Early Plotopteridae from Japan (1 Viewer)

[Fred Ruhe]

Hirotsugu Mori & Kazunori Miyata, 2020

Early Plotopteridae specimens (Aves) from the Itanoura and Kakinoura Formations (latest Eocene to early Oligocene), Saikai, Nagasaki Prefecture, western Japan

Paleontological Research in press
oi:10.2517/2020PR018

Abstract and pdf: http://www.palaeo-soc-japan.jp/publications/pr/2020PR018_Mori_Miyata.pdf

Plotopterids, commonly known as “penguin-like birds”, are wing-propelled diving birds known from the latest Eocene to middle Miocene in the eastern and western Pacific Rim. Here, we describe two new specimens of the family Plotopteridae, a right femur from the Itanoura Formation (latest Eocene to earliest Oligocene), and a distal half of a right tibiotarsus from the Kakinoura Formation (early Oligocene), both at the lower part of the Nishisonogi Group, Saikai City, Nagasaki Prefecture, Kyushu, southwestern Japan. The femur is slightly younger than or potentially as old as the earliest plotopterid known from Japan and the U.S.A. CT scanning revealed that it has a dense cortical bone, justifying its taxonomic assignment to the family Plotopteridae. It resembles the femur of Olympidytes, which is endemic to North America previously, in its femoral neck, well developed trochanter femoris, and straight facies articularis antitrochanterica, but is not assignable to any known genus. The tibiotarsus resembles that of Olympidytes in the presence of a well-developed trochlea catilaginis tibialis, a large embossment lateral to the pons supratendineus, and a deep incisura intercondylaris, and therefore referable to Olympidytes. The possibility of hindlimb-propelled diving in the family Plotopteidae was also discussed. These new specimens suggest the early diversity of the family in Japan was higher than previously thought.

Enjoy,

Fred

 

[Fred Ruhe]

Systematic Paleontology

Class Aves Linnaeus, 1758
Family Plotopteridae Howard, 1969
Plotopteridae gen. et sp. indet.
Material.–SM–SKT–940, a right femur Localities and geological settings—The femur (SM–SKT-940) was discovered from the Shirikusari Beach, Setofukushima, Saikai City. 32°55’36” N, 129°38’16” E (WGS 84 datum), middle part of the Itanoura Formation, latest Eocene or earliest Oligocene. It was found in a medium-grained sandstone, about 40 m above the bottom of the Itanoura Formation (about 90 m in total thickness), from the basal part of the Nishisonogi Group (Figure 2BC). The Itanoura Formation is mainly composed of cross-laminated sandstones and yields numerous marine molluscan fossils reflecting shallow marine depositional environments (e.g. Nagao, 1928a; Mizuno, 1964; Hattori et al., 1993). Miyachi and Sakai (1991) reported the fission track dating (37.8±4.0 Ma and 37.9±3.5 Ma) of the tuff and tuffaceous layers interbedded in the basal part of the Itanoura Formation, which supports the correlation to the late Eocene in age. In addition, Okada (1992) assigned the calcareous nannoflora from the lower part of the Itanoura Formation to the CP16a subzone (ca. 34.44-33.43 Ma, the latest Eocene to earliest Oligocene; Vandenberghe et al., 2012) and the upper part of the formation to the following early Oligocene CP16b subzone (ca. 33.43-32.92 Ma). These fission track and nannofloral ages support that the Eocene–Oligocene boundary is potentially positioned within the lower half of the Itanoura Formation; thus, the horizon of SM–SKT–940 in the middle part of the formation should be viewed as close to the Eocene–Oligocene boundary. However, no evidence clarifies the position of the Eocene–Oligocene boundary in the formation, and some workers believe the Itanoura Formation is younger than the Eocene–Oligocene boundary without reasonable geochronological evidence (cf. Yamaguchi, 2004; Yamaguchi et al., 2006). Measurements.—Length=136.5 mm; Proximal width=41.5 mm; Caput femoris depth=11.3 mm; Shaft width=18.4 mm; Shaft depth= 14.8 mm; Distal width=35.9 mm.

Cf. Olympidytes sp. Mayr and Goedert, 2016
Material.– SM–SKT–1153, a distal portion of the right tibiotarsus
Locality and geological setting—The tibiotarsus (SM–SKT–1153) was discovered from the Yanagi Coast, Saikai City, Nagasaki Prefecture, lower part of the Kakinoura Formation (Nishisonogi Group, Figure 2BC). The specific horizon is a shell bed of Ostrea lunaeformis, reported by Hattori et al. (1993). Calcareous nannofossils of the Kakinoura Formation (Okada, 1992) are of CP16b-c (33.9-32.0 Ma; Anthonissen and Ogg, 2012), indicating the early Oligocene. This result is congruent with the detrital zircon age, 33.9±3.3 Ma, of a tuff layer of the Sari Formation (Kishima Group; Miyachi and Sakai, 1991), which correlates to the Tokuman Formation overlying the Kakinoura Formation.

Fred

Figure 1. Photograph SM–SKT–940, a right femur of Plotopteridae gen. et sp. indet. Cranial (A), caudal (B), medial (C), lateral (D), proximal (E) and distal (F) views. Breakage of the specimen in cranial (G), caudal (H), and lateral (I) views.

Figure 2. SM–SKT–1153, a distal portion of the right tibiotarsus of cf. Olympidytes sp. Cranial (A), caudal (B), medial (C), proximal (D), and distal (E) views.