Systematic Paleontology
Class AVES Linnaeus, 1758
Subclass NEORNITHES?
Gen. et sp. indet.
Material. MPEF-PV 7758 Distal end of left ulna
Provenance. Mirasol Chico stream; Southeastern margin of the Somún Curá Plateau, Chubut Province, Argentina. Maastrichtian (Upper Cretaceous), La Colonia Formation.
Description. The material is relatively large (see measurements), comparable in size with
Chauna torquata and
Coscoroba coscoroba among others and rounded in section. The condylus dorsalis ulnaris is prominent and distally extended not surpassing the condylus ventralis ulnaris. The caudal margin of the condylus dorsalis presents an acute margin. This margin confluences with the caudal margin of the facies ventralis tracing a large (obtuse) external angle.
The condylus ventralis ulnaris is blunt, ventrally located and positioned along the long axis in dorsal view. The condylus dorsalis is subrounded, as in most of the birds compared on caudal perspective. Subtle differences in the margin (the outline) of the fossil and other birds can be appreciated in Fig. 3, where in MPEF-PV 7758 it is slightly dorsoventrally elongated. The incisura tendineus is deep and proximo-distally elongated, associated with a thin and oblique bony flange for the insertion of the extensor metacarpi ulnaris. The condylus ventralis ulnaris is more distally extended than the condylus
209 dorsalis ulnaris, and both are in contact over a wide area at the most distal and dorsal part.
The tuberculum carpale is rounded and barely diverges from the shaft, although the real shape in the fossil can be obscured by the damage to its tip. The cranial margin of the tuberculum carpale is parallel to the shaft, and it is largely extended distally. The incisura tuberculum carpale is particularly wide and shallow. The articular facets of the condylus dorsalis, condylus ventralis, and tuberculum carpali are more ventrally oriented than those of the other specimens examined here. The depression radialis is not developed as a defined depression since this area consists of a flat surface barely differentiated from the rest of the facies ventralis. Measurements (expressed in mm). Cranio-caudal distal width 16.85, cranio-caudal width of the shaft 11.86, dorso-ventral width of the shaft 8.95, dorso-ventral distal width (corresponding to dorso-ventral width of the condylus dorsalis) 14.65, maximum extension of
222 the incisura tendineus 8.13.
Discussion: Among the Cretaceous Antarctic birds, the only specimen with a preserved distal portion of ulna is
Antarcticavis capelambensis SDSM 78147, although it is partially crushed and still attached to the radius. It is difficult to examine, and the features described above for MPEF-PV 7758 cannot be observed. Unfortunately, the elements known for the Paleocene
Conflicto antarcticus MLP 07-III-1-1, the Maastrichtian
Polarornis gregorii TTU P9265, and
Neogaeornis wetzelli GPMK 123, do not include the ulna. Finally, the holotype of
Vegavis iaai MLP 93-I-3-1 only preserves the proximal end of the ulna, whereas the second specimen 233 MACN-PV 19.748 includes a partial ulna still in contact with the radius and non-comparable at all. In the latter, the tuberculum carpale is less ventrally and distally projected than in the ulna of MPEF-PV 7758 (La Colonia bird hereafter), and the dorso-ventral width of the shaft is by far smaller (4.5 mm in MACN-PV 19.748 and 8.95 mm in La Colonia bird). However, according to the size comparisons made by West et al., (2019), the femur of
Vegavis sp. SDSM 78247, which would be larger than that of
Polarornis gregorii and nearly twice the femur of
Vegavis iaai, could be consistent in size with La Colonia bird.
The development of two distinctive condyles (condylus ventralis and condylus dorsalis) and the extension of the tuberculum carpale precludes the belonging of MPEF-PV 7758 to a non-avialan Paraves. Even in the most primitive Avialae, the distal ulna has a single widened end with undefined articular condyles. Most of the taxonomic features of their ulna refer to the relationship in length with the humerus and the radius, as an indicative parameter of the flight abilities, which are not applicable here. However, the features observed in the preserved portion of some Avialae permit extensive comparisons. For example, the incisura tendineus and the proximal small tuberosity described in the ulna of La Colonia bird, are not present in
Archaeopteryx lithographica or
Confuciusornis sanctus and are barely developed in
Limenavis patagonica and
Ichthyornis (Clarke and Chiappe, 2001; Clarke, 2004; Benito et al., 2022). These structures were extensively described in Enantiornithes, in which the condylus dorsalis is rounded and larger than the condylus ventralis (e.g., Walker and Dyke, 2009), and vary in depth and extension among neornithine birds. The incisura constitutes a deep and proximo-distally elongated depression in the La Colonia bird, as in Aix,
Phalacrocorax brasilianus,
Caracara plancus,
Larus dominicanus,
Milvago chimango,
Tachyeres patagonicus,
T. leucocephalus, and
Cygnus melanocoryphus among the modern species. In Macronectes giganteus, this incisura is strongly developed (the proximal end is deeper and narrower, and the proximo-dorsally end remains opened), whereas it is poorly delimited or absent in other compared taxa (e.g.,
Chauna torquata;
Chloephaga picta), and probably in
Martinavis (see Walker and Dyke, 2009). This incisura is short and proximally restricted in
Coscoroba coscoroba,
Dendrocygna viduata, and
Lophonetta specularioides, and more distally located and shallower in
Vultur gryphus. Proximal to the incisura tendinous there is a small and oblique crest in the La Colonia bird. It forms a more conspicuous bony flange in
Macronectes giganteus, it is slightly developed in
Stercorarius antarcticus, and is absent in
Ichthyornis and
Limenavis.
The ulna of Enantiornis, the most frequent bird in the Cretaceous of Argentina, is characterized by the presence of a pneumatic foramen located at the distal end of the ulna, which is not present in MPEF-PV 7758. Besides, as in other enantiornithine birds (including the Argentinian specimens from El Lecho Formation from El Brete; Walker and Dyke, 2009: the condylus dorsalis of
Enantiornis is rounded and more proximo-distally enlarged than that of MPEF-PV 7758. The tuberculum carpale (=carpal tuberosity) is barely developed in comparison with the robust tuberculum of the fossil described here
Also, although not conclusive, MPEF-PV 7758 is notably larger than in most Enantiornithine birds (e.g., Lawver et al., 2011).
The condylus dorsalis
of Limenavis is rounded and more transversally developed regarding the ulnar shaft, although with a minimal widening. The distal extension of this condyle is ventrally and dorsally equivalent, whereas in the La Colonia bird, it is more distally extended at the dorsal edge. The condylus dorsalis of
Ichthyornis is rounded and more dorso-ventrally symmetrical regarding the shaft, although distally truncated (Clarke, 2004; Benito et al., 2022). In the La Colonia bird, the condylus dorsalis is sub-rounded and more ventrally developed, as in
Cygnus melanocoryphus and
Coscoroba coscoroba, although the distal extension is less important in the fossil. In addition, it is more circular in
Vultur gryphus, and more proximo-distally elongated in
Aix galericulata,
Chloephaga picta, and
Macronectes giganteus.
The caudal margin of the condylus dorsalis forms an angle with the facies ventralis which varies greatly, being broad or obtuse in La Colonia bird, as well as in a broad spectrum of extant birds (e.g.,
Aix galericulata,
Chauna torquata,
Cygnus melanocoryphus,
Coscoroba coscoroba,
Phalacrocorax brasilianus,
Vanellus chilensis,
Fulica leucoptera,
Tachyeres, and
Chloephaga), and the Presbyornithidae
Telmabates antiquus. This angle is higher in other taxa such as
Lophortyx valifornica,
Odontophorus capueira,
Asio flameus,
Nothura paludosa,
Larus dominicanus,
Milvago chimango,
Mareca (
Anas)
sibilatrix,
Stercorarius antarcticus, and
Macronectes giganteus; and lower in
Aratinga leucophtalma,
Colaptes sp.,
Caracara plancus,
Buteo sp.,
Ortalis sp., Columbidae,
Lophonetta specularoides,
Dendrocygna viduata, and
Vultur gryphus.
The condylus ventralis ulnaris of the La Colonia bird resembles that of
Vultur gryphus,
Cygnus melanocoryphus,
Coscoroba coscoroba, and that of
Chauna torquata due to its blunt shape, and to
Cygnus melanocoryphus and
Phalacrocorax brasilianus regarding the relative position concerning the distal end and the ventral surface, and the perfect alignment with shaft axis. However, the shape of the condylus ventralis (in a ventral perspective) shows a wide variation. It appears notably pointed in
Phalacrocorax brasilianus and
Vanellus chilensis, more distally extended in
Aratinga leucophtalma, and cranially displaced in
Nothura paludosa,
Lophortyx californica,
Ortalis sp., and
Stercorarius antarcticus (just to mention some taxa). Besides, as in
Nycticorax nycticorax and
Chunga burmeisteri, the condylus ventralis is more asymmetrical and cranially inclined.
The relative distal extension of the condyles is also highly variable among taxa (and it also depends on the perspective). They are sub-equally extended (the ventralis is hardly more appreciably elongated) in the La Colonia bird, the Presbyornithidae
Wilaru,
Odontophorus capueira, and
Chauna torquata, condition more marked in
Chloephaga burmeisteri,
Aratinga leucophtalma,
Asio flameus, and
Gallus gallus, among crown birds Neornithes. On the other hand,
Telmabates,
Cygnus melanocoryphus, and
Coscoroba coscoroba, also with the condyles subequally extended, present the condylus dorsalis a little more projected than the ventralis.
The morphology of the tuberculum carpale seems more relevant regarding the clear differences observed with other ulnae. In fact, the tuberculum carpale presents a unique condition among all specimens compared. It is a robust and cranio-caudally wide structure in the La Colonia bird, whereas it appears more gracile and cranially deflected in
Ichthyornis and is, unfortunately, not observable in
Limenavis. The tuberculum carpale is craniodistally deflected, as in Presbyornithidae (Ericson, 1999, 2000), and resembles that of
Telmabates antiquus (Howard, 1955: fig. 5),
Wilaru tedfordi (de Pietri et al., 2016: fig. 1u), and
Cygnus in the marked distal elongation. Whereas in the La Colonia bird, the condyles and the tuberculum are similarly projected, the opposite condition appears, for example, in
Stercorarius antarcticus and
Colaptes sp., in which the condyla dorsalis and ventralis are significantly more distally extended than the tuberculum carpale.
The distalmost part of the tuberculum is parallel to the shaft, whereas in all the other families compared, it acquires different shapes but always diverges cranially from the diaphysis A similar configuration is developed in Presbyornithidae, whereas the Pelagornithidae exhibits an intermediate condition between the La Colonia bird and
Cygnus melanocoryphus, in which the divergence of the tuberculum carpale is insignificant. The incisura tuberculum carpale is also unique in shape in the fossil, more distally extended than in
Cygnus melanocoryphus and even more than in Pelagornirhidae (Bourdon et al., 2010
. It is almost as wide and shallow as in
Stercorarius antarcticus,
Lophonetta specularoides,
Cygnus melanocoryphus, and
Dendrocygna viduata. In
Chauna torquata, the incisura is shallow but narrow and it constitutes an intermediate condition between the La Colonia bird and most of the birds here compared.
In summary, we observed some similarities between
Limenavis and the La Colonia bird: both have the axis of major expansion of the condylus dorsalis transverse to the shaft. However, we also noticed stronger differences: in the La Colonia bird, the condylus dorsalis (the only one that can be compared) is more ventrally expanded, the distal margin is rounded and not truncated, and the incisura tendineus is more pronounced in extension and depth. In addition, similarities in the general configuration and relative extension and width of both condyles and the tuberculum carpale were noted between
Telmabates and the La Colonia bird. After a vast comparison with species of different taxawe discarded the possibility to include the la Colonia bird in a reliable phylogenetic analysis for a further discussion due to the lack of scorable characters in the datasets of Worthy et al. (2017). We selected that matrix, because it was extensively used during the analysis of different taxa such as
Conflicto antarcticus in Tambussi et al. (2019),
Asteriornis maastrichtensis in Field et al. (2020), and
Vegavis iaai in Acosta Hospitaleche and Worthy (2021), among others. The results are clearly affected by the fact that only three characters (152 to 154 from Worthy et al., 2019) could be certainly observed in the new ulnaial 2). Besides, our observations suggest that the features scored in the La Colonia bird are not informative enough for taxonomic purposes. .
The presence of all the features described above is consistent with a neornithine identity. Among them, the ulnae of Palaeognathae birds are completely variable. It is extremely derived in flightless paleognathous birds, as for example,
Struthio camelus,
Casuarius casuarius, and
Dromaius novaehollandiae, in which the shaft gradually widens toward the ends, constituting a convex structure, not comparable with the distal end of the Neognathae birds. In the Lithornithidae (e.g.,
Lithornis promiscuus USNM PAL 336535 and
L. plebius USNM PAL 336534), on the contrary, the condylus dorsalis ulnaris is sub-rounded and distally truncated (Houde, 1988: fig. 19), and in Tinamiformes, the condyles and the tuberculum carpale are not as well defined as in the La Colonia bird and the Neognathae birds in general.
In summary, an overall resemblance with Anseriformes, and particularly with Presbyornithidae, is noted. It could be indicating possible affinities with these groups or be interpreted as superficial similarities due to convergences or retention of primitive characters.
Fred
Fig. 1. Ulna of La Colonia bird MPEF-PV 7758. A, dorsal; B, caudal; C, transversal section of the shaft; D, ventral; E, cranial; F, distal views. Abbreviations: bf, bony flange; cdu, condylus dorsalis ulnaris; cvu, condylis ventralis ulnari; it, incisura tendinosa; itc, incisura tuberculum carpale; lrui, Lig. radioulnare interosseum; luud, Lig. ulno-ulnocarpale distale; luri, Lig. ulno-radiocarpale inter 575 osseum; luup, Lig. ulno-ulnocarpale proximale; si, sulcus intercondylaris; tc, tuberculum carpale.
Scale bar: 10 mm. [planned for double column width]
