The genus Euryurus was originally recognized by Gervais and Ameghino (1880). Later, Ameghino (1888: 493) recognized the species E. antiquus on the basis of materials (associated osteoderms of the dorsal carapace) coming from his “Piso hermósico de la formación araucana” (Ameghino, 1889: 842–843; pl. 62, fig. 6) that corresponds to the Monte Hermoso Formation (Early Pliocene, Montehermosan Stage/Age; see Tomassini, 2012 and Tomassini et al., 2013) in the Buenos Aires Province. Several years later, the generic name was replaced by Neuryurus Ameghino, 1889, as the first was preoccupied by Euryurus Koch, 1847. Ameghino (1888) characterized the exposed surface of the osteoderms of “E. antiquus” as having an evident rough aspect and a large number of small foramina; in some osteoderms, it is possible to observe a poorly defined circular central figure. In 1889, Ameghino carried out the first detailed description and illustrated the materials, which included the associated osteoderms of the dorsal carapace, a relatively well preserved skull and a left hemimandible (Ameghino, 1889: pl. 62, figs. 6 and 7; pl. 63, figs. 1 and 2). As in the original description, the species was mainly characterized by a particular morphology of the exposed surface of the osteoderms. In turn, the internal surface of the osteoderms was characterized as having fewer foramina and showing some resemblance with those observed in the Pleistocene Doedicurinae Doedicurus (Ameghino, 1889; pl. 62, fig. 6). The first to doubt the validity of this species was Lydekker (1895: 15; pl. 9, figs. 1–3), who interpreted the material that Ameghino identified as “N. antiquus” as belonging to juvenile specimens of P. figuratus. The taxonomic conclusion of Lydekker (1895) was based on an erroneous conception about the ontogenetic evolution of the osteoderms in glyptodonts (see Ameghino, 1895 and Zurita et al., 2011). This originated a heated discussion between Lydekker (1895) and Ameghino (1895), who strongly rejected the interpretation of Lydekker. To support his position, Ameghino (1895: 523) mentioned that the associated osteoderms of the dorsal carapace (Ameghino, 1889; pl. 62, fig. 6) belong to the same specimen as the skull and hemimandible (Ameghino, 1889; pl. 63, figs. 1 and 2). Finally, Castellanos (1926) transferred “N. antiquus” to his Neogene genus Urotherium (“U. antiquum”) (see also Kraglievich, 1934). Since then, the taxon was considered valid by most authors (e.g. Cabrera, 1944, Castellanos, 1926, Fernicola, 2008 and Fernicola and Porpino, 2012) and even included in cladistics analyses. A recent study concerning the diversity of Late Neogene Glyptodontidae (see Zurita et al., 2016) offers the opportunity to reevaluate the status of this species. In the following sections, we provide a justification for the new proposed synonymy.

Skull (MACN A-229) (Fig. 2A–D). Ameghino (1889, pl. 63, figs. 1 and 2) described and illustrated a partial skull and left hemimandible that he identified as “N. antiquus”. Only the skull was found (MACN A-229) and it lacks the right zygomatic arch. Among the recognized diversity of Montehermosan (Early Pliocene) and Chapadmalalan (Late Pliocene) Glyptodontidae of the Pampean region of Argentina, only three species have known skulls, “U. antiquum”, cf. Eleutherocercus antiquus (Doedicurinae) and P. figuratus (“Hoplophorinae” Plohophorini). The latter two represent the most frequently recorded taxa in Neogene strata (Zurita et al., 2014 and Zurita et al., 2016). The skull (MMP 4676) recovered from the Chapadmalal Formation was tentatively assigned by Oliva et al. (2010) to the Glyptodontinae cf. Paraglyptodon chapadmalensis (Ameghino), and is currently under revision; it is possible that this identification needs a correction (Zurita et al., 2016).

The morphology of the skull of “U. antiquum” (MACN A-229) is not concordant with that known of Doedicurinae (i.e. cf. E. antiquus and Doedicurus), excluding its inclusion in this subfamily. In fact, the morphology of the molariforms and palate of this clade is very particular (see Zurita et al., 2014: fig. 2). Additionally comparison to MMP 4676 shows significant differences. Thus, the discovery of new and more complete Plohophorus specimens (MMP 4823; Xen 72) allows a detailed comparison and, as emphasized by Lydekker (1895) MACN A-229 shows a remarkable similarity with P. figuratus, one of the most recorded taxa in the Montehermosan and Chapadmalalan levels. In fact, the general morphology of the skull and molariforms are almost identical.

In anterior view (Fig. 2A), it is possible to observe that the most distal borders of the narines are not preserved in MACN A-229. However, the morphology is clearly similar to that of P. figuratus (Fig. 2E), and also shows some resemblance to cf. Eleutherocercus antiquus. Furthermore, it is possible to see that the orbital notch is completely different than cf. E. antiquus, in which this structure is more laterally expanded, with evident bony ridges on its ventral edge.

In lateral view (Fig. 2B, F) P. figuratus and “U. antiquum” show the same dorsal profile. The occipital and parietal areas are clearly elevated with respect to the nasal region. The zygomatic arch presents the same graceful morphology in both MACN A-229 and P. figuratus, and completely differs from that of cf. Eleutherocercus antiquus, in which this structure is much more developed. The orbital notch is subeliptical and dorso-ventrally elongated, as in P. figuratus, and does not present the evident bony ridges on the ventral edge that are observed in cf. Eleutherocercus antiquus and Doedicurus. The descending process of the maxillae is not preserved in its ventral half, but the general morphology is almost identical to that of P. figuratus. Ahead of the orbital notch, the dorsal profile of the rostral region shows an angle of ca. 46° with respect to the palatal plane, as in P. figuratus (ca. 42°); this angle is 54° in cf. E. antiquus.

In dorsal view, “U. antiquum” (Fig. 2C) and P. figuratus (Fig. 2G) also show the same morphology, which is very different than cf. Eleutherocercus antiquus and MMP 4676. The parietal area presents several large foramina on both sides of the sagittal crest, which is clearly developed in P. figuratus and was not preserved in “U. antiquum”. This led several authors to mistakenly interpret it as absent in phylogenetic analyses. In MMP 4676, the parietal region does not show the evident foramina. The orbital notch is posteriorly opened, lacks a postorbital bar, and the left zygomatic arch is identical to that of Plohophorus. The rostral area located behind the orbital notch is subtriangular, showing the same morphology as that seen in P. figuratus. This morphology completely differs from that of Doedicurus and cf. E. antiquus, in which the rostral area shows a sub quadrangular contour and a strong postorbital bar is present. Additionally, the zygomatic arch and the orbital notch are evidently more laterally expanded.

In occlusal view (Fig. 2D), it is possible to observe a transversal enlargement of the palate at the level of its proximal and distal borders, and a minimum at level of Mfs 4-5, as in P. figuratus (Fig. 2H). In cf. Eleutherocercus antiquus, this morphology is also present but is much more evident (Table 1). This particular morphology is not present in MMP 4676, and both molariform series are almost parallel. As in P. figuratus, the palate of MACN A-229 has several large foramina at level of the Mfs3 and 4.

The left hemimandible (MACN A-230) described and illustrated by Ameghino (1889: pl. 63, fig. 2) was not found in the collection. However, the general morphology is very similar to that figured by Lydekker (1895, Pl. VIII) and Pascual et al. (1966, Pl. XXXVII), which belongs to P. figuratus.

Upper molariforms. The general morphology of the molariforms (Fig. 2I) completely matches those of P. figuratus (Fig. 2J), and is different than Doedicurus, cf. Eleutherocercus antiquus and MMP 4676. The Mf1 is simple, has a subeliptical contour and is almost identical to that of P. figuratus. The Mf2 shows an incipient trilobulation, even more evident than that observed in cf. E. antiquus but less developed compared to MMP 4676; the Mf3 presents the transversal axis of the first lobe oriented ca. 54.5° with respect of the palatal plane; the first lobe shows a similar bilateral development, unlike cf. E. antiquus where the lingual side is much more developed. Both in P. figuratus and MACN A-229, the labial margin of the third lobe shows a “slot”. The Mfs 4–6 are similar in morphology and the third lobe presents in its labial side a “slot”, as in cf. E. antiquus and P. figuratus. Finally, the Mfs 7 and 8 do not show any differences when compared to P. figuratus.

Osteoderms (MACN A-231) (Fig. 3A–E; see also Ameghino, 1889: pl. 62, figs. 6, 6a and 7). According to Ameghino (1889), the osteoderms show two different morphologies, characterizing both the internal and external (= exposed) surfaces. The internal surface (Fig. 3B, C) shows several (5–12) large, uniformly distributed foramina, whereas the articular area between the osteoderms is clearly evident (Fig. 3 D, E). In turn, the exposed surface presents rugose aspect and larger number of very small foramina (Fig. 3A). Our observations coincide with the descriptions of Ameghino. However, some interesting observations can be added, suggesting that the osteoderms belong to a juvenile specimen. For this, the material was compared to those of juvenile glyptodonts previously studied by Luna and Krapovickas (2011) and Zurita et al. (2011) in order to find evidences of this ontogenetic stage. The internal surface is slightly concave, consisting of a finely spongy tissue, especially in the central region (Fig. 3C); although this character could be attributed to taphonomic factors, it has been also observed in some juvenile specimens of Glyptodon Owen (MCNC-PV-246) (Fig. 3F). This differs from the typical condition observed in adult specimens because the internal surface is smooth, flat or very gently concave, exhibiting macroscopic meshwork of mineralized fiber bundles (Hill, 2006), which are deposited during more advanced ontogenetic stages. Another feature, also observed in juveniles of some genera (e.g. Glyptodon and Neosclerocalyptus Paula-Couto), is the presence of a particular flat surface in the articular area (under study by one of the authors), which is related to the ossification mechanism among osteoderms for the development of a rigid dorsal carapace (Fig. 3 E, F). In the adult phase, this structure disappears or is limited to a few relicts through resorption processes. In addition, the development of the articular area (Fig. 3E) shows a very similar morphology compared to juvenile specimens of Glyptodon (Fig. 3F).

Additionally, some years later Ameghino (1895) referred new materials to this species, specifically the specimens illustrated by Lydekker (1895: pl. 9, figs. 1–4) and classified by this author as belonging to P. figuratus. The morphology of the exposed surface of this osteoderms is clearly rugose, and is almost identical to that originally described by Ameghino (1889).