Gavia sp.

Wu, Siao-Man, Worthy, Trevor H., Chuang, Chih-Kai & Lin, Chien-Hsiang, 2023, New Pleistocene bird fossils in Taiwan reveal unexpected seabirds in East Asia, Acta Palaeontologica Polonica 68 (4), pp. 613-624 : 616-618

Gavia sp.

Figs. 2A , 4A .

Material.—CMM 2123 left humerus, Bazhang River, Niubu area, Taiwan; Liuchungchi Formation ( Fig. 3 ); 1.95–1.35 Ma.

Measurements. —See Table 1.

Description.—The shaft of the distal left humerus CMM 2123 is stout and rounded in cross section, and is notched craniodorsally proximal to the tuberculum supracondylare dorsale. Distally, the shaft forms an angle of 150° with respect to the proximal side of the tuberculum supracondylare dorsale. The tuberculum supracondylare dorsale is dorsally prominent and proximodistally overlaps the condylus dorsalis such that the apex is located distal to the most proximal part of the condyle; the sharp and elevated apex extends smoothly to the shaft. The fossa m. brachialis is shallow, proximally elongate, and has its narrow proximal end close to the ventral margin of the fossil, making the fossa remarkably broad. The non-elevated tuberculum supracondylare ventrale is elongated and narrow, and has a narrow proximal margin close to the fossa m. brachialis. The facet where the ligamentum collaterale ventrale attaches to the tuberculum supracondylare ventrale is poorly defined, and slight ventral expansion distally. The proximal end of the facet lies only slightly proximal to the level of the proximal apex of the condylus dorsalis. The condylus ventralis is rounded with a proximodistal length about half of the condylus dorsalis. The two condyli are separated by a narrow incisura intercondylaris. On the caudal side, the sulcus scapulotricipitalis is well-defined and deep, with the two margins slightly converging proximally, and the processus flexorius is missing ( Figs. 2 , 4A 3 ).

Remarks. —CMM 2123 is referred to Gavia based on (i) broad fossa m. brachialis that comes close to the medial margin proximally, (ii) the elongate, narrow, and non-elevated tuberculum supracondylare ventrale, (iii) pronounced dorsally projected tuberculum supracondylare dorsale, and (iv) shaft notched craniodorsally proximally-adjacent to the tuberculum supracondylare dorsale. CMM 2123 represents an adult as displayed by the smooth surface, and the two condyli and processes with clear shapes as displayed in the modern adult specimens ( Fig. 4 ). In addition, CMM 2123 shares the following qualitative features with humeri of modern Gavia adamsii G.R. Gray, 1859 ): the relatively sharp (in cranial aspect) apex of tuberculum supracondylare dorsale, the smooth dorsal surface on the proximal side of tuberculum supracondylare dorsale, narrow (less than half width of the tuberculum) and non-elevated proximal end of the facet for the attachment of ligamentum collaterale ventrale on the tuberculum supracondylare ventrale, and the sulcus scapulotricipitalis with deep distal end and slightly proximally converged margins ( Figs. 2 , 4 ). However, the fossil humerus is rather smaller than those of the examined G. adamsii specimens, and is also relative larger than the examined G. stellata ( Pontoppidan, 1763) specimens Fig. 4B, E ; Table 1). This excludes the possibility of identifying the humerus as either Gaviaadamsii or G. stellata . Gavia artica ( Linnaeus, 1758) ( Fig. 4C ) and Gavia pacifica Lawrence, 1858 ) ( Fig. 4D ), while of similar size to the fossil Table 1), differ by having larger facets for the attachment of ligamentum collaterale ventrale on the tuberculum supracondylare ventrale, having prominent scars for the palmar branch of m. extensor carpi radialis that interrupt the dorsal profile proximal to the apex of the tuberculum supracondylare dorsale, and a much shallower sulcus scapulotricipitalis; G. immer ( Fig. 4F ) is larger, and has a much more massive facet for the attachment of ligamentum collaterale ventrale on the tuberculum supracondylare ventrale that more greatly separates the fossa brachialis from the ventral shaft facies, a prominent scar for the palmar branch of m. extensor carpi radialis that interrupts the dorsal profile proximal to the apex of the tuberculum supracondylare dorsale, a less rounded condylus ventralis, and a much broader sulcus scapulotricipitalis that is bound ventrally by a lower ridge.

Gavia stellata ( Pontoppidan, 1763)

Figs. 2B , 5A .

Material.— CMM 2124 tibiotarsus, Bazhang River, Niubu area, Taiwan; Liuchungchi Formation ( Fig. 3 ); 1.95–1.35 Ma.

Measurements. —See Table 1.

Description. —The shaft of the tibiotarsus CMM 2124 is slightly compressed craniocaudally with an oval cross section, and the distal end expands medially, with and the condylus medialis extending slightly further distally than the condylus lateralis. The articular scar for the distal fibula is, most proximally, on the lateral side of the shaft. It distally locates increasingly cranially and terminates near the proximal end of the medial tuberculum retinaculum m. fibularis. The medial tuberculum retinaculum m. fibularis is more elevated cranially and extends farther proximally than its lateral counterpart, which is prominent laterally, projecting slightly further laterally than the condylus lateralis. Proximally, both tubercula converge cranially, and distally, they converge laterally, which makes the sulcus m. fibularis shallow. The sulcus m. fibularis sits between the tubercula and faces slightly laterally. Distally, the sulcus m. fibularis passes caudally to a small lateral prominence at the distal end of the lateral tuberculum retinaculum m. fibularis. The medial tuberculum extends distally to join a small narrow laterally-facing crest and hence to the condylus lateralis at its proximolateral point. The effect of this is that proximally the sulcus m. fibularis is enclosed by medial and lateral tubercula, but at the level of the proximal margin of the pons supratendineus, the sulcus passes onto the lateral shaft facies. A distinct sulcus sits adjacent to the proximal end of condylus lateralis and is bounded laterally by the distal end of the medial tuberculum retinaculum m. fibularis. This sulcus is interpreted as the distolateral insertion scar of the transverse ligament. The pons supratendineus is wide proximodistally, and its proximomedial side has a rounded and elevated insertion scar, for the proximomedial insertion of the transverse ligament. The canalis extensorius is centered on the shaft and opens widely proximodistally between the condyli, and is close to the condylus medialis.

Remarks. —The fossil tibiotarsus displays several characteristics that are typical of Gavia : (i) parallel condyli that are proximodistally shorter than the distal width of the bone and are medially inclined distally, (ii) proximally-elevated medial and laterally-projected lateral tuberculi retinaculum m. fibularis, with a shallow sulcus m. fibularis in-between, (iii) a proximodistally wide pons supratendineus, (iv) a rounded scar abutting the proximomedial margin of the pons supratendineus for the proximomedial insertion of the transverse ligament, (v) a pit sits beside the end of the thin ridge immediately proximal to the condylus lateralis for the distolateral insertion of the transverse ligament, (vi) a wide opening of the canalis extensorius which is aligned proximodistally between the condyli (not proximal to the condyli) and is medially close to the condylus medialis, and (vii) a proximodistally short incisura intercondylaris with respect to the adjacent condyli. The fossil is similar to G. stellata , especially regarding the fact that the caudomedial margin of the condylus medialis lacks a caudomedially prominent rounded projection as seen in all the other modern gaviids compared ( Fig. 5 ). This characteristic clearly separates the fossil and G. stellata from the other extant species of Gaviidae . While the mediolateral width of the shaft with respect to the distal end of the fossil in the cranial view seems to be smaller than G. stellata (see Table 1), we think this disparity falls within intraspecific scope and, therefore, we prefer to identify the fossil as G. stellata ( Table 1).

Figures

Fig. 2. The fossils of gaviid seabirds from Niubu, Chiayi, Taiwan, Liuchungchi Formation, Lower Pleistocene. A. Gavia sp., CMM 2123, distal left humerus in cranial (A1, A2), dorsal (A3), caudal (A4), ventral (A5), and distal (A6) views. B. Gavia stellata (Pontoppidan, 1763), CMM 2124, distal left tibiotarsus in cranial (B1, B2), lateral (B3), caudal (B4), medial (B5), and distal (B6) views. Arrows highlight the articular scar for the distal fibula.

Fig.4.The humeri of the fossil gaviid seabird Gavia sp. (A) and modern Gaviidae (B–F) in cranial (A1–F1), dorsal (A2–F2), caudal (A3–F3), ventral (A4–F4), and distal (A5–F5) views. A. CMM 2123 from Niubu, Chiayi, Taiwan, Liuchungchi Formation, Lower Pleistocene. B. Gavia stellata (Pontoppidan, 1763) (YIO 79598).C. Gavia arctica (Linnaeus, 1758) (YIO 72000). D. Gavia pacifica (Lawrence, 1858) (YIO 64763).E. Gavia adamsii (G.R. Gray, 1859) (YIO 63205).F. Gavia immer (Brünnich, 1764) (NHMUK S/1987.13.1).The images of CMM 2123 were obtained from CT.The image order of modern Gaviidae follows the taxonomy established by Sprengelmeyer (2014) and the photos of G. immer are courtesy of Junya Watanabe.

Fig. 3. Stratigraphy of the Western Foothills of southern Taiwan (modified after Chen 2016; Lin et al. 2021). Grey block highlights the Liuchungchi Formation, wherein the bird fossils were found. Only the Early Pleistocene of southern Taiwan is shown.

Fig. 5. Tibiotarsi of the fossil gaviid seabird Gavia stellata (Pontoppidan, 1763) (A) and modern Gaviidae (B–F), in cranial (A1–F1), lateral (A2–F2), caudal (A3–F3), medial (A4–F4), and distal (A5–F5) views. A. CMM 2124 from Niubu, Chiayi, Taiwan, Liuchungchi Formation, Lower Pleistocene. B. Gavia stellata (Pontoppidan, 1763) (YIO 79598). C. Gavia arctica (Linnaeus, 1758) (YIO 79597). D. Gavia pacifica (Lawrence, 1858) (YIO 60621). E. Gavia adamsii (G.R. Gray, 1859) (YIO 74354); F. Gavia immer (Brünnich, 1764) (NHMUK S/1987.13.1).Arrows on C4–F4 and C5–F5 indicate the caudomedial process present in these taxa that is lacking in G. stellata. The image order of modern Gaviidae follows the taxonomy established by Sprengelmeyer (2014) and the photos of G. immer are courtesy of Junya Watanabe.