Delphinornis gracilis (Wiman, 1905) Jadwiszczak & Mörs, 2011

Jadwiszczak, Piotr & Mörs, Thomas, 2011, Aspects of diversity in early Antarctic penguins, Acta Palaeontologica Polonica 56 (2), pp. 269-277 : 271-272

Delphinornis gracilis (Wiman, 1905) comb. nov.

Fig. 2 .

1905 Ichtyopteryx gracilis sp. nov.; Wiman 1905a: 251, pl. 12: 4. 2002 Delphinornis gracilis sp. nov.; Myrcha et al. 2002: 30–31, fig. 11;

new synonymy.

Holotype: NRM−PZ A.20, incomplete right 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−0279a (complete right tarsometatarsus; type specimen of Delphinornis gracilis Myrcha, Jadwiszczak, Tambussi, Noriega, Gaździcki, Tatur , and del Valle, 2002), IB/P/B−0492 (incomplete left tarsometatarsus), IB/P/B−0549 (incomplete left tarsometatarsus), IB/P/B−0408 (incomplete left tibiotarsus) and IB/P/B−0130 (incomplete right femur).

Emended diagnosis.—Tarsometatarsus small and slender (Table 1; Myrcha et al. 2002: table 1). The medial hypotarsal crest sloping towards the medial margin of the bone, but the slope steeper than in Delphinornis larseni , though not than in Delphinornis arctowskii ( Myrcha et al. 2002: figs. 10–12). It also differs from D. larseni in having the intercondylar eminence narrow and prominent and trochleae not massive. The distal vascular foramen poorly developed in comparison with that of D. larseni (Wiman 1905b: pl. 2: 2; Myrcha et al. 2002: fig. 10a; Ksepka et al. 2006: fig. 15). The articular surface of the trochlea III, unlike its counterpart in other species of Delphinornis , markedly narrowing towards the plantar surface of the shaft in plantar view (Wiman 1905b: pl. 2: 5 a and Myrcha et al. 2002: figs. 11b, 12b).

Remarks.—Wiman (1905a, b) erected six monotypic genera of Sphenisciformes from the La Meseta Formation, Seymour Island. Type specimens for five species are tarsometatarsi, Ichtyopteryx gracilis ( Fig. 2A ) and D. larseni being decidedly the smallest penguins within this assemblage. Ichtyopteryx gracilis was placed by Simpson (1971) in “dubious taxa”, because of the badly preserved holotype (distal tarsometatarsus). Myrcha et al. (2002) supplemented the genus Delphinornis with two species: D. gracilis and D. arctowskii , both based on tarsometatarsi ( Fig. 2B, C ) and representing small−bodied fossil penguins. Myrcha et al. (2002) proposed also a new generic diagnosis for Delphinornis based on features of the proximal tarsometatarsus. Unfortunately, this part is not preserved in Wiman’s (1905) specimen assigned to I. gracilis . Additionally, tarsometatarsi belonging to Delphinornis share a characteristic shape of a distal part of the doi:10.4202/app. 2009.1107

Table 1. Metric comparisons of the sphenisciform Delphinornis gracilis (Wiman, 1905) comb. nov. with other species of small−sized penguins from the Eocene La Meseta Formation, Seymour Island.

lateral extensor (intermetatarsal) sulcus ( Fig. 2 ): it is generally well marked with a U−shaped cross−section (slight in Marambiornis , moderately marked and V−shaped in Mesetaornis ) (PJ's personal observation; see also Myrcha et al. 2002). Interestingly, this feature is also conspicuous in I. gracilis .

The specimen assigned to I. gracilis , like those of D. arctowskii and D. gracilis , possesses a poorly developed distal vascular foramen. This is contrary to the condition (a specific feature; Myrcha et al. 2002) observed in Delphinornis larseni . Tarsometatarsi of D. larseni are also clearly larger than their counterparts in the above−mentioned taxa. Further investigation of tarsometatarsi assigned to D. gracilis and Ichtyopteryx gracilis revealed that they are closest to each other in terms of dimensions (Table 1). They also share a unique shape of the articular surface of the trochlea III in plantar view and this is the only new feature added to the specific diagnosis by Myrcha et al. (2002; see above). The assumption of their conspecificity is the most parsimonious explanation, hence the synonymisation ( I. gracilis has priority at specific level, Delphinornis has priority at generic level). Interestingly, the specific names in the above−mentioned binominals are homonyms (secondary homonymy; ICZN 1999: Art. 53.3 and 57.3).

Figures

Fig. 2.Tarsometatarsi assigned to the sphenisciforms from the late Eocene, upper La Meseta Formation of Seymour Island, in dorsal view. Assignments as proposed in this paper. A, B. Delphinornis gracilis (Wiman, 1905) comb. nov.; NRM−PZ A.20 (A), IB/P/B−0279a (B). C. Delphinornis arctowskii Myrcha, Jadwiszczak, Tambussi, Noriega, Gaździcki, Tatur and del Valle, 2002, IB/P/B−0484 (reversed to facilitate comparison).