Cariamiforms from the Submeseta Formation (1 Viewer)

[albertonykus]

Acosta Hospitaleche, C. and W. Jones (2024)
Were terror birds the apex continental predators of Antarctica? New findings in the early Eocene of Seymour Island
Palaeontologia Electronica 27: 13
doi: 10.26879/1340

Two ungual phalanges attributed to large birds were collected in the Ypresian (early Eocene) levels of the Cucullaea Allomember (Submeseta Formation). Both materials were found in localities in proximity on Seymour Island in West Antarctica. The pronounced curvature, considerable size robustness, and the extension of the flexor tubercle provide compelling evidence for their classification within Cariamiformes. Additionally, the results of quantitative analyses strongly support this assignment to Phorusrhacidae or a Phorusrhacidae-like bird resembling Phorusrhacos longissimus. These phalanges belonged to a large or even giant predator, estimated to have had a substantial body mass of around 100 kg. It is highly likely that this bird was an active predator, hunting and feeding on small marsupials and medium-sized ungulates. This finding fundamentally changes our understanding of the dynamic within the Antarctic continental ecosystems during the early Eocene. It reveals that large carnivorous birds assumed the role of continental apex predators apparently sub-occupied by mammals.

 

[Fred Ruhe]

SYSTEMATIC PALEONTOLOGY

Class Aves Linnaeus, 1758
Subclass Neognathae Pycraft, 1900
Order Cariamiformes Verheyen, 1957
?Phorusrhacidae Ameghino, 1889

Material 1. MLP-PV 13-XI-28-546 complete right ungual phalanx

Provenance. Fossil locality IAA 2/13, Seymour
Island (Antarctic Peninsula, West Antarctica). Ypresian (early Eocene) Cucullaea I Allomember, La Meseta Formation.

Description. The specimen consists of a robust, laterally compressed, and curved ungual phalanx with a small dorsal fragment of the tip missing. The neurovascular sulcus is present along the entire length of both lateral and medial surfaces, though it is slightly wider and deeper on the medial side. This sulcus is situated closer to the plantar surface than to the dorsal margin and is proximally shallower and wider. On the medial side, the sulcus bifurcates in the proximal region, with the lower branch extending further proximally. As it approaches the tip, the sulcus becomes more dorsally positioned and is partially covered by a thin bony layer in certain areas. The claw becomes increasingly latero-medially compressed toward the tip.
The articular surface exhibits a sub-triangular to ovoidal outline, with a main dorso-ventral axis. The plantar side is rounded and approximately twice as wide as the dorsal margin, making the lateral and medial margins of the articular
surface visible in dorsal view. The extensor tubercle is short and rounded, with minimal proximal projection. The articular surface is divided into two cotylae of similar size, slightly asymmetrical and dorsoventrally elongated. The lateral cotyla leans toward the lateral side, while the medial cotyla leans medially. Between them, the articular surface gently rises as a rounded ridge which is more pronounced in the area that contacts the flexor and extensor tubercles. The flexor tubercle is robust, rounded, and extends more toward the plantar and proximal directions than the extensor tubercle.

Fred


FIGURE 1. Fossil cariamiforms examined here. Ungual phalanx of the second right digit MLP-PV 13-XI-28-546 (A, C, E, G) in lateral (A), dorsal (C), medial (E), and proximal (G) views, and ungual phalanx of second digit MLP-PV 14-I-10-199 (B, D, F) in lateral or medial (B, F) and dorsal (D) views. Scale bar: 10 mm.

 

[Fred Ruhe]

Material 2. MLP-PV 14-I-10-199 incomplete ungual phalanx

Provenance. GPS S64°14’24.3’’, W56° 40’ 1.9’’, Seymour Island (West Antarctica). Cucullaea I Allomember, La Meseta Formation.

Description. The overall appearance of this specimen 0 closely resembles MLP-PV 13-XI-28-546, although it is more fragmentary, and some features cannot be observed. The entire proximal face and dorsal surface are lacking, and the entire surface is covered with diagenetic cracks. Only the lateromedial and plantar facies of the phalanx body are
preserved. As a result, it is not possible to obtain linear and angular measurements. The neurovascular sulcus is present on
both sides, like in MLP-PV 13-XI-28-546

Anatomical identification. The ungual phalanges display characteristic morphologies for each digit with minor differences between families. The ungual of digit I is the smallest, relatively gracile, and weakly curved, and presents a long flexor tubercle. The ungual of digit II is highly curved, lateromedially compressed, dorsoventrally tall (with a sub-elliptic transverse section) and possesses a well-developed plantarly rounded flexor tubercle. The ungual of digit III is less curved and dorso-ventrally lower than digit II, with a short and plantarly flattened flexor tubercle (although it is caudally expanded in Mesembriornis and Chunga burmeisteri and develops lateral wings in Brontornis burmeisteri); consequently, it has a sub-triangular section. The ungual of digit IV is less curved and plantarly wider than those of digits II and III.

Fred