Colbertia lumbrerense Bond, 1981

Armella, Matías A., García-López, Daniel A., Babot, M. Judith, Deraco, Virginia, Herrera, Claudia M., Saade, Luis & Bertelli, Sara, 2020, Postcranial remains of basal typotherian notoungulates from the Eocene of northwestern Argentina, Acta Palaeontologica Polonica 65, pp. 1-17 : 4-5

publication ID	https://doi.org/ 10.4202/app.00664.2019
persistent identifier	https://treatment.plazi.org/id/456987D6-FFFF-FF9B-AF35-FAE4FBEFD904
treatment provided by	Felipe
scientific name	Colbertia lumbrerense Bond, 1981
status

Treatment

Material.— PVL 6227 View Materials ; right calcaneum and left and right astragali, part of a skeleton of an adult individual also preserving the skull and mandibles, from Pampa Grande , Guachipas Department , Salta Province, Argentina ( Fig. 1 View Fig ); Lower Lumbrera Formation, middle Eocene (Vacan subage of the Casamayoran SALMA; del Papa et al. 2010) .

Description.—The calcaneum is poorly preserved. This element has suffered a dorsoplantar compression that altered its main features and it will not be described here ( Fig. 3A View Fig ). The astragali are complete, but the right one is also compressed dorsoplantarly); the following description is based on the better-preserved left element. In dorsal view, the tibial trochlea is shallow, asymmetric (see Table 1), and trapezoidal in shape, the lateral crest being more developed and conspicuous than the medial crest, which is more rounded ( Fig. 3B View Fig 1 View Fig ). The trochlear fossa is shallow and almost smooth. The astragalar neck is short compared to other Paleogene taxa (e.g., Thomashuxleya externa Ameghino, 1901 , Notopithecus adapinus Ameghino, 1897 ) and other specimens here studied (see below), representing almost 28% of the total length of the astragalar body ( Table 1). The oblique dorsal crest (nuchal crest or tibial stop) is robust, oriented posterolateral-anteromedially and it occupies the entire surface of the neck (it forms an angle of approximately 62° with the anteroposterior axis of the astragalus; Fig. 3B View Fig 1 View Fig ). The head is slightly eroded on its medial side; however, it can be observed that it is spherical, being roughly as wide as the neck but not expanded (a shallow groove surrounding the head separates both structures; Fig. 3B View Fig 1 View Fig ). The navicular facet occupies the entire anterior surface of the astragalar head and it is slightly extended on the medial surface ( Fig. 3B View Fig 1 View Fig ).

In plantar view, the astragalus shows some eroded areas; nevertheless, the articular facets are well-preserved ( Fig. 3B View Fig 2 View Fig ). The ectal facet is subtrapezoidal, with the major axis anteroposteriorly oriented. It is represented by a deep concavity that faces lateroplantarly ( Fig. 3B View Fig 2 View Fig ). In contrast, the sustentacular facet is convex, oval, and it occupies the plantar surface of the astragalar neck. This facet exhibits well-defined edges. Although a small, smooth surface connects the sustentacular and navicular facets, the strong concavity of this surface indicates that both facets are functionally separated ( Fig. 3B View Fig 2 View Fig , B 4 View Fig ).

The interarticular sulcus is deep and anterolateral-posteromedially oriented. Moreover, in its posterior area, the inferior astragalar foramen is small and faces anteroplantarly ( Fig. B 2 View Fig ). The groove for the tendon of the flexor hallucis longus muscle is poorly developed compared to other specimens such as PVL 4292 (see below) and it is roughly continuous with the trochlea ( Fig. B 3 View Fig ). The lateral crest that limits the groove is broken; nevertheless, the rest of the structure is clearly visible. The dorsal astragalar foramen is present but very small and located near the lateral crest of the trochlea ( Fig. 3B View Fig 1 View Fig , B 3 View Fig ). On the medial area, the astragalar medial plantar tuberosity (medial protuberance in Cifelli 1993; medial process in Shockey and Flynn 2007; tibial protuberance in Vera 2012) is small but well pronounced, defined by the anteromedial portion of the medial crest ( Fig. 3B View Fig 2 View Fig ). The orientation of the facet for the medial malleolus of the tibia is similar to the astragalar medial plantar tuberosity, but the former is smooth and less projected ( Fig. 3B View Fig 5 View Fig ). The attachment for the tibioastragalar ligament is located between these two structures, forming a shallow groove.

In lateral view, the fibular facet is flat, crescent-like (with a blunter, although somewhat eroded, posterior end), and lacking projections ( Fig. 3B View Fig 4 View Fig ). The attachment for the fibuloastragalar ligament is represented by a deep and conspicuous kidney-shaped depression, with the major axis being anteroposteriorly oriented. In medial view, the medial aspect of the astragalar medial plantar tuberosity is elliptical, with the major axis being anteroposteriorly oriented ( Fig. 3B View Fig 5 View Fig ). Although there is an eroded area on the medial surface, this structure is not connected with the navicular facet (this is more evident on the right astragalus; Fig. 3C View Fig ).

Remarks.—The genus Colbertia includes two species of small-sized notoungulates: Colbertia magellanica , from the early Eocene of Brazil (Itaboraí)and Colbertia lumbrerense , recorded from middle Eocene rocks of the Lower Lumbrera Formation in Salta Province, Argentina. Although Bergqvist et al. (2007) and Bergqvist and Fortes Bastos (2009) studied postcranial remains of both species, these authors did not provide detailed descriptions of the tarsal bone morphology, since these elements were not fully prepared and poorly preserved in the specimens available to them. In the particular case of C. lumbrerense , the specimen analyzed by Bergqvist et al. (2007) was the postcranium PVL 6218, currently not available to us, and proximal tarsals were not figured in detail. The recently prepared PVL 6227 allows us to compare in detail the two species of Colbertia . Concerning the astragalus, the shared features between C. magellanica ( MCT 2456M; Bergqvist et al. 2007: fig. 5; Bergqvist and Fortes Bastos 2009: fig. 2) and C. lumbrerense ( Fig. 3 View Fig ) are the spherical astragalar head, where the sustentacular and navicular facets are separated by a deep sulcus, and the navicular facet slightly extended on the medial surface, which is not connected with the astragalar medial plantar tuberosity.

Bergqvist et al. (2007) considered that, at least regarding the features exposed in the specimen PVL 6218, the trochlea and ectal facet morphology are traits shared by C. magellanica and C. lumbrerense . Nevertheless, PVL 6227 shows an asymmetric and trapezoidal tibial trochlea ( Fig. 3B View Fig 1 View Fig ), in contrast to C. magellanica ( MCT 2456M), where the trochlear crests are parallel and inclined to the lateral side ( Bergqvist et al. 2007: fig. 5; Bergqvist and Fortes Bastos 2009: fig. 2). Other traits of C. lumbrerense differing from C. magellanica are the ectal and fibular facets not projected laterally, smaller astragalar medial plantar tuberosity on the medial face, longer neck (made of the oblong sustentacular facet), and very small dorsal astragalar foramen (in C. magellanica this foramen is well developed and located in a depression on the trochlea).

In summary, the mentioned differences are opposed to a previous point of view that considers the morphologies of Colbertia magellanica and Colbertia lumbrerense being similar ( Bergqvist et al. 2007). Hence, these comparisons indicate a source of diagnostic traits potentially useful to the further characterization of both species.

Stratigraphic and geographic range.—Lower Lumbrera Formation, middle Eocene (Lutetian); Pampa Grande, Salta Province, Argentina.