Xenarthra, Bergqvist & Abrantes & Avilla, 2004, Bergqvist & Abrantes & Avilla, 2004
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publication ID |
https://doi.org/ 10.5281/zenodo.5375880 |
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persistent identifier |
https://treatment.plazi.org/id/03DD87BC-4D62-FFF0-090A-FC70FC84FBC8 |
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treatment provided by |
Marcus |
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scientific name |
Xenarthra |
| status |
incertae sedis |
XENARTHRA incertae sedis
REFERRED MATERIAL. — Humeri (MCN-PV 1780, 1781, MCT 2396-M, 2397-M); ulna (MCN-PV 3606); astragali (MCN-PV 1340, 1380, MCT 2394- M, 2395-M).
DESCRIPTION
Humeri (MCN-PV 1780, 1781, MCT 2396-M, 2397-M)
The humeri can be separated into two morphotypes, slightly distinct in shape but clearly distinct in size.
The larger humeri (MCT 2396-M, 2397-M – HUM-morph 1) are powerfully built ( Figs 3 View FIG ; 4 View FIG ). The head has an elliptical outline and is more posteriorly directed than dasypodids, in a way similar to Manis pentadactyla Linnaeus, 1758 . The articular area for the scapular acromion, a very characteristic feature of fossorial armadillos, is shallow. As in other xenarthrans, the shaft is flat and wider than deep in its proximal half. The greater tuberosity is higher than the lesser (but not projected above the head), but less transversely expanded. There are well marked impressions for the infraspinatus and subscapularis muscles on the greater and lesser tuberosities, respectively. The tubercle for insertion of the teres major and latissimus dorsi muscles is more prominent than in any known Xenarthra , although less than in Metacheiromys Wortman, 1903 , and is placed at the level of the deltoid tuberosity, as in this genus. These two muscles are frequently well developed and fused in burrowers, in which they have an important digging function ( Reed 1951 apud MacPhee 1994; MacPhee 1994). The bicipital groove differs from that of any known “edentate” in being wide, shallow and well defined in the proximal third of the shaft. A similar condition is present in most pilosans and some derived cingulates, but the groove is not as defined in the shaft as in the Itaboraí humeri. The deltopectoral crest, as in Eurotamandua Storch, 1981 , Palaeanodon Matthew, 1918 and Metacheiromys is broad, shelf-like and extends more than halfway down the shaft, unlike the condition in most dasypodids. In Priodontes and Cabassous McMurtrie, 1831 , the deltopectoral crest is quite longer but not as robust as in the Itaboraí humeri. However, they form a derived monophyletic group ( Priodontina sensu Abrantes 2002 ), and this feature may represent a new acquisition compared to the pattern observed in other cingulates. The shelf is proximodistally concave, but transversely concave only on the distal half; its outline is very close to that of Palaeanodon ignavus Matthew, 1918 ( Rose 1999: figs 2, 3). Unlike Dasypodidae , the medial (pectoral) border of the crest overhangs the shaft. This is a distinctive feature of palaeanodonts, even though the pectoral tuberosity is more marked. It is distal to the deltoid tuberosity, which is also well marked, but more distally located on the lateral border than in Eurotamandua and palaeanodonts. As in palaeanodonts, manids and Eurotamandua , but unlike xenarthrans, the deltopectoral shelf is anteromedially oriented.
The distal extremity is incomplete in both specimens, but its preserved portion suggests that the distal end was very broad transversely, more than in most Xenarthra currently known, with the possible exception of Priodontes . The entepicondyle is prominent, a typical feature of “edentates” ( Rose 1999). A small portion of the supinator crest is preserved in specimen MCT 2397-M, showing that it was very prominent, probably like Eurotamandua and palaeanodonts. Its shape recalls Palaeonodon ignavus ( Rose 1999) in having a straight border parallel to the shaft. However, its proximal end forms an angle of almost 90° with the proximodistal axis of the shaft, and is straighter than in P. ignavus . Tamandua Gray, 1825 also presents a straight supinator crest, but it is considered here a parallelism, as this genus is a derived Myrmecophagidae , which is a derived group within Xenarthra ( Gaudin & Branham 1998) .
The other humeri (MCN-PV 1780, 1781 – HUM-morph 2) are shorter than HUMmorph 1 ( Fig. 5 View FIG ). They are very similar to morphotype 1, except in having the tuberosities more equally developed and transversely expanded, and the tuberosity of the teres major muscle less prominent. Compared to morphotype 1, the deltoid tuberosity is more proximal, less projected, more rounded and rougher.
Ulna (MCN-PV 3606)
The ulna, like the smaller humerus, is gracile ( Fig. 6 View FIG ). Only the proximal portion is preserved. The olecranon process, like that of Dasypus Linnaeus, 1758 , is straight and long, with a broad and medially inflected epiphysis, in the same way as in some Dasypodidae . The enlargement and inflection of the olecranon is associated with great enlargement of the triceps muscles, and perhaps also the digital flexor (K. Rose pers. comm.), which powerfully extend the forearm during the power stroke of digging ( Puttick & Jarvis 1977 apud Rose & Emry 1983). The medial surface of the olecranon is concave and the lateral is flat, but from the trochlear notch to the distal preserved portion, it bears a deep and narrow concavity. The trochlear notch is shallow and very similar in orientation to that of Proeutatus Ameghino, 1891 . The anconeal process is little projected. The radial facet is comparable to Tolypeutes , but with its lateral portion in a more distal position than in this taxon. As in other “edentates”, the ulnar shaft is deeper than wide.
Astragali (MCN-PV 1340, 1380, MCT 2394-M, 2395-M)
There are two different patterns and sizes of astragali ( Fig. 7 View FIG ). The larger (MCN-PV 1340, MCT 2395-M – AST-morph 1) is morphologically similar to Utaetus Ameghino, 1902 . The smaller (MCN-PV 1380, MCT 2394-M – ASTmorph 2) in some ways (especially in the shape of trochlea) resembles Peltephilus Ameghino, 1887 . They were first described by Cifelli (1983), whose description is partially transcribed here with additions provided by two new specimens not seen by this author.
The body of AST-morph 1 is low, transversely broad and anteroposteriorly shortened, as in several cingulates. The trochlea is moderately deep, with the medial crest shorter and less defined than the lateral one, as seen in Utaetus . A rugose fossa, possibly a remnant of the superior astragalar foramen, is present posterolaterally on the trochlea. The lateral wall of the astragalar body is vertical, and a small fibular shelf protrudes from the anteroinferior angle of the lateral wall, a very common feature of dasypodids, but less so in other “edentates”. The medial wall bears a discrete protuberance for the medial collateral ligament. Both ectal and sustentacular facets are posterolaterally-anteromedially aligned. The alignment of ectal and sustentacular facets on the same axis of rotation was proposed by Szalay & Schrenk (1998) as a synapomorphy of Xenarthra and Palaeanodonta. The sustentacular facet extends from the poorly defined groove for the digital flexor tendons, which is more distinct from the trochlea in the specimen MCN-PV 1340. Distally, the sustentacular facet touches the navicular facet by a medial prolongation. The ectal facet has the typical triangular shape and concavity of cingulates. It is separated from the sustentacular facet by a developed sulcus tali.
As in other cingulates, but differently from palaeanodonts and pholidotans, the neck is relatively wide, short, shallow and markedly oblique to the anteroposterior axis of the trochlea. A well defined crest occupies the dorsodistal extremity of the neck, which, as in living dasypodids, would have given rise to the astragalonavicular ligament. The head is transversely narrow, as in other dasypodids, and extends posteriorly medially, almost reaching the groove for the digital flexor tendons, as in Utaetus , for example.
The AST-morph 2, like morphotype 1, lacks the superior astragalar foramen. The trochlea is deeper than in the AST-morph 1 and strongly constricted, as in Peltephilus , Propraopus Ameghino, 1881 and most Glyptodontidae , as well as Myrmecophaga Linnaeus, 1758 . Its lateral border is condyloid and does not extend anteriorly beyond the middle of the astragalus. Both medial and lateral walls of the body are vertical, and the protuberance for the medial collateral ligament is poorly developed in specimen MCT 2394-M. Compared to AST-morph 1, the fibular shelf is more prominent. The ectal and sustentacular facets are placed the same way as in other astragali. The latter facet is rounded and does not contact either the navicular facet or the trochlea (the digital flexor groove is not distinct from the posteroinferior margin of the trochlea). The ectal facet is narrower than morphotype 1 and more concave.
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