Primozygodactylus eunjooae, Mayr & Zelenkov, 2009

Mayr, Gerald & Zelenkov, Nikita, 2009, New specimens of zygodactylid birds from the middle Eocene of Messel, with description of a new species of Primozygodactylus, Acta Palaeontologica Polonica 54 (1), pp. 15-20 : 16-17

Primozygodactylus eunjooae sp. nov.

Figs. 1 , 2 .

Etymology: The species is dedicated to the wife of the senior author, Eun−Joo Shin (pronounce “oon−joo−ae”).

Holotype: SMF−ME 1074, incomplete postcranial skeleton ( Fig. 1 ).

Type locality: Messel near Darmstadt, Hessen, Germany.

Type horizon: Early middle Eocene, MP 11 ( Legendre and Lévêque 1997).

Diagnosis.—Slightly larger than Primozygodactylus danielsi ( Table 1), and with proportionally shorter proximal phalanges of the second and third toes ( Fig. 2 ). Whereas the tarsometatarsus measures 21/ 21.7 mm (versus 18.3/ 19.6 mm in the holotype of P. danielsi ), the length of the proximal phalanx of the second toe is only 3.1 mm (versus 4.8 mm in the holotype of P. danielsi ), and that of the third toe 4.1 mm (versus 5.4 mm in the holotype of P. danielsi ). The other species of Primozygodactylus are distinctly larger ( Table 1).

Measurements.—See Table 1.

Description and comparisons.—As far as comparisons are possible, the wing bones are similar to those of the other species of Primozygodactylus , and do not exhibit features which were not already described for the latter ( Mayr 1998).

The distal end of the right tarsometatarsus, however, is preserved in a completely uncrushed condition, and for the first time allows its detailed examination in one of the Messel zygodactylids. The bone is visible in plantar view, and one of its most unusual features is the presence of a very deep fossa distal of the large foramen vasculare distale. This fossa is steeply sloping medially and laterally, and distally undercuts the lateral rim of the asymmetric trochlea metatarsi III ( Fig. 2A 2 ). The trochlea metatarsi II is small and has an odd, squarish shape, with an almost flat distal surface; its lateral portion is damaged. In contrast to Zygodactylus ( Mayr 1998: fig. 28B) and Primozygodactylus danielsi ( Fig. 2 ), there is no plantarly directed projection on this trochlea. The large trochlea accessoria is separated from the trochlea metatarsi IV by a marked furrow. Distally, it only extends to the base of the trochlea metatarsi III, whereas it reaches its midst in Zygodactylus . The rims of the trochlea metatarsi III are widely separated by a marked furrow. The fossa metatarsi I is shallow.

The toes are less slender than those of P. danielsi , and the second and third toes have proportionally shorter proximal phalanges. Whereas the tarsometatarsus of P. eunjooae is slightly longer than that of the holotype of P. danielsi ( Table 1), the proximal phalanx of the second and third toes are shorter ( Fig. 2 and measurements above). The second toe is much thinner than the third and fourth toes.

The specimen is of particular interest because the tail feathers are exceptionally well preserved ( Fig. 1 ). These were so far only known from the holotype of P. major , where they do not allow the recognition of details ( Mayr 1998). In the P. eunjooae holotype there are two long central rectrices with broad vanes and a rounded tip, which have a length of about 84 mm. A pair of smaller adjacent feathers are visible which measure 40 mm. The outer tail feathers appear to have been very short, but only one of these is clearly visible in the specimen and measures about 20 mm. Although this possibility cannot be excluded, there is likewise no indication that the specimen is a moulting individual. A similar accentuation of the central pair of rectrices is found in few extant birds, such as emu−wrens ( Stipiturus spp. , Passeriformes : Maluridae ).

The specimen also exhibits remains of the wing feathers, which indicate a rather short and broad wing, with a rounded tip. On the left wing, the distalmost, i.e., tenth, primary measures about 27 mm, but the length of the other wing feathers cannot be unambiguously determined (as preserved, the longest primary of the left wing measures about 35 mm, but its tip may not be completely preserved in the specimen).

A medium−sized (3.5 × 2.5 mm) seed is situated close to the specimen, and in all likelihood stems from the stomach or gut content of the animal.

Figures

Fig. 1. Zygodactylid bird Primozygodactylus eunjooae sp. nov., holotype (SMF−ME 1074) from the middle Eocene of Messel in Germany. A. Specimen with remains of feathering. B. Specimen coated with ammonium chloride.

Fig. 2. Foot of zygodactylid bird Primozygodactylus eunjooae sp. nov. (A) from the middle Eocene of Messel in Germany in comparison to the feet Primozygodactylus danielsi Mayr, 1998 (B) from Messel and Zygodactylus luberonensis (C) from the early Oligocene of the Lubéron in France. A. Left (A1) and right (A2) foot of P. eunjooae sp. nov., holotype (SMF−ME 1074). B. Left (B1) and right (B2) foot of P. danielsi Mayr, 1998, holotype (SMF−ME 2522a+b). C. Left (C1) and right (C2) foot of Zygodactylus luberonensis Mayr, 2008 (SMF Av 519). Note the proportionally shorter proximal phalanges of the second and third toes of P. eunjooae. Specimens in A and B coated with ammonium chloride.