Introduction

One of the most informative characters allowing the differentiation among glyptodont genera and species has been the morphology and ornamentation of the osteoderms of the dorsal carapace (Owen 1845; Nodot 1856; Burmeister 1864; Ameghino 1889) and the caudal tube (Burmeister 1864; Ameghino 1889). Ameghino (1889), Castellanos (1959), Hoffstetter (1958), and Paula Couto (1979) were the first authors to realize a systematic analysis based in characters of the caudal tube and carapace. Recently, other authors (Zurita et al. 2005; Fernicola 2008; Zurita et al. 2009) have based their systematic works solely on cranial characters; however, it is important to incorporate the available information from the ornamentation of the dorsal caparace and caudal tube into such analyses. In this context, it becomes necessary to define the most important characters of these regions for the different genera and species.

Within Glyptodontia, the tribe Panochthini (Castellanos 1927) is known from Upper Pliocene to Pleistocene strata from southern South America, including Argentina, Uruguay, Bolivia (Tonni et al. 2009), and Brazil (Porpino and Bergqvist 2002; Porpino 2009). The species of the tribe Panochthini differ from the other glyptodonts by the presence in most of the dorsal carapace of osteoderms with only small polygonal figures on the superficial surface and, in some sections of the carapace, one central figure. The caudal tube is characterized by its large size, depressed shape, terminal figures at its distal end, lateral figures on the lateral side, and marginal and central figures on the dorsal and ventral surfaces (see Castellanos 1925b, 1942). The surrounding area of these figures is covered by small polygonal figures similar to those present in the carapace osteoderms.

Panochthus is a very important genus within Panochthini because it represents the last lineage of this tribe and also the largest size. The latest revision of the genus Panochthus (Burmeister 1866) was carried out by Castellanos (1942), who considered nine valid species, including six previously known taxa: P. intermedius (Lydekker 1894), P. subintermedius Castellanos, P. frenzelianus (Ameghino 1889), P. morenoi (Ameghino 1881), P. vogti (Ameghino 1889), P. tuberculatus (Owen 1845), and three new species: P. rusconii, P. oliveira-roxoi, and P. greslebini, the latter two from Brazil. He also stated that Panochthus bullifer (Burmeister 1870–1874) represented a different genus, Propanochthus (Castellanos 1925a). Later, two new species from Brazil were defined: Panochthus rochai (Paula Couto 1954) and P. jaguaribensis (Moreira 1965). Moreira (1971) and Porpino and Bergqvist (2002) synonymized P. oliveira-roxoi and P. rochai with P. greslebini, in a revision of Brazilian panochthines. However, the taxonomic status of P. jaguaribensis is debatable, therefore Moreira (1971) proposed the exclusion of this species from Panochthus and transferred it a new genus, Parapanochthus. However, Porpino and Bergqvist (2002) returned this species to genus Panochthus, but this assignment is still controversial (Porpino 2009). Within all species of Panochthus, only one was differentiated by cranial characters (Panochthus vogti Ameghino, see the section “Materials and methods”), from P. frenzelianus and P. tuberculatus. In contrast, the last two and the remaining species are recognized by characters of the osteoderms from the dorsal carapace and the caudal tube.

No exhaustive revisions of the group have been made since the work of Castellanos (1942). However, the species P. vogti, P. rusconii, and P. subintermedius have not been mentioned in different faunal lists from southern South America for the Ensenadan (early-middle Pleistocene)–Lujanian (late Pleistocene–early Holocene) interval (Cione and Tonni 2005, and references therein). Only P. intermedius has been cited for the Ensenadan, although Castellanos (1933) mentioned a new species, P. subintermedius, without providing figures or a formal description. Afterwards, Castellanos (1936: 24–25 pp.), provided a brief differential description in a footnote: “…difiere del P. intermedius en lo que respecta a la coraza en que las figuritas centrales de las placas son más pequeñas y el número de filas más reducido y limitado a las zonas marginales” (“…differ from P. intermedius with respect to the carapace because the central figures of the osteoderms are smaller in size and have fewer rows and limited to the marginal zones”). Moreover, he designated the holotype (MACN-Pv 5130) and mentioned that he would describe the latter in detail in another work, which was eventually published in 1942.

The aim of this contribution is to provide a formal diagnosis and redescription of the holotype of Panochthus subintermedius, and to discuss which year should be considered as the valid name-establishing date for this species. In addition, we analyze its biostratigraphic implications and provide some characters from the ornamentation of the carapace and caudal tube that should be incorporated into future phylogenetic analyses of glyptodonts.

Materials and methods

The fossil specimens studied are housed in the following institutions: Museo Argentino de Ciencias Naturales “Bernardino Rivadavia,” Sección Paleontología Vertebrados, Buenos Aires, Argentina (MACN-Pv); Museo de La Plata, División Paleontología Vertebrados, La Plata, Buenos Aires, Argentina (MLP); Museo Universitario Florentino y Carlos Ameghino, Rosario, Santa Fé, Argentina (MUFyCA); Divisão de Geologia e Mineralogia, Colección de mamíferos fósiles, del Departamento Nacional da Produção Mineral, Rio de Janeiro, Brazil (DGM-M); Muséum National d’Histoire Naturelle, colección Pampéen, Paris, France (MNHN-PAM); American Museum of Natural History, New York, NY, USA (AMNH).

An exhaustive comparative examination of taxonomically informative characters of the carapace and caudal tube of several specimens (including the holotypes of the Panochthus species) was made (see Appendix). Regarding the type specimens, DGM-M 1 (Panochthus greslebini) and AMNH 11243–11245 (Panochthus frenzelianus) were compared through photographs taken specifically for this analysis. The holotype of P. vogti could not be examined and may possibly be housed in the National Museum of Geneva (Switzerland) (Ameghino 1889 and personal inquiry). Ameghino distinguished the species P. vogti in 1889 based on cranial characters observed in photographs and indicated that the caudal tube of this new species, although smaller, was very similar to that of P. tuberculatus. Accordingly, P. vogti may be considered as a valid species on the basis of the cranial characters described by Ameghino (1889), which are not available for comparison in the species studied here.

The systematic scheme follows Fernicola (2008). The terminology used for the description of the caudal tube was taken from Castellanos (1925a, 1942). The description and terminology for osteoderms follow Hill (2006) and Krmpotic et al. (2009), while the scheme for the different regions of the carapace is based on Porpino (2009) (see Fig. 1).

Fig. 1
figure 1

a Schematic representation of the different regions of the glyptodontid carapace. b Dorsal view of the carapace of P. subintermedius (MACN-Pv 5130). c Lateral view of P. morenoi (MLP 16-382) and representation of the different regions. AD Anterodorsal region, MD mid-dorsal region, PD posterodorsal region, AB anterior border, LB lateral border, PB posterior border (modified from Porpino 2009)

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Systematic paleontology

  • Magnorder Xenarthra Cope, 1889

  • Order Cingulata Illiger, 1811

  • Suborder Glyptodontia Ameghino, 1889

  • Infraorder Glyptodontoidei Gray, 1869

  • Superfamily Gliptodontoidea Gray, 1869

  • Family Panochthidae Castellanos, 1927

  • Subfamily Panochthinae Castellanos, 1927

  • Tribe Panochthini Castellanos, 1927

  • Genus Panochthus Burmeister, 1866

  • Type Species: Panochthus tuberculatus Owen, 1845

  • Panochthus subintermedius Castellanos, 1936

  • Figs. 2, 3

    Fig. 2
    figure 2

    Comparison of the holotype carapaces of P. intermedius (MLP 16-36), P . subintermedius (MACN-Pv 5130), and P. morenoi (MLP 16-382). Details of the anterodorsal (AD), mid-dorsal (MD), and posterodorsal (PD) regions of a P. intermedius, b P. subintermedius, and c P. morenoi. HF Hexagonal figure from the superficial osteoderm surface, RES reticulating sulci, SS sutural surface (modified from Hill 2006), other abbreviations as in Fig. 1. Scale: 10 cm

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    Fig. 3
    figure 3

    Caudal tube of P. subintermedius (MACN-Pv 5130). a Schematic representation of dorsal view. FT Terminal figure, FA apical figure, FD dorsal figure of the dorsoventral pair, FL lateral figure, FC central figure, FM marginal figure, FP polygonal figure, Ha annular row (the black circles indicate reconstructed sections). b Dorsal view. c Right lateral view. d Ventral view. Scale 10 cm

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Holotype: MACN-Pv 5130. Several carapace fragments corresponding to anterodorsal (two rows only), mid-dorsal, and posterodorsal regions, complete caudal tube fused to last distal ring, proximal fragment of right humerus, fragment of pelvis, and left femur.

Referred material: MUFyCA 342. Distal fragment of the caudal tube.

Geographical and Stratigraphic provenance of the holotype: Ciudad Autónoma de Buenos Aires, Argentina, Ensenadan (early-middle Pleistocene).

Diagnosis: Large species, similar in size to P. intermedius. Superficial surface of the osteoderms of the carapace ornamented with polygonal figures without distinct central figure, differing from P. intermedius in which this central figure is present in almost all osteoderms from the anterodorsal and posterodorsal regions of the carapace. Osteoderms of penultimate row without central figure in contrast to those of P. intermedius and P. tuberculatus in which a central figure is present. General shape of the carapace similar to that of P. tuberculatus; in both species, the mid-dorsal outline of the carapace presents a horizontally elongated S-shape in lateral view, different from the convex dorsal outline of the carapace in P. intermedius, P. morenoi, and P. frenzelianus. Caudal tube with a strongly sharpened tip, differing from the less sharp morphology of P. greslebini and P. rusconii, and the blunt tip of P. tuberculatus, P. morenoi, and P. frenzelianus; dorsal surface of tube with one apical figure at its distal end, between the first pair of dorsoventral figures, a character shared only with P. greslebini; central figures distributed over entire dorsal surface, differing from P. tuberculatus, P. morenoi, and P. rusconii in which central figures are restricted to the distal region of the tube; terminal figures separated in dorsal view and united in ventral view, in contrast to P. frenzelianus in which the terminal figures are separated on both surfaces.

Comparative description

Carapace (Figs. 1, 2)

A large portion of the carapace from specimen MACN-Pv 5130 is preserved, from the last row of the anterodorsal region to the posterior border, but all lateral margins are missing. The general shape of the carapace is similar to that of P. tuberculatus, and both species present a characteristic concave-convex outline (oblong S-shape) at the anterior section of the mid-dorsal region. This convexity corresponds to the rise of the sacrolumbar crest and of the transversal crest developed by the superior part of the iliac bones. This carapace morphology contrasts with those of P. intermedius, P. morenoi, and P. frenzelianus in which the carapace presents a convex dorsal outline.

Most of the osteoderms are fused so that their limits are not visible on the superficial surface, but the sutures are evident on the deep surface. The osteoderms (except those from the posterior border) present several polygonal figures of similar size, each of them flat or slightly depressed at the center, without central figure (see Fig. 2). In contrast, Castellanos’s (1942: 492) original description stated: “Su escultura externa se caracteriza por presentar en cada placa figuras centrales, como en Panochthus intermedius Lyd., pero se diferencian de las de éste porque son de menores dimensiones y la zona de la coraza que ofrecen estas figuras es más reducida. Por consiguiente, resulta una transición entre el Panochthus intermedius y los P. frenzelianus y P. tuberculatus, estos dos últimos con la ornamentación de sus caparazones muy semejante” (“the external sculpture is characterized by a central figure in each plate, such as in P. intermedius Lyd., but smaller and in a more restricted region of the carapace. Consequently, this morphology is a transition between Panochthus intermedius and that of P. frenzelianus and P. tuberculatus, the last two with very similar carapace ornamentation”). The carapaces of the holotypes of P. intermedius and P. subintermedius differ in that, in the former, the osteoderms of the anterodorsal and posterodorsal regions have a clearly distinguishable central figure, whereas in P. subintermedius no central figure is present in the last anterodorsal row or in the posterodorsal region. These areas present only small polygonal figures; a central figure is only present in the posterior border of the carapace.

The ornamentation of the osteoderms on the posterior border and the penultimate row is another noteworthy character that differs among some species within Panochthus. The osteoderms of the penultimate row of P. subintermedius do not bear central figures, in contrast to those of P. intermedius and P. tuberculatus in which a central figure is present. The osteoderms on the posterior border comprise a central figure that is almost the size of the osteoderm. This figure is surrounded by a row of small polygonal figures, and in the anterior region of the osteoderm, also by a second row of peripheral figures. Although only a few osteoderms of the posterior border are preserved, it is possible to observe that this character differs depending on the carapace region: the central figure is smaller closer to the lateral borders, and the rows of polygonal figures increase toward the anterior region. In P. morenoi no polygonal figures are distinguishable, only a rugose area surrounds the anterior half of the central figure, and in contrast to Ameghino’s (1889) statement, this region differs from the comparable area in P. tuberculatus. On the other hand, and in accordance with the description of Ameghino (1889), the posterior border and the penultimate row of P. frenzelianus are highly similar to those of P. subintermedius. Nevertheless, in the holotype of P. frenzelianus, most, if not all, of the osteoderms corresponding to these rows have been reconstructed, and it was not possible to observe them.

Caudal tube (Figs. 3, 4)

The caudal tube is 96 cm long, including the last caudal ring, which is fused to the caudal tube (see Table 1 for the rest of the measurements). The proximal opening is almost circular, but the dorsal surface of the caudal tube descends abruptly in the first third of its length and the tube becomes depressed. There are five lateral figures on the right side of the tube, whereas there are only four figures on the left lateral region. This character is similar to the condition in some caudal tubes of P. tuberculatus and P. morenoi, but other tubes of both species have only four on each side, whereas in Propanochthus bullifer there are five figures on each side. These figures are conical tubercles and become larger toward the tip. Between these conical figures, the lateral surface bears small polygonal figures, as in the superficial surface of the osteoderms. At its distal end, the tube presents two terminal figures that are very extended anteroposteriorly and compressed, making for a sharper apex in contrast to the less sharp morphology of P. greslebini and P. rusconii, and the blunt tip of P. frenzelianus, P. morenoi, and P. tuberculatus.

Fig. 4
figure 4

Dorsal view of the caudal tube of a holotype of Propanochthus bullifer (MACN-Pv 1761), b holotype of Panochthus subintermedius (MACN-Pv 5130), c holotype of P. greslebini (DGM-M 1), d P. tuberculatus (MACN-Pv 1745), e holotype of P. nodotianus (MNHN-PAM 291), f P. morenoi (MNHN-PAM ?), g P. sp. (MLP 16-381), h P. sp. (MLP 16-32)

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Table 1 Measurements of the caudal tube of P. subintermedius (MACN-Pv 5130)
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The dorsal surface (Fig. 3a, b) bears one apical figure between both terminal figures, a character shared only with P. greslebini, and dorsoventral pairs of figures: five on the left side and four on the right side. Five to eight marginal figures are located between these figures. Finally, the dorsal surface among all these figures is covered by small polygonal figures and numerous central figures without a defined pattern. Of note, the central figures are not arranged in transverse rows as in the caudal tube of the Propanochthus bullifer, but present greater continuity than the ones in the caudal tubes of Panochthus morenoi, P. tuberculatus, and P. rusconii (Figs. 3a, b, 4).

The proximal end presents two annular rows of helicoidal figures; the first one includes four figures similar to those of the dorsoventral pairs. The second row bears only three figures, smaller and similar in shape to the central figures. The ventral surface of the caudal tube does not differ from that of other species; the only major difference is that the terminal figures are adjacent, they touch each other, whereas they are separated in Propanochthus bullifer and Panochthus frenzelianus.

Results and discussion

Our results highlight two useful carapace characters for distinguishing some species of Panochthus: (1) the general shape of the carapace (Figs. 1 and 2) and (2) the presence of the central figure in the superficial ornamentation of the osteoderms from the anterodorsal and posterodorsal regions, and from the penultimate and/or posterior border (Fig. 2). Thus, as a summary of the observations made of different species of panochthines, the following can be concluded. (1) Based on the general shape of the carapace in lateral view, two groups can be distinguished among Panochthus species: (a) carapace with oblong S-shape of the mid-dorsal region (P. subintermedius and P. tuberculatus) (Fig. 1a) and (b) carapace uniformly convex (P. intermedius, P. morenoi, and P. frenzelianus) (Fig. 1c). (2) The osteoderms from Propanochthus bullifer (posterodorsal region) and from Panochthus intermedius (anterodorsal and posterodorsal region) are easily recognizable because they have a distinct central figure different from the peripheral polygonal figures. In the remaining species for which almost complete carapaces are known, namely P. subintermedius, P. tuberculatus, P. morenoi, and P. frenzelianus, the osteoderms only have a central figure in the posterior border of the carapace and/or in the osteoderms from the penultimate row, and in the three or four osteoderms from the lateral borders, in agreement with some previous descriptions (Burmeister 1870–1874; Ameghino 1889, 1895; Castellanos 1942). Nevertheless, this character cannot be evaluated with full certainty because in the case of P. morenoi and P. frenzelianus¸ many of the osteoderms in these regions have been restored.

Regarding the characteristics of the caudal tube, some important features that allow distinguishing among the species of Panochthus are as follows (Figs. 3, 4): (1) shape of the distal end of the tube, (2) distribution of the central figures on the dorsal surface, (3) number of apical figures, and probably (4) size. The number of lateral figures on the caudal tube is a more variable character because in some cases, e.g., P. subintermedius (MACN-Pv 5130) and P. morenoi (MNHN-PAM, unnumbered), each side of the tube has a different number of figures.

The results suggest that the caudal tube of Panochthus subintermedius differs from the other species by a combination of characters: strongly sharpened apex, only one apical figure, and central figures irregularly arranged on the entire dorsal surface of the tube. The annular rows of helicoidal figures on the proximal end of the tube and their arrangement should probably be considered as a combination of particular characters rather than as individual exclusive features, as is the case for most tube-related characters.

As previously pointed out, because Castellanos (1933) was the first to mention Panochthus subintermedius, some authors have accepted the validity of this species from that publication (Paula Couto 1954; Soilbelzon 2008), others considered it as a nomen nudum and accepted its validity as from 1942 (Mones 1986), yet others considered it in 1933–1936 (Castellanos 1942), and finally, some authors informally considered this species as a synonym to P. intermedius. Thus, taking into account Article 13 of the International Code of the Zoological Nomenclature (1999) and considering that it was in 1936 that Castellanos provided a brief description and designated the holotype and its repository, we propose the 1936 publication as the one in which the species was named.

Finally, there is a general consensus, expressed in faunal lists, to recognize only some of the species of the genus Panochthus for southern South America (e.g., P. intermedius from the Ensenadan and P. frenzelianus, P. tuberculatus, and P. morenoi from the Bonaerian-Lujanian interval (Scillato-Yané and Carlini 1998; Zamorano 2005; Cione and Tonni 2005, and references therein). Thus, P. subintermedius represents a well-characterized species for the Mesotherium cristatum Biozone, biostratigraphic base of the Ensenadan (Cione and Tonni 2005; Tolypeutes pampaeus Biozone from Cione and Tonni 1995, and references therein), which increases the diversity of glyptodons in sediments from the early-middle Pleistocene of southern South America.

Conclusions

Panochthus subintermedius differs from the remaining species of the genus (P. intermedius, P. tuberculatus, P. rusconii, P. frenzelianus, P. greslebini, P. morenoi, and P. vogti) by a unique combination of characters found in the dorsal carapace and caudal tube: the general shape and ornamentation of the carapace are similar to those of P. tuberculatus, although it is larger and comparable to that of P. intermedius; and it lacks central figures in most of the osteoderms. The distal end of the caudal tube of P. subintermedius is sharper than in other species, with only one apical figure on the dorsal surface and central figures along the entire dorsal surface.

Panochthus subintermedius (Castellanos 1936) is a valid species according to the International Code of the Zoology Nomenclature (1999). Consequently, two species of Panochthus are recorded for the Ensenadan of southern South America: P. intermedius and P. subintermedius.