Panochthus is an extinct genus of glyptodont, which lived in the Gran Chaco-Pampean region of Argentina (Lujan, Yupoí and Agua Blanca Formations), Brazil (Jandaíra Formation), Bolivia (Tarija and Ñuapua Formations), Paraguay and Uruguay (Sopas and Dolores Formations) during the Pleistocene epoch. The first specimen of Panochthus consisted of two carapace (shell) fragments, now lost, recovered from Buenos Aires. In 1845, the fragments were referred by Sir Richard Owen to the genus Glyptodon. In 1864, working from more complete remains, Karl Hermann Konrad Burmeister erected Panochthus as a subgenus. Three years later, he elevated it to the rank of genus. The species named by Owen, now P. tuberculatus, stands as the type species, though many others have since been named. The internal systematics of Panochthus have long been debated. At least twenty species have been named. While some have been reclassified, rendered invalid, or synonymised with existing species, at least nine remain valid. Mitochondrial DNA analyses suggest that Panochthus, like all other glyptodonts, is part of the armadillo family Chlamyphoridae. In 2022, glyptodonts were divided into two main clades: "traditional glyptodontines", and the "Austral clade"; Panochthus is part of the latter, and specifically the tribe Hoplophorini, which also includes Hoplophorus (and possibly Propanochthus, although that may be a species of Panochthus). Panochthus was generally a large glyptodont, though body size varied between species. The biggest skulls known from the genus have been assigned to P. tuberculatus, measuring 394–442 mm (15.5–17.4 in), while the smallest, that of P. frenzelianus, instead measured 330 mm (13 in). As a genus, Panochthus is characterised by having a skull far deeper than it is long, a downturned nasal region, and three-cusped, molar-like teeth. In some species, the back of the orbit (eye socket) was encircled by a so-called postorbital bar, though this was not true for others. The armour of Panochthus, as in other glyptodonts, consisted of four primary structures: the cephalic shield, which topped the head; the dorsal carapace, which covered the body; the caudal rings, which encircled the base of the tail; and the caudal tube, a rigid mass which covered the last half or so of the tail. In P. intermedius, the caudal tube bears large depressions similar to those seen in Doedicurus, suggesting the presence of conical spines.
Taxonomy
Early history
The two syntypes of Panochthus, consisting of two dorsal carapace (shell) fragments recovered in the pampas of Buenos Aires, Argentina were described by English biologist and palaeontologist Sir Richard Owen, in an 1845 work cataloguing the bird and mammal fossils housed in the Royal College of Surgeons of England. Owen assigned them to the existing genus Glyptodon as a new species, G. tuberculatus. The holotype was subsequently lost, though has since been replaced by a neotype. Ten years after Owen's paper, naturalist Léonard Nodot re-examined the fossils. He described additional elements, also from the carapace. Noting a degree of carapace flexibility not observed in Glyptodon, he reassigned it to the genus Schistopleurum, which has itself been subsumed into the former genus. Between 1864–1874, Karl Hermann Konrad Burmeister (writing under the name Carlos Germán Conrado Burmeister) published extensively on the taxon published on by Nodot and Owen. His first study, published in 1864, focused on a specimen recovered in 1851 from the Luján River, by Comandante Albornoz. The specimen in question consisted of a complete caudal tube, the arrangement of scutes lining the caudal (tail) vertebrae. In that first study, Burmeister sunk S. tuberculatus back into Glyptodon, this time as a subgenus (Panochthus) of its own. Soon after, he became aware of a more complete specimen, recovered from Villa Mercedes. The specimen consisted of a complete skeleton, the cephalic shield (the scutes on top of the skull), the dorsal carapace, and the caudal tube. Following the discovery of this specimen, Burmeister would, in 1867, elevate Panochthus to genus level. In the last of his papers, published in 1874, Burmeister named a new Panochthus species, P. bullifer, whose remains were recovered from the Sierras de Córdoba mountains. While this species was briefly moved to Propanochthus, it has since been removed from that genus, and is once again considered a member of Panochthus.
Internal systematics Since the description of P. tuberculatus, multiple species of Panochthus have been described. While many are valid, many others are either junior synonyms of others (i.e. P. oliveiraroxoi and P. rochai, both synonyms of P. greslebini), meaning that they are misidentified members of existing taxa, or nomina nuda (i.e. P. beyrichi and P. vogti), meaning that they were not properly described, and that their names thus do not apply to a specific taxon.
Classification
While initially believed to form a family of their own, glyptodonts are currently regarded as a subfamily of the armadillo family Chlamyphoridae, based on mtDNA analysis. Glyptodontinae can be further divided, per Daniel Barasoain et al. (2022), into two clades: traditional glyptodontines, including genera close to Glyptodon, and the "Austral clade", containing the majority of glyptodont diversity and likely originating in South America. There is some disagreement over where Panochthus falls in the glyptodontine tree. In 2013, Martín Zamorano and Diego Brandoni recovered it as the sister genus to Hoplophorus in all trees, with their analysis suggesting that the two genera sat apart from other glyptodonts in what is now defined as the Austral clade. Barasoain et al. (2022), however, recovered a different topology. In their phylogeny, the Austral clade consists of multiple loosely assorted genera, and two smaller clades: Doedicurinae, and most relevantly, Hoplophorini. This tribe includes Hoplophorus, Panochthus, and Propanochthus (or Panochthus bullifer). This contradicts the topology recovered by Zamorano and Brandoni, who recovered P. bullifer as part of "Plohoplophorini".
A genus-level cladogram of glyptodonts, based on the results of Barasoin et al. (2022), is as follows:
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