Chloephaga dabbenei, Agnolín & Álvarez Herrera & Tomassini, 2024

Agnolín, Federico L., Álvarez Herrera, Gerardo P. & Tomassini, Rodrigo, 2024, Pleistocene record of Chloephaga Eyton, 1838 (Anseriformes: Anatidae) in the Argentine Pampas, with the description of a new species, Comptes Rendus Palevol 23 (18), pp. 241-255 : 244-250

Chloephaga dabbenei n. sp.

( Figs 2-9)

urn:lsid:zoobank.org:act:96

TYPE MATERIAL. — Holotype. Argentina • PV-UNS-54, left tarsometatarsus with slightly abraded hypotarsus.

REFERRED MATERIAL. — Argentina • PV-UNS-149 , right coracoid lacking sternal end; PV-UNS-364, left ulna lacking proximal end; PV-UNS-562, proximal end of left ulna; PV-UNS-428, left femur. The specimens are referred to C. dabbenei n. sp. because they came from the same site, are congruent in most characters and they are notably robust and large, falling among the largest specimens of C. picta .

STRATIGRAPHIC AND GEOGRAPHIC PROVENANCE. — Facies Gm (holotype) and Sp (referred materials), Lower Section of the San José Sequence. Middle Pleistocene. Argentina, Buenos Aires province, Bajo de San José.

DIAGNOSIS. — Large species of the genus diagnosable on the basis of the following unique combination of characters: 1) very elongate tarsometatarsus with proximal end showing a thick, prominent, and proximodistally extended (more than one third of the entire bone length) surface for the lateral collateral ligament; 2) distal trochleae subparallel to each other and trochlea II strongly posteriorly retracted; and 3) articular surfaces of distal trochleae proximodistally elongated (particularly evident on the elongate subrectangular contour of trochlea III in posterior view).

ETYMOLOGY. — The specific epithet honors the great ornithologist Roberto Dabbene (1864-1938).

LOCALITY. — Argentina, Buenos Aires province, Bajo de San José.

DESCRIPTION AND COMPARISONS

All the specimens assigned to Chloephaga dabbenei n. sp. are notably large and robust ( Table 1 ).

The coracoid shows a notably thick neck (much thicker than any known species). Chloephaga poliocephala has the narrowest and more elongated neck among species of the genus. In medial view the coracoid neck is notably thick and shows a proximodistally extended and obliquely oriented groove, a feature shared with C. poliocephala (see “groove of the coracoid neck” in Figure 2). Muscular crests are more marked than in other species (e.g. C. poliocephala , C. picta , C. rubidiceps ), but similar to Oressochen . The triosseal canal is poorly excavated and non-pneumatic. The scapular facet is relatively small and subtriangular in contour, similar to other species, but different from the more excavated and rounded condition present in C. picta .

The humeral facet is very wide, as occurs in C. picta . In contrast to the latter, the distal end of the facet gradually merges with the coracoidal neck. The procoracoid process is long and distally deflected, as occurs in C. poliocephala , but not in C. picta . In C. dabbenei n. sp. the procoracoid is also slightly medially oriented. The recessus infra-acrocoracoideus is deeply marked, as is present in C. poliocephala , but not in C. picta and C. rubidiceps .

The ulna is notably robust. The radial articular surface of the dorsal cotyle is relatively transversely wide, but does not extends distally, as occurs in C. picta , in which it is tongue-like. A relatively wide and concave surface (see Fig. 4) is located between the ventral cotyle, and proximal to the ventral collateral ligament tuberosity, similar to C. picta , whereas in other species it is relatively small, shallower and not well-defined. The brachial impression is also more defined, deeper, and more proximodistally elongated than in C. picta . The nutritious foramen of the shaft is located at the proximal half of the bone, whereas it is more distally positioned in C. poliocephala . The distal end of the bone distinguishes from other species in being more robustly built. The distal crest in C. rubidiceps is notably sharp and extensive, which differs from the condition observed in other species of the genus.

The femur shows the anterior and posterior muscular lines more marked than other species, with the possible exception of C. poliocephala . Proximal to the posterior intermuscular line exists a marked muscular surface, as occurs in C. picta , whereas it is absent in C. rubidiceps and C. poliocephala . Both C. rubidiceps and C. poliocephala show a narrower (especially towards the distal end of the bone) and more curved shaft; these features are especially evident in C. rubidiceps . In C. dabbenei n. sp. and C. picta the shaft is straighter and thicker than in other species. The surface for the gastrocnemius muscle is present but smooth, as occurs in C. picta , whereas in C. rubidiceps and C. poliocephala is much prominent and well-separated from the shaft.

The tarsometatarsus is elongated and shows non-divergent distal trochleae. The proximal end of the bone in anterior view shows a deeply excavated extensor groove with poorly defined tubercles of the muscle tibialis cranialis, as occurs in C. picta . In other species, the anterior surface of the bone is poorly excavated and is nearly flat, with the tubercles of the muscle tibialis cranialis very well-developed and well-defined. Distal trochleae are notably proximodistally elongate. In posterior view the articular surface of the III trochlea is notably elongate and subrectangular in contour, a condition that resembles C. poliocephala and C. rubidiceps . Distal trochleae are much more robust and divergent in C. picta . The trochlea II is notably posteriorly retracted, in contrast to other Chloephaga species. Trochlea IV shows the posterolateral wing proximodistally extended and prominent, differing from the much shorter exhibited by C. picta . The tarsometatarsus clearly differs from that of Oressochen , which shows very short and divergent distal trochleae, and with posterior wings of II and IV trochleae reduced ( Fig. 9). In C. dabbenei n. sp. the distal vascular foramen is very wide and ellipsoidal in contour, as occurs in C. picta , whereas in C. rubidiceps and C. poliocephala it is smaller and subcircular in contour.

REMARKS

Chloephaga dabbenei n. sp. is a very distinctive species of the genus. It is much larger than extant species (with exception of C. picta ) and has very elongated hindlimb, particularly the tarsometatarsus. In this sense, tarsometatarsus with straightness and uniformity of the shaft throughout its length, and with elongated trochleae that are in line with the shaft (poorly spreading), are features associated with cursorial habits ( Miller 1937). This morphology differs from the much more robust and divergent distal trochleae present in other members of the genus Chloephaga , as well as the closely related taxa Neochen jubata (Spix, 1825) and Oressochen melanopterus.

In spite that C. dabbenei n. sp. may fall within the range of the largest specimens of C. picta (see Table 1 ), there are several features that clearly distinguish both taxa. The coracoid of C. dabbenei n. sp. shows a notably thick coracoidal neck that is medially excavated by a groove (the neck is very narrow and the groove is absent in C. picta ), relatively small and subtriangular-shaped scapular cotyla (wide, deep and rounded cotyla in C. picta ), distal end of humeral facet gradually merges with the coracoidal neck (forms a marked step in C. picta ), procoracoid process long and distally deflected (much shorter and straight in C. picta ), recessus infra-acrocoracoideus deeply marked (shallow in C. picta ), ulna with radial articular surface

of the dorsal cotyle distally straight (in C. picta , is distally extended and tongue-like), brachial impression is also more defined, deeper, and more proximodistally elongated than in C. picta , tarsometatarsus with distal trochleae notably proximodistally elongate, in particular the articular surface of the III trochlea in posterior view is notably elongate and subrectangular in contour (distal trochleae are much more robust and divergent in C. picta ), the trochlea II is notably posteriorly retracted (in contrast to other Chloephaga species, including C. picta , in which the trochlea II is nearly in line with trochlea IV), and trochlea IV with posterolateral wing proximodistally extended and prominent (much shorter in C. picta ). This combination of characters, and particularly distal tarsometatarsus shape, clearly distinguish C. dabbenei n. sp. from C. picta and any known species belonging to the genus.

The ulna of the extinct Pliocene species “ Chloephaga ” robusta is much larger than that of C. dabbenei n. sp. and shows a more extensive distal crest, separated by a very deep groove and that it smoothly contacts the ulnar shaft ( Tambussi 1998). This combination of characters, that separates “ Chloephaga ” robusta from C. dabbenei n. sp., are reminiscent to the genera Neochen and Alopochen ( Agnolín 2006) .

Neochen pugil is an extinct goose coming from Upper Pleistocene of Lagoa Santa, Minas Gerais State, Brazil ( Winge 1887; Nascimento & Silveira 2020). In spite that it was referred to the genus Neochen , N. pugil is similar to C. dabbenei n. sp. in having relatively gracile tarsometatarsus, coracoid with strong transverse muscular lines, and thick coracoid neck with a medially located and obliquely oriented groove. However, it differs in having much larger size (tarsometatarsus total length is 13.5 cm, when compared with the 11 cm of C. dabbenei n. sp.), tarsometatarsus with proportionally short distal trochleae, a more strongly divergent II trochlea and coracoid with pneumaticity below brachial tuberosity and a more deeply excavated triosseal canal. Taking into account the similarities between C. dabbenei n. sp. and Neochen pugil , it is not improbable that, in fact, the latter belongs to the genus Chloephaga .

Neochen debilis ( Ameghino, 1891) is a species described by Ameghino (1891) on the basis of an incomplete tarsometatarsus coming from the Pleistocene of Buenos Aires province. It

was originally included under the genus Chenalopex Stephens, 1824 and lately regarded as belonging to Neochen ( Agnolín 2006) . The description made by Ameghino is very brief and lacks figures, and the material in which the species is based is currently lost. However, Ameghino points that the maximum width between the distal trochleae of the tarsometatarsus is of 8 mm, being much smaller than the 18 mm of C. dabbenei n. sp.