Anthropornis nordenskjoeldi Wiman, 1905

Jadwiszczak, Piotr & Mörs, Thomas, 2011, Aspects of diversity in early Antarctic penguins, Acta Palaeontologica Polonica 56 (2), pp. 269-277 : 272-273

Anthropornis nordenskjoeldi Wiman, 1905

Fig. 3 .

1905 Anthropornis nordenskjöldii sp. nov.; Wiman 1905a: 249, pl. 12: 6.

1905 Orthopteryx gigas sp. nov.; Wiman 1905b: 27–28, pl. 8: 2–2b; new synonymy.

Holotype: NRM−PZ A.45, incomplete left tarsometatarsus.

Type locality: NE Seymour Island (Antarctic Peninsula).

Type horizon: La Meseta Formation, Telm6–7 of Sadler (1988; see also Marples 1953: fig. 1 [“Swedish locality”], and Myrcha et al. 2002: fig. 1), late Eocene.

Material.—IB/P/B−0070, IB/P/B−0085, IB/P/B−0287, MLP 84−II−1−7, MLP 83−V−20−50, MLP 83−II−1−19, BMNH A3358 (incomplete tarsometatarsi); IB/P/B−0091, IB/P/B−0092, IB/P/

B−0307, IB/P/B−0478, IB/P/B−0711, NRM−PZ A.37, MLP 93 −X−1−4, MLP 82 −IV−23−4, MLP 83 −I−1−190, MLP 88 −I− 1−463, BMNH A3338 , SAMA P14157 b, SAMA P14157 c, SAMA P14158 a (incomplete humeri), IB/P/B−0119, NRM−PZ A.43 (nearly complete humeri); NRM−PZ A.23 (incomplete synsacrum, type specimen of O. gigas Wiman, 1905 ) .

Diagnosis.—Tarsometatarsal features as listed by Myrcha et al. (2002) (but see the “Skeletal sexual dimorphism and fossil penguins” section).

Remarks.—Wiman (1905b), by erecting Orthopteryx gigas , had departed from his principle of basing fossil penguin species on tarsometatarsi, and this led to long lasting confusion in the systematics of this group ( Simpson 1946, 1971; Jadwiszczak 2009). In his opinion (Wiman 1905b), the type specimen of O. gigas , a partial synsacrum being the sole member of the so−called Group 1, was too large to belong to Anthropornis nordenskjoeldi (another “giant” penguin he described, placed in Group 3), and due to other morphological details, was rather doubtfully spheniscid.

The total length of specimen NRM−PZ A.23 is 212 mm, and taking into account its missing ends, was originally somewhat longer (approximately 230 mm long, Simpson 1946). For comparison, the complete synsacrum of the extant Aptenodytes forsteri (NRM−VE A611330; Fig. 3A ) is 177 mm long (data for A. nordenskjoeldi do not exist, because no other synsacrum can be reasonably assigned to this species; see previous paragraph). However, the length of a synsacrum depends, in part, on the number of vertebrae within it. According to Pycraft (1898), one to three caudal vertebrae can be included in the synsacrum of modern penguins, depending on age. Moreover, Wiman’s (1905b) arguments for the uncertain status of O. gigas as a penguin species are not too serious (as stated by Simpson [1946], and we agree with this statement). First, specimen NRM−PZ A.23 originally comprised at least 14 vertebrae; too many according to Wiman (1905b), but present−day sphenisciforms have between 12 and 14 synsacral vertebrae ( Pycraft 1898; Simpson 1946; Stephan 1979). Although Waimanu Jones, Ando, and Fordyce, 2006 (the basal penguin from the Palaeocene of New Zealand) had 11 fused synsacral vertebrae ( Slack et al. 2006), it would be premature to conclude that the primitive state was to have a lower number of such skeletal elements. Even so, O. gigas is considerably younger in terms of geologic time than Waimanu . Second, the lack of a dorsal keel is arguable, as it is conspicuous in the preserved fragment of the dorsal surface (Wiman 1905b: pl. 8: 2). On the other hand, the lack of a ventral keel was considered by Simpson (1946: 39) as a sole “really distinctive” feature (to some degree, at least), and placed in the generic diagnosis of the “dubious taxon” Orthopteryx (see Simpson 1971). However, the ventral keel, as noticed by Simpson (1946), is not equally well developed in all modern penguins, and is usually restricted to the cranial part of the synsacrum. A reduced ventral keel is also observed in the Eocene penguins, but some specimens (e.g., IB/P/B−0102 and IB/P/B−0149) possess the keel extending to the caudal part of the bone (PJ personal observation; Jadwiszczak 2006a: fig. 18e). In the Palaeocene Waimanu , the synsacrum does not form such a structure but keeps a columnar shape ( Slack et al. 2006: fig. 1; Tatsuro Ando, personal communication).

The cranial part of specimen NRM−PZ A.23 is clearly elongated. Additionally, there is a conspicuous swelling of the bone, just caudal to the (missing) articular surface ( Fig. 3B ). It seems to be a structure supporting the cranial end of the synsacrum, evolved to compensate for the huge body mass of the bird. Interestingly, such a swelling is also observed in the Palaeocene Waimanu (“large robust birds”; Slack et al. 2006: fig. 1). The only Eocene penguin known to have analogous supportive structures, but within its hind−limb skeleton (evolutionarily sensitive to mass−related forces), is Anthropornis nordenskjoeldi . Birds assigned to this species had massive tarsometatarsi with well pronounced convexity in the centre of their (otherwise concave) medial margins (e.g., Wiman 1905b: pl. 2: 3, 6; Myrcha et al. 2002: 17).

In our opinion premises discussed above justify the synonymisation of Orthopteryx gigas with Anthropornis nordenskjoeldi (the latter having priority). Neither the synsacral length of O. gigas is inconsistent with that of holotype tarsometatarsus of A. nordenskjoeldi nor the lack of the ventral keel in O. gigas does not exclude the species from sphenisciforms.

We agree with Simpson’s (1971) view that the synsacrum NRM−PZ A.9 ( Fig. 3D ), assigned by Wiman (1905b) to his Group 3 (the one containing the holotype of A. nordenskjoeldi ), could have belonged to a clearly smaller bird, most probably from the genus Palaeeudyptes . The systematic position of the synsacrum from Wiman’s (1905b) Group 2 (NRM−PZ A.47; Fig. 3C ) remains open to question, however.

Figures

Fig. 3. Synsacra (in ventral views) of the Recent sphenisciform and fossil penguins. A. Recent Aptenodytes forsteri Gray, 1844, NRM−VE A611330. B–D. Fossil penguins from the Eocene La Meseta Formation, Seymour Island. Assignments as proposed in this paper. B. Anthropornis nordenskjoeldi Wiman, 1905, NRM−PZ A.23. C.?Anthropornis sp., NRM−PZ A.47. D.?Palaeeudyptes sp., NRM−PZ A.9.