length, in mm; in brackets the dimension of Cyrilavis colburnorum after Ksepka et al., 2011): Right coracoid, 17.9 [19.5–20.0]; right carpometacarpus, 15.0 [16.2– 16.4]; right tarsometatarsus, 16.7 [17–18.1].

Remarks: NMS.Z.2021.40.67 differs from Cyrilavis olsoni (Feduccia and Martin, 1976) in the longer carpometacarpus (15.0 mm vs. 13.1 mm in C. olsoni; Ksepka et al., 2011). It is distinguished from Pulchrapollia gracilis Dyke and Cooper, 2000, in that the femur is proportionally larger and with a proximodistally longer condylus lateralis. It differs from ?Pulchrapollia eximia nov. sp. in that the sternum is craniocaudally longer, with a cranially projected apex carinae, the carpometacarpus has a proportionally longer os metacarpale alulare. NMS.Z.2021.40.67 is differentiated from ?Pulchrapollia tenuipes nov. sp. in that the carpometacarpus has a proportionally longer os metacarpale alulare. Differs from
Pseudasturides macrocephalus (Mayr, 1998) in that the tip of the
beak is mediolaterally narrower, the sternum is proportionally longer (subequal to tarsometatarsus length in P. macrocephalus,
but much longer in NMS.Z.2021.40.67), the humerus has a more
strongly proximally projected tuberculum dorsale, the tsometatarsus has a slenderer shaft. Distinguished from Serudaptus pohli Mayr, 2000 in that the humerus has a more strongly oximally
projected tuberculum dorsale, the tarsometatarsus is proportionally
longer, and the ungual phalanges of the foot are shorter and have laterally open neurovascular sulci.

Fred

 

[Fred Ruhe]

Gen. et sp. indet. A

Referred specimen: NMS.Z.2021.40.68 (Fig. 2(G)), a partial skeleton including partial mandible, a few vertebrae, extremitas omalis of right coracoid, partial furcula, distal portion of ri t ulna, distal portion of left femur, left tarsometatarsus, proximal portion of right tarsometatarsus; collected in 1993 by M. Daniels (original collector’s number WN 93800).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Mb. of the London Clay Fm.; early Eocene.

Measurements (maximum length, in mm): Left tarsometatarsus,
15.8. See Table 1.

Remarks: The mandible, of which the rostral portion is preserve corresponds well to that Cyrilavis olsoni in its shape, whereas the mandibles of Pseudasturides macrocephalus and that of an unnamed halcyornithid from the Geisel Valley; Mayr, 2020b) have a wider pars symphysialis. The furcula has more robust scapi avicularum and a wider extremitas sternalis than that of those halcyornithids from Walton-on-the-Naze we refer to Pulchrapollia. Unlike in Cyrilavis colburnorum and most other halcyornithids from Walton-on-the-Naze, the coracoid has a shallow facies articularis scapularis. The femur is larger than that of the Pulchrapollia gracilis holotype (Table 1) and the condylus lateralis is proximodistally longer. Unlike in Cyrilavis, the tarsometatarsus exhibits a lateral foramen vasculare proximale. The trochlea metatarsi II reaches farther distally than in most other halcyornithids from Walton-on-the-Naze. The trochlea metatarsi III is dorsoplantarly narrower than in the specimen we tentatively refer to C. colburnorum MS.Z.2021.40.67). Even though NMS. Z.2021.40.68 is therefore distinguished from the halcyornithids described above, the limited skeletal material available makes it inadvisable to erect a new species, especially since the postcranial osteology of Halcyornis toliapicus is unknown (see discussion).

Fred

 

[Fred Ruhe]

Gen. et sp. indet. B

Referred specimen: NMS.Z.2021.40.69 (Fig. 2(H)), right tarsometatarsus; collected in 2004 by M. Daniels (original collector’s number WN 041067).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Mb. of the London Clay Fm.; early Eocene.

Measurements (maximum length, in mm): Right tarsometatarsus,

Remarks: This tarsometatarsus is distinctly shorter than that of
the Pulchrapollia gracilis holotype (15.0 mm vs. 16.3 mm; Dyke and
Cooper, 2000). It is distinguished from other halcyornithids from
Walton-on-the-Naze in the proportionally shorter crista lateralis
of the hypotarsus. The section between the foramen vasculare distale and the incisura intertrochlearis lateralis is longer than in the superficially similar (albeit somewhat larger) tarsometatarsus of the undetermined halcyornithid classified below as gen. et sp. indet. C (NMS.Z.2021.40.70).

Fred

 

[Fred Ruhe]

Gen. et sp. indet. C

Referred specimen: NMS.Z.2021.40.70 tarsometatarsus;
collected in 1990 by M. Daniels (original collector’s number WN 90661).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Mb. of the London Clay Fm.; early Eocene.

Measurements (maximum length, in mm): Left tarsometatarsus,
15.6.

Remarks: This tarsometatarsus has a stouter shaft and a oportionally wider distal end than the corresponding bone of other halcyornithids from Walton-on-the-Naze. In its proportions, it sembles the tarsometatarsus of Pseudasturides macrocephalus from Messel, with which it agrees in size (the tarsometatarsus of the P. macrocephalus holotype measures 15.5 mm; Mayr, 1998). NMS.Z.2021.40.70 also corresponds well to a tarsometatarsus from the middle Eocene of the Geisel Valley in Germany in its overall shape (Fig. 5(J)), which was likened to P. macrocephalus by Mayr (2002b, 2020b). However, just based on these similarities in size and proportions, an unambiguous assignment to P. macrocephalus, or even the taxon Pseudasturides, is not possible.

Fred

 

[Fred Ruhe]

Halcyornithidae indet.

Referred specimen: NMS.Z.2021.40.71 (Fig. 2(C)), a partial skeleton including right coracoid, cranial extremity of right sca- pula, extremitas omalis of furcula, cranial portion of sternum, right
humerus; collected in 1988 by M. Daniels (original collector’s
number WN 88590).

Locality and horizon: Walton-on-the-Naze, Essex, United Kingdom; Walton Mb. of the London Clay Fm.; early Eocene.

Measurements (maximum length, in mm): Right coracoid, 17.0; right humerus, 28.5.

Remarks: Because this specimen lacks a tarsometatarsus, an unambiguous assignment to any of the other halcyornithids from Walton-on-the-Naze is not possible.

Fred