CARIAMIDAE AND

Hospitaleche, Carolina Acosta & Jones, Washington, 2024, Were terror birds the apex continental predators of Antarctica? New findings in the early Eocene of Seymour Island, Palaeontologia Electronica (a 13) 27 (1), pp. 1-31 : 13-15

COMPARISONS WITH CARIAMIDAE AND PHORUSRHACIDAE

MLP-PV 13-XI-28-546 and MLP-PV 14-I-10- 199 fall within a size range comparable to Phorusrhacos longissimus (MLP-PV 20-572, MLP-PV 67- VIII-28-1, and MLP-PV 20-139). Both, MLP-PV 13- XI-28-546 and MLP-PV 14-I-10-199, are larger than Psilopterus colzecus , Patagornis marshi , and Andrewsornis abbotti , but smaller than Devincenzia pozzi ( Figure 8 ).

The latero-medial compression of MLP-PV 13-XI-28-546 and MLP-PV 14-I-10-199 is less pronounced compared to Psilopterus but comparable to compression observed in Phorusrhacos and Devincenzia . This feature is closely related to the shape of the articular (proximal) surface. The articular surface is dorso-ventrally (=dorso-plantarly) lower than that of Devincenzia , Ps. colzecus , Patagornis , and Procariama , which have an ovoidal outline. MLP-PV 13-XI-28-546 instead approaches the condition observed in Phorusrhacidae MPEFPv 12156, and Psilopterinae MLP-PV 90-III-5-56, which have a wider plantar surface and a more sub-triangular outline.

From a qualitative point of view, the internal and external curvatures can differentiate hooked phalanges. Among Cariamiformes , the most pronounced curvature (or lower inner and outer curvature angles) is found in Cariama cristata , followed by Chunga burmeisteri and Ps. colzecus . Although phalanges MLP-PV 13-XI-28-546 and MLP-PV 14- I-10-199 may appear as hooked as Phororhacus, based on t the preserved portions, they exhibit a more moderate curvature (see Table 1).

A parameter closely related to the inner and outer curvature is the robusticity index, which was calculated at 0.41. This value supports the inclusion of MLP-PV 13-XI-28-546 (and Phorusrhacos longissimus ) within the category of ground and predatory birds.

In MLP-PV 13-XI-28-546, the flexor tubercle is elongated and rounded, although narrower and proximo-plantarly more projected than in other members of Cariamiformes . The flexor tubercle of MLP-PV 13-XI-28-546 is distally wider and more proximally projected, whereas it projects more plantarly in Titanis . In Devincenzia and P. colzecus , the flexor tubercle is wider and sturdier, and slightly more plantarly projected, while in Brontornis is also wider but shorter and less elongated. In Procariama simplex , the flexor tubercle is similarly elongated, but has truncated tip. In extant Cariamidae , the flexor tubercle is elongated and plantarly pointing, slightly more rounded in Cariama cristata and sharp in Chunga burmeisteri . The flexor tubercle is shorter, and not elongated in Patagornis , and Messembriornis, and slightly bulkier towards the sides in MLP-PV 13-XI-28-546 and Andrewsornis abbotti (MLP-PV 59-II-26-83) than in other phorusrhacids and cariamids. It is plantarly flat in Phorusrhacos longissimus MLP-PV 67-VIII-28-1 and rounded in MLP-PV 20-572, Cariama cristata , Psilopterus colzecus , and Devincenzia pozzi . Unfortunately, these features cannot be objectively compared in MLP-PV 14-I-10-199 ( Figure 8 ).

The extensor tubercle of MLP-PV 13-XI-28- 546 is short and not proximally projected like in Chunga burmeisteri and Cariama cristata and Procariama simplex and Psilopterus colzecus . In Phorusrhacos longissimus , the extensor tubercle moderately projected, but definitively more than in MLP-PV 13-XI-28-546. In Brontornis the extensor tubercle is also proximally expanded, but more rounded and robust, delimiting dorsally the articular proximal facets. In a proximal view, the extensor tubercle coincides with separation of the medial and lateral cotylae. Elongation of the extensor tubercle of MLP-PV 13-XI-28-546 is less pronounced than in Titanis and resembles Paraphysornis and Patagornis marshi .

The neurovascular sulcus ( Figure 9 ) is open and marked in MLP-PV 13-XI-28-546 and MLP-PV 14-I-10-199, a feature also observed in Patagornis marshi MLP-PV 20-164, but not in Patagornis marshi MLP-PV 20-184, where the sulcus is shallower and barely marked. In Andrewsornis abbotti MLP-PV 59-II-26-83 and Brontornis burmeisteri MLP-PV 20-570, the neurovascular sulcus is absent, probably due to weathering or development of an internal channel (not externally visible) rather than an open sulcus. In Psilopterus colzecus , the sulcus is not visible in digit II, but is present in digit III. This sulcus is proximally shallower in Phorusrhacos longissimus MLP-PV 67-VIII-28-1.

Figures

FIGURE 8. Antarctic ungual phalanx compared with different Phorusrhacidae species in lateral view. A, MLP-PV 13- XI-28-546; B, Phorusrhacos longissimus (AMNH 9497 taken from Sinclair and Farr 1932 and mirrored); C, Procariama simplex MACN 8275; D, Phorusrhacus (MLP-PV 20-572); E, Paraphysornis brasiliensis; F, Patagornis marshi MLP-PV 20-184; G, Procariama simplex MACN 8225; H, Devincenzia pozzi MACN Pv 6681; I, Titanis walleri (calcotype UF 10417); J, Psilopterinae indet. MLP-PV 90-III-5-56; K, Mesembriornis milneedwardsi MACN Pv 5944; L, Patagornis marshi MLP-PV 20-164; M, Psilopterus colzecus MLP-PV 76-VI-12-2; N, Psilopterinae indet. MPEF-PV 12256; O, MMP s/n Phorusrhacidae (re-drawn from Cenizo et al., 2012). Scales bar: 10 mm (except for C, G, M, and N where the scale represents 20 mm).

FIGURE 9. Three-dimensional reconstructions (not scaled) of different ungual phalanges. The dotted lines indicate the transversal cut areas shown below each phalanx. The arrowheads mark the path of the neurovascular sulcus and/or neurovascular canal along the phalanx. A, Patagornis marshi (MLP-PV 20-85, digit III); B, Patagornis marshi (MLP-PV 20-86, digit III); C, Psilopterus colzecus (MLP-PV 76-VI-12-2, digit II); D, Brontornis burmeisteri (MLP-PV 20-570, digit III); E, Phorusrhacos longissimus (MLP-PV 67-VIII-28-1, digit II); F, Andrewsornis abbotti MLP-PV 59-II-26-83 (digit II).