Agnopterus

DYKE, GARETH J., 2001, The Fossil Waterfowl (Aves: Anseriformes) from the Eocene of England, American Museum Novitates 3354, pp. 1-16 : 5-9

Agnopterus ? hantoniensis Lydekker ( 1891)

Agnopterus ? hantoniensis Lydekker ; listed by Lambrecht ( 1933)

Agnopterus ? hantoniensis Lydekker ; listed by Brodkorb ( 1964)

Headonornis hantoniensis Harrison and Walker ( 1976)

The genus Agnopterus was erected by Milne­Edwards (1869–1871) for the reception of the distal portion of a right tibiotarsus from the Upper Eocene of Montmartre, France. Milne­Edwards (1869–1871) named the species Agnopterus laurillardi on the basis of this material, and he noted a resemblance with the extant Phoenicopterus (flamingo). Later, Lydekker ( 1891) named the species Agnopterus ? hantoniensis on the basis of a partially complete right coracoid (BMNH PAL 30325; fig. 4A ), and referred a cast of a proximal left femur (BMNH PAL 144) to this taxon. Harrison and Walker ( 1976) removed this material from the genus Agnopterus , making the coracoid the holotype of the new genus and species Headonornis hantoniensis within the Anseriformes . They also referred an incomplete right humerus (BMNH A 3686; fig. 4B ). The specimen (BMNH PAL 144) is a cast of an original held in the Institute of Geological Sciences, London (IGS GSM 113109; formerly the Geological Survey Museum, London; Harrison and Walker, 1976).

MATERIAL: BMNH PAL 30325, a partially complete right coracoid lacking extremities of processus lateralis and angulus medialis (holotype; Harrison and Walker, 1976; fig. 4A ), purchased from the Hastings Collection in 1855 but collected sometime prior to this from the Upper Eocene of Hordle (Hordwell), Hampshire. BMNH PAL 3686, an incomplete right humerus lacking the distal end (referred specimen: Harrison and Walker, 1976; fig. 4B ), collected from Upper Eocene deposits near Milford, Hampshire, and presented by J. Athersuch in 1970. BMNH PAL 4989, the proximal end of a right scapula (referred specimen: Harrison and Walker, 1979a), collected from the Lower Oligocene Hampstead Beds on the Isle­of­Wight by S. L. Wood in 1925 (presented in 1933). SMC C20412, the sternal end of a coracoid (referred specimen: Harrison and Walker, 1979a; collection data unknown). As noted by Harrison and Walker (1979a), this specimen was described by Seeley ( 1866) under the generic name Ptenornis (no species name given), that was later regarded by Lydekker ( 1891) as a nomen nudum. BMNH PAL 5105, the distal end of a left humerus (referred specimen: Harrison and Walker, 1979a; fig. 4C ), collected from the Lower Oligocene Bembridge Marls at Burnt Wood, Isle of Wight, by R. Ford in 1978.

TAXONOMIC REMARKS: As discussed above, the original holotype specimen of H. hantoniensis is a partially complete right coracoid (Harrison and Walker, 1976).

As noted by Harrison and Walker ( 1976), this specimen is very similar to the coracoids of Recent anseriforms, such as Cygnus and Anseranas . The cotyla scapularis is deep and rounded (as appears to be primitive for modern birds in general; Mayr, 1999), the impressio m. sternocoracoidei is poorly developed and shallow, and the processus procoracoideus is blunt, having some ventral curvature at its tip. The morphology of this specimen agrees with that of Presbyornis pervetus (as coded by Livezey, 1997, and figured by Ericson, 1999a) in the presence of a small foramen pneumaticum on the processus procoracoideus and in the absence of a large and excavated foramen on the sternal face (dorsal surface). More generally, BMNH PAL 30325 has a small and semi circular processus acrocoracoideus and a non excavated articularis clavicularis. With respect to these specific features, and overall, this element is very similar (other than in size) with the known coracoids of P. pervetus (Ericson, 1999b) . Hence, the holotype of H. hantoniensis can be referred to the family Presbyornithidae Wetmore.

A similar­sized member of this family is known from the Tertiary of North America. Olson ( 1994) described a ‘‘giant’’ species of the genus Presbyornis , P. isoni , on the basis of a left humerus lacking the distal end ( USNM 294116: holotype) and the left phalanx of the alar digit ( USNM 294117) from the Palaeocene of Maryland. Olson ( 1994) noted that, in the morphology of the distal humerus, this species does not differ from P. pervetus other than in size and the distinctiveness of the fossa olecrani. The latter character may also be related to the large size of the specimen. In particular, on the distal end of the humerus of both Presbyornis pervetus and P. isoni , the fossa m. brachialis is deep and marked, and the condylus dorsalis is prominent and strongly developed proximally (Olson, 1994; Ericson, 1999a).

Harrison and Walker (1979a) noted that Headonornis from the London Clay is ‘‘a member of the Presbyornithidae’’. However, on the basis of the material avialable to them at the time, their conclusion cannot be confirmed. As discussed above, since the holotype on Headonornis hantoniensis is a coracoid, it is not certain whether the two referred humeri (Harrison and Walker, 1976, 1979a) can be associated to the taxon on any basis other than their large size; the coracoid and humeri are clearly from birds of a similar size. Subsequent investigation, however, shows that the second referred humerus, BMNH PAL 5105 (Harrison and Walker, 1979a), is identical both in size and preserved morphology to the holotype of Presbyornis isoni (Olson, 1994) .

At the time of his description of P. isoni, Olson ( 1994) omitted discussion of the London Clay material from his paper. However, the morphology of another specimen, BMNH PAL 6240, does serve to confirm the suspicions of Harrison and Walker (1979a) that at least some of the London Clay material should be referred to the Presbyornithidae .

BMNH PAL 6240 was collected from the Totland Bay Member of the Middle­Upper Eocene Headon Hill Formation at Hordle, Hampshire, in 1995 by A. Jord. This material consists of a partially complete, but fragmented, right humerus with both the proximal and distal ends, as well as much of the shaft. The proximal end preserves about onethird of the crus dorsale fossa (broken beneath the fossa pneumotricipitalis), and the distal end lacks the medial area lateral to the condylus dorsalis ( fig. 5 ). Nevertheless, the morphology of this specimen confirms the association between the two specimens originally referred to H. hantoniensis ( BMNH PAL 3686 and PAL 5105; Harrison and Walker, 1976, 1979a), as does the presbyor­ nithid nature of this material on the basis of the osteological features discussed by Ericson (1999a). On the basis of the hypothesis of Ericson (1997, 1999a) , BMNH PAL 6240 exhibits the presence of four derived pres­ byornithid characters ( fig. 5 ), including the presence of an excavated area beneath the caput humeri and a prominent scar for the attachment of M. flexor carpi ulnaris .

In addition, in size and morphology BMNH PAL 6240 is identical to both the originally referred specimens of H. hantoniensis and the holotype of P. isoni ( figs. 4 , 5 ). This match is further confirmed by comparison with the illustrations of a complete (but crushed) humerus referred to P. isoni from the late Paleocene of North Dakota by Benson ( 1999).

Having confirmed that these fossil humeri from the Eocene of England are similar to those described for Presbyornis isoni (see Olson, 1994; Benson, 1999), the synonymy of this material remains. Since the holotype of the species hantoniensis within the genus Headonornis is a coracoid that can only be placed tentatively within the Presbyornithidae (see above) and cannot be associated with certainty to the originally referred humeri (other than by size), the original genus and species is retained here on the basis of the holotype alone. The three fossil humeri ( BMNH PAL 3686, 5105, and 6240) are therefore referred to the taxon Presbyornis isoni Olson. It is likely, however, that the generic name Headonornis Harrison and Walk­ er is a junior synonym of Presbyornis Wetmore , and that the species name isoni Olson is preoccupied by the earlier name hantoniensis Harrison and Walker ; however, the recovery of additional fossil material will be required to confirm this.

The remaining fossil specimen referred to H. hantoniensis by Harrison and Walker (1979a: BMNH PAL 4989, the proximal end of a right scapula) is regarded as Aves incertae sedis. This specimen cannot be referred to the holotype of H. hantoniensis with certainty.

Figures

Fig. 4. Some of the fossil material referred to Headonornis hantoniensis Harrison and Walker. A, holotype right coracoid in dorsal view (BMNH PAL 30325); B, referred distal end of left humerus in cranial view (BMNH PAL 5105); C, referred right humerus in caudal view (BMNH PAL 3686). Scale bars are 5 mm.

Fig. 5. Additional fossil material referred to Headonornis hantoniensis (BMNH PAL 6240). A, right proximal humerus in caudal view; B, cranial view; C, referred distal end of left humerus in cranial view (BMNH PAL 5105). Characters based on the hypothesis of Ericson (1999) are labeled. Scale bars are 5 mm. Abbreviations as follows: a, incisura capitis humeri; b, impression of M. scapulohumeralis cranialis; c, tuberculum supracondylare ventrale; d, epicondylus dorsalis.