Emended diagnosis

Humerus stout and with a wide proximal end (ratio proximal width : total length of bone, 0.34–0.37); caput humeri proximally protruding and distal margin abruptly set off from the caudal surface of bone; tuberculum dorsale marked and proximodistally long; tuberculum ventrale prominent; ulna only slightly longer than humerus; extremitas sternalis of coracoid with short processus lateralis with markedly convex lateral margin; tibiotarsus with wide distal end and widely splayed condyles.

LSID

urn:lsid:zoobank.org:act:E0437199-0CB4-44ED-8C90-DCA4973F9ED7

Derivation of name

The genus name is derived from archaios (Greek): ‘old’, and dromos (Greek): ‘racetrack’, a suffix that appears in the names of two distantly related extant aerial avian insectivores (Nyctidromus (Caprimulgidae) and, in a slightly modified form, Aerodramus (Apodidae)). The name refers to the presumed foraging method of the animal and has also been chosen because of its phonetic closeness to Archaeotrogon.

Type species

Archaeodromus anglicus sp. nov.

Diagnosis

Mandible with cotyla lateralis situated far caudally; quadrate with cotyla quadratojugalis cup-shaped and facing dorsally; humerus with transverse ridge in the incisura capitis and with the tuberculum ventrale separated from the crista bicipitalis by a notch; and coracoid with a distinct medial protrusion in the sternal section of the shaft, with the facies articularis clavicularis being bipartite and forming two projections.

Differential diagnosis

Distinguished from Archaeotrogon Milne-Edwards, 1892, by the medial margin of the proximal end of the humerus not forming a point (Fig. 1C); the coracoid having a proportionally shorter processus acrocoracoideus and a bipartite facies articularis that forms two distinct projections, the more ventral one of which gives the processus acrocoracoideus a hook-like shape; the coracoid shaft being more slender and having a prominent medial projection (Fig. 1G); and the facies articularis sternalis being mediolaterally wider; it further differs from Archaeotrogon venustus, A. zitteli and A. cayluxensis in that the humerus is proportionally longer and has a more slender shaft (ratio humerus : coracoid 1.49 vs 1.31–1.36; based on measurements in Mourer-Chauviré 1980, tables 2, 3).

It is distinguished from Hassiavis Mayr, 1998, by the humerus being proportionally longer and having a more slender shaft (ratio humerus : coracoid 1.49 vs c. 1.39 in the holotype of H. laticauda); the tuberculum dorsale being proportionally shorter (Fig. 1A–C); the processus acrocoracoideus of the coracoid less markedly hook-shaped; the scapula having a proportionally narrower shaft; and the phalanx proximalis digiti majoris having a small but distinct processus internus indicis.

Remarks

Most of the previously described birds from Walton-on-the-Naze are clearly distinguished from Archaeodromus, but the humerus of the putatively coliiform taxon Eocolius Dyke & Waterhouse, 2001 shows a superficial similarity. Eocolius differs from Archaeodromus in the coracoid not having a foramen nervi supracoracoidei; the acromion of the scapula being proportionally longer and not forming a narrow, pointed tip; and the humerus with the tuberculum ventrale (proximal end) being proportionally larger, the caput humeri more protruding, and the tuberculum supracondylare ventrale (distal end) being proportionally longer (Dyke & Waterhouse 2001).

The humerus and coracoid of Archaeodromus also resemble the corresponding bones of trogons (Trogoniformes). In both, trogons and archaeotrogons, the humerus is stout and has a wide proximal end, but in archaeotrogons, the tuberculum ventrale is more prominent, the incisura capitis wider, and the sulcus transversus more marked; the coracoid of archaeotrogons has a reduced processus lateralis with a convex lateral margin. Archaeodromus is furthermore clearly distinguished from trogons in the morphology of the quadrate (see description and discussion).

LSID

urn:lsid:zoobank.org:act:EA5FC86F-EF95-4A0B-9120-536D2735EE5D

Derivation of name

The species epithet refers to the geographic provenance of the new species.

Holotype

SMF Av 654: partial skeleton including the caudal portion of the right mandibular ramus, right quadrate, at least seven vertebrae or fragments thereof, both coracoids, right scapula, right humerus, left humerus lacking proximal end, proximal end of right ulna, fragment of distal end of left ulna, right os carpi ulnare, left phalanx proximalis digiti majoris, right phalanx distalis digiti majoris, and various unidentifiable bone fragments. The fossil was found in 1988 by Paul Bergdahl (original collector’s number BC 8814).

Diagnosis

As for genus. The new species is slightly larger than Hassiavis laticauda (humerus length 24.2 vs 20.3–20.6 mm; Mayr 1998) and its humerus measures only slightly less than that of Archaeotrogon venustus, the smallest Archaeotrogon species, whose humerus has a length of 25.0–29.7 mm (Mourer-Chauviré 1980; table 2). Compared with extant birds, the new species is about the size of the Australian owlet-nightjar (Aegotheles cristatus).

Type locality and horizon

Walton-on-the-Naze, Essex, UK; Walton Member of the London Clay Formation (previously Division A2; Jolley 1996; Aldiss 2012), early Eocene (early Ypresian, 54.6–55 Ma; Collinson et al. 2016).

Measurements

Right coracoid, length, 16.2 mm. Right humerus, length, 24.2 mm; proximal width, 8.9 mm; distal width, 5.0 mm. Phalanx proximalis digiti majoris, length, 8.9 mm. Phalanx distalis digiti majoris, length, 7.7 mm.

Description and comparisons

The right quadrate is nearly complete, with only the tip of the processus orbitalis being damaged (Fig. 2A–F). The most unusual feature of the bone is the deeply excavated and dorsally facing, cup-like cotyla quadratojugalis (Fig. 2G). In most birds, including the Steatornithidae, Podargidae and Trochilidae among the Strisores, the cotyla quadratojugalis faces laterally. It is dorsally directed in the Caprimulgidae, Nyctibiidae, Aegothelidae, Apodidae and Hemiprocnidae, but in the latter three taxa it forms a shallow articulation facet rather than a distinct cup-shaped cotyla (in the Caprimulgidae and Nyctibiidae the cotyla is cup-like, even though it is less excavated than in Archaeodromus). The capitulum squamosum is continuous with a small protrusion on the lateral surface of the processus oticus (tuberculum subcapitulare of Elzanowski & Stidham 2010). The capitulum oticum is not strongly medially projecting, unlike in the Caprimulgidae (Fig. 2M), Nyctibiidae, Aegothelidae (Fig. 2L), and Apodiformes; the incisura intercapitularis is very shallow. As in non-apodiform Strisores, but unlike most other neornithine birds, the condylus lateralis faces ventrolaterally. There are small pneumatic foramina along the caudal margin of the tip of the processus oticus, whereas a foramen pneumaticum mediale is absent; with regard to the distribution of pneumatic foramina, the fossil agrees with the quadrate of the Aegothelidae and apodiform birds. The well-developed processus orbitalis is dorsoventrally deep and slightly medially bent; its medial surface is concave. In extant Caprimulgidae (Fig. 2J), Nyctibiidae, Aegothelidae (Fig. 2I), Apodidae and Hemiprocnidae the processus orbitalis is greatly reduced. As in other Strisores but unlike, for example, the Trogoniformes (Fig. 2H) and Coraciiformes, the condylus medialis is proximodistally extensive but only weakly protruding in the ventral direction; its lateral surface forms a distinct bulge, which also occurs in the Podargidae, Nyctibiidae and Aegothelidae. The condylus pterygoideus is well-defined. An unusual feature of the bone is a distinct, scar-like sulcus on the caudomedial surface next to the base of the condylus lateralis (Fig. 2B); if this is a true osteological feature, this sulcus would distinguish the quadrate of Archaeodromus anglicus from that of other neornithine birds.

The dorsoventrally shallow caudal end of the mandible (Fig. 2N, O) has a distinctive shape in that the cotyla lateralis is situated caudally rather than laterally so that, in dorsal or ventral view, the caudal margin has a markedly convex outline and forms a distinct bulge (Fig. 2N). The sulcus intercotylaris is shallow and the tuberculum intercotylare has a crest-like shape. The cotyla medialis forms a deep but narrow trough. The well-developed processus medialis bears a pneumatic foramen. The caudal bulge is also present in the Podargidae and Apodidae, whereas the caudal end of the mandible of the Caprimulgidae (Fig. 2R), Nyctibiidae and Aegothelidae (Fig. 2S) is proportionally smaller and has a straight caudal margin.

In the holotype, a short section of the caudal portion of the right mandibular ramus is preserved (Fig. 2P), which most closely resembles the corresponding section of the mandible of the Aegothelidae. The fragment bears a small fenestra caudalis (absent in the Caprimulgidae and Nyctibiidae) and is not as wide mediolaterally as the caudal section of the mandibular ramus of the Caprimulgidae and Nyctibiidae.

The vertebral series preserved in the holotype includes at least seven vertebrae or fragments thereof (Fig. 3A–J). Two nearly complete cervical vertebrae (Fig. 3A–F) are from the cranial series (4th–8th cervicals). Both bear long and thin processus costales. The processus transversi are moderately long and exhibit ridge-like tori dorsales. In one of the two vertebrae, the ventral surface of the corpus forms a narrow ridge (Fig. 3E); in the other vertebra (Fig. 3A–C), the facies articularis cranialis has a convex surface (rather than being saddle-shaped). Both vertebrae are dorsoventrally compressed as in the Aegothelidae. The foramina transversaria are small, also as in the Aegothelidae and Podargidae (larger in the Caprimulgidae). Two further fragmentary cervicals are from the more caudal series. A fragmentary thoracic vertebra appears to be the 14th (if compared to the Steatornithidae) or 13th (if compared to other Strisores); its corpus bears lateral depressions and there is a pair of wing-like lateral projections at the base of the processus ventralis.

The coracoid (Fig. 4A–D) is distinguished from the corresponding bone of Archaeotrogon in the shape of the processus acrocoracoideus and the more slender shaft. Overall, the bone resembles the coracoid of the Trogonidae (Fig. 4N). With regard to the more slender shaft, the bone also differs from the coracoid of Hassiavis with which it, however, agrees in that the processus acrocoracoideus has a hook-like outline. In Archaeodromus, the facies articularis clavicularis is bipartite and forms two protrusions; the larger, ventrally situated one is for the attachment of the ligamentum acrocoracoclaviculare superficiale and contributes to the hook-like appearance of the processus acrocoracoideus. As in Archaeotrogon, the cotyla scapularis is cup-like but very shallow. The processus procoracoideus appears to have been well developed and of subtriangular shape, even though its tip is broken in both coracoids. A foramen nervi supracoracoidei is absent. The sternal portion of the shaft forms a large medial projection, which is situated next to an indistinct notch. In Archaeotrogon the medial margin of the shaft also exhibits a shallow notch, but there is no pronounced medial projection even though in some specimens a minute protuberance is present (e.g. Mourer-Chauviré 1980, fig. 3c). A raised, elongated scar is situated in the medial portion of the impressio musculi sternocoracoidei. The extremitas sternalis is nearly complete in the right coracoid, in which only a short portion of the midsection of the rim of the processus lateralis is damaged. As in Archaeotrogon, the processus lateralis is short and has a strongly convex outline; the facies articularis sternalis is, however, mediolaterally wider than in Archaeotrogon (the morphology of the sternal end of the coracoid is poorly known for Hassiavis).

The scapula (Fig. 4F, G) has a narrow shaft; the caudal end is only slightly widened and moderately deflected. The acromion is proportionally longer than in Archaeotrogon.

Of the sternum, the left margo costalis with four processus costales as well as various small fragments are preserved (Fig. 3L, M). Another plate-like bone fragment that tapers into a narrow tip (Fig. 3K) defies a straightforward identification and may either be another part of the sternum or, more likely, a skull fragment.

The humerus (Fig. 4H–M) is a comparatively short and stout bone with a wide proximal end and closely resembles the humerus of Archaeotrogon hoffstetteri in its proportions (Fig. 1A, C). As in all Archaeotrogon species and in Hassiavis, the prominent caput humeri protrudes proximally. Also as in Hassiavis and the species of Archaeotrogon, the distal margin of the caput humeri is abruptly set off from the caudal surface of the bone (Fig. 4L). There is a transverse ridge in the incisura capitis (Fig. 4L), which is also found in Archaeotrogon hoffstetteri (Mourer-Chauviré 1980, fig. 10), but which is absent in other Archaeotrogon species. The sulcus transversus is long and distinct. The well-developed and caudally protruding tuberculum ventrale is separated from the crista bicipitalis by a notch (Fig. 1A); the incisura capitis is wide. Unlike in Archaeotrogon (Fig. 1C), the medial margin of the proximal humerus does not form a distinct point. The base of the fossa pneumotricipitalis is damaged so that it cannot be discerned whether there were pneumatic openings. The crus dorsale fossae is broken. The tuberculum dorsale is large and proximodistally elongated; in extant Strisores a pronounced tuberculum dorsale occurs only in the Caprimulgidae and in some apodiform birds. The crista deltopectoralis is damaged but appears to have had a convex margin. The shaft of the bone is narrower than the humerus shaft of all Archaeotrogon species except for A. hoffstetteri, which has a proportionally more slender shaft (Fig. 1C, F; Mourer-Chauviré 1980); on its caudal surface there is an elongate scar for the insertion of the musculus latissimus dorsi pars caudalis. The sulcus scapulotricipitalis, on the distal end of the bone, is very marked; its distolateral edge forms a distally projected prominence. The fossa musculi brachialis is shallow. A tuberculum supracondylare dorsale is absent, but there is a longitudinal insertion scar for the attachment of the musculus extensor carpi radialis (Fig. 4M). The scar for the attachment of the tendon of musculus pronator superficialis, on the ventral surface of the distal humerus, is a raised tubercle. The distoventral portion of the bone (processus flexorius and epicondylus ventralis) forms a short but well-defined projection. The large tuberculum supracondylare ventrale reaches as far proximally as the condylus dorsalis. The condyli are of similar shape to those of Archaeotrogon and are separated by a distinct incisura intercondylaris.

The proximal end of the right ulna (Fig. 5A–C) has a distinctive morphology. It resembles the proximal ulna of the Caprimulgidae (Fig. 5D) and Nyctibiidae, except for the short olecranon, which has a broadly rounded tip in Archaeodromus and is more similar to the olecranon of the Podargidae and Aegothelidae (Fig. 5E), whereas the olecranon of the Caprimulgidae and Nyctibiidae is narrower. The tuberculum ligamenti collaterale ventrale is small. A marked, oblique ridge extends distally past the processus cotylaris dorsalis. The cotyla ventralis is large. On the cranial surface of the bone, distal of the cotyla ventralis, there are two tubercles for the attachment of the tendon of musculus biceps brachii; these tubercles are a derived characteristic of the ulna of the Caprimulgidae (Fig. 5A, D). Also as in the Caprimulgidae, the impressio scapulotricipitalis forms a deeply marked pit. A midline ridge runs along the caudal surface of the bone.

The holotype includes a fragment of the caudal portion of the distal end of the left ulna (Fig. 5G). The condylus dorsalis does not exhibit the derived shape of the Caprimulgidae, in which it is caudally splayed, and the incisura tendinosa is very distinct.

In its shape, the os carpi ulnare (Fig. 5H, I) resembles the corresponding bone of the Caprimulgidae (5J). As in all other Strisores (Mayr 2010), the crus longum is pronounced.

The phalanx proximalis digiti majoris (Fig. 5L, M) is similar to that of the Aegothelidae (Fig. 5P) in its shape and morphology. It bears a well-defined, albeit very small, processus internus indicis (Stegmann 1963). There is a small fenestra in the distal portion of the bone, which represents a true osteological feature rather than a breakage owing to damage of the bone; the proximal section of the phalanx exhibits only two minute perforations. The phalanx distalis digiti majoris (Fig. 5N, O) is proportionally longer than in the Aegothelidae, being as long as the phalanx proximalis digiti majoris.

Referred specimen

SMF-ME 11702A + B (postcranial skeleton on two slabs; Fig. 6).

Locality and horizon

Messel near Darmstadt, Germany; latest early or earliest middle Eocene (48 Ma; Lenz et al. 2015).

Measurements

Given as: left/right, maximum length [measurements of the holotype of H. laticauda from Mayr 1998] (all in mm). Humerus, 22.1/c. 22 [c. 20.5/20.5]. Ulna, 24.0/c. 24.6 [c. 24.1/c. 23.9]. Carpometacarpus, 13.8/13.6 [c. 12.7/13.1]. Femur, c. 14.5/c. 14.5. Tibiotarsus, c. 23.0/c. 23.0 [25.0/24.6]. Tarsometatarsus, c. 13.5/12.4 [12.0/12.2]. Pedal phalanges: I1, c. 4.8 [4.4]; I2, 2.5 [2.5]; II1, c. 4.2 [3.9]; II2, c. 4.0 [3.9]; II3, 2.2 [2.8]; III1, 4.1 [3.2]; III2, 3.1 [3.8]; III3, >3.6 [4.1]; III4, 2.4 [2.9]; IV1, 3.6 [2.8]; IV2, 2.4 [2.5]; IV3, 2.1 [2.5]; IV4, 2.8 [2.5]; IV5, 2.2 [c. 2.3].

Description and comparisons

The new fossil is the best-preserved skeleton of Hassiavis found so far and the only one in which the postcranial bones are not substantially flattened and crushed. Whereas the overall shape of the major limb bones could be assessed in previous fossils, the new specimen reveals several osteological details that were unknown for Hassiavis and that are described in the following. The new specimen closely resembles previously reported fossils of H. laticauda, but has a proportionally slightly longer humerus and carpometacarpus, and a slightly shorter tibiotarsus. Whether these differences indicate distinctness at the species level or are due to the taphonomic distortion of the fossils is difficult to determine, so that the new fossil is classified as Hassiavis cf. laticauda.

In the new specimen, the hook-like shape of the processus acrocoracoideus of the coracoid is less apparent than it is in previously described specimens (Mayr 1998, 2004), which is due to the fact that the processus acrocoracoideus lacks the ventral portion in the right coracoid and is distorted in the left one (Figs 1H, I, 7I). However, the processus acrocoracoideus of the right coracoid shows that the facies articularis clavicularis was bipartite, as it is in Archaeodromus. The shaft of the bone appears to be somewhat stouter than that of Archaeodromus, although it is not as stout as in Archaeotrogon. The processus procoracoideus is not fully preserved in the fossil, but seems to have been broad and short. The medial margin of the sternal end is damaged, but the right coracoid allows the recognition of the base of a broken medial projection (Fig. 1H); whether this projection was as pronounced as in Archaeodromus remains, however, unknown. The processus lateralis is likewise damaged in both coracoids and although it appears to have been short, its exact shape cannot be discerned.

The humerus (Fig. 7A–D) closely resembles that of Archaeodromus. As in the latter, the tuberculum dorsale is distinct and proximodistally elongated and the sulcus transversus is deeply marked. The tuberculum dorsale is proportionally longer than in Archaeodromus (Fig. 1A–C; the humerus length is 24.3 vs 22.1 mm for Archaeodromus and Hassiavis, respectively, whereas the length of the tuberculum dorsale is 1.7 vs 2.0 mm, respectively). As far as comparisons are possible, the distal end of the bone resembles the distal humerus of Archaeodromus. The distolateral edge of the sulcus scapulotricipitalis forms a distally projected prominence. The scar for the attachment of the tendon of musculus pronator superficialis forms a raised tubercle.

The proximal end of the ulna agrees with Archaeodromus in that there is a deeply marked impressio scapulotricipitalis. The visible part of the distal end of the bone is also similar to the fragment of the distal ulna preserved in the Archaeodromus anglicus holotype, and exhibits a distinct incisura tendinosa.

The comparatively short carpometacarpus (Fig. 7E–G) has similar proportions to that of Archaeotrogon. Unlike in the latter, however, the long and cranially prominent processus extensorius does not form a spur-like, pointed tip.

The os carpi ulnare (Fig. 7F) resembles that of Archaeodromus in its shape and has a long crus longum. The os carpi radiale is similar to the corresponding bone of the Caprimulgidae, Nyctibiidae and Aegothelidae, whereas the bone has a distinctive derived morphology in the Trogoniformes and in coraciiform birds (compare Fig. 7E with Mayr 2014, figs 3, 6). Unlike in Archaeodromus, the phalanx proximalis digiti majoris is not fenestrated.

As in Archaeotrogon, the tibiotarsus has a very wide distal end, with widely splayed condyli (Fig. 7K). The trochlea cartilaginis tibialis is proximodistally extensive and has a distinctly concave articular surface.

The tarsometatarsus (Fig. 7J–M) likewise has similar proportions to that of Archaeotrogon, and in concordance with the wide distal end of the tibiotarsus, it has a wide proximal end. The tuberositas musculi tibialis cranialis is situated near the medial margin of the bone. The hypotarsus forms a broad platform (Fig. 7N), which is distally continuous with a short crista medianoplantaris; deep hypotarsal sulci or canals for the flexor tendons of the toes appear to be absent. The trochleae metatarsorum resemble those of Archaeotrogon in their proportions. The foramen vasculare distale is not clearly visible. The proximal phalanges of the fourth toe are not abbreviated.

The tail feathers are well preserved in the fossil (Fig. 6C, D), with the longest one measuring c. 62 mm. Although the tail is therefore as long as that of other Hassiavis specimens with preserved tail feathers, the new fossil shows no banding pattern, which is pronounced in previously reported fossils (Mayr 1998, 2004).