| Panochthus | |
|---|---|
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| P. frenzelianus | |
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| Skeleton and shell ofPanochthus tuberculatus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Cingulata |
| Family: | Chlamyphoridae |
| Subfamily: | †Glyptodontinae |
| Genus: | †Panochthus Burmeister, 1867 |
| Type species | |
| †Panochthus tuberculatus Owen, 1845 | |
| Species | |
 | |
| Inferred range of the genusPanochthus based on known localities | |
| Synonyms | |
Synonyms ofP. tuberculatus
Synonyms ofP. greslebini
| |
Panochthus is an extinct genus ofglyptodont, which lived in theGran Chaco-Pampean region ofArgentina (Lujan,Yupoí andAgua Blanca Formations),Brazil (Jandaíra Formation),Bolivia (Tarija andÑuapua Formations),Paraguay andUruguay (Sopas andDolores Formations) during thePleistocene epoch.[1][2][3] The first specimen ofPanochthus consisted of two carapace (shell) fragments, now lost, recovered from Buenos Aires. In 1845, the fragments were referred bySir Richard Owen to the genusGlyptodon. In 1864, working from more complete remains,Karl Hermann Konrad Burmeister erectedPanochthus as a subgenus. Three years later, he elevated it to the rank of genus. The species named by Owen, nowP. tuberculatus, stands as thetype species, though many others have since been named.
The internal systematics ofPanochthus 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 thatPanochthus, like all other glyptodonts, is part of the armadillo familyChlamyphoridae. In 2022, glyptodonts were divided into two mainclades: "traditional glyptodontines", and the "Austral clade";Panochthus is part of the latter, and specifically thetribe Hoplophorini, which also includesHoplophorus (and possiblyPropanochthus, although that may be a species ofPanochthus).
Panochthus was generally a large glyptodont, though body size varied between species. The biggest skulls known from the genus have been assigned toP. tuberculatus, measuring 394–442 mm (15.5–17.4 in), while the smallest, that ofP. 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 theorbit (eye socket) was encircled by a so-called postorbital bar, though this was not true for others. The armour ofPanochthus, as in other glyptodonts, consisted of four primary structures: the cephalic shield, which topped the head; the dorsalcarapace, 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. InP. intermedius, the caudal tube bears large depressions similar to those seen inDoedicurus, suggesting the presence of conicalspines.
The twosyntypes ofPanochthus, consisting of two dorsalcarapace (shell) fragments recovered in thepampas ofBuenos Aires, Argentina[4] were described by English biologist and palaeontologistSir Richard Owen, in an 1845 work cataloguing the bird and mammal fossils housed in theRoyal College of Surgeons of England. Owen assigned them to the existing genusGlyptodon as a new species,G. tuberculatus.[5] The holotype was subsequently lost, though has since been replaced by aneotype.[4] 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 inGlyptodon, he reassigned it to the genusSchistopleurum,[6] which has itself been subsumed into the former genus.[7] 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 theLuján River, by Comandante Albornoz. The specimen in question consisted of a complete caudal tube, the arrangement ofscutes lining thecaudal (tail)vertebrae.[4] In that first study, Burmeister sunkS.tuberculatus back intoGlyptodon, this time as asubgenus (Panochthus) of its own.[8] Soon after, he became aware of a more complete specimen, recovered fromVilla Mercedes. The specimen consisted of a complete skeleton, the cephalic shield (the scutes on top of the skull), thedorsal carapace, and the caudal tube.[4] Following the discovery of this specimen, Burmeister would, in 1867, elevatePanochthus to genus level.[9] In the last of his papers, published in 1874, Burmeister named a newPanochthus species,P. bullifer, whose remains were recovered from theSierras de Córdoba mountains.[10][11] While this species was briefly moved toPropanochthus,[3] it has since been removed from that genus, and is once again considered a member ofPanochthus.[12]
Since the description ofP. tuberculatus, multiple species ofPanochthus have been described. While many are valid, many others are eitherjunior synonyms of others (i.e.P. oliveiraroxoi andP. rochai, both synonyms ofP. greslebini), meaning that they are misidentified members of existing taxa, ornomina nuda (i.e.P. beyrichi andP. vogti), meaning that they were not properly described, and that their names thus do not apply to a specific taxon.[3]
| Taxon | Status | Author(s) of taxon | Taxon publication year | Countr(ies) of origin |
|---|---|---|---|---|
| P. beyrichi | Nomen nudum[3] | Roth | 1888 | |
| P. brocherii | Nomen nudum[3] | Moreno | 1888 | |
| P. bullifer | May be a genus of its own,Propanochthus,[3][15] or aPanochthus species[12] | Burmeister | 1874 | Argentina |
| P. eocenus | Nomen nudum | Scalabrini | 1887 | |
| P. florensis | Valid | Brambilla, Lopez & Parent | 2020 | Argentina |
| P. frenzelianus | Valid | Ameghino | 1889 | Probably Argentina |
| P. greslebini | Valid | Castellanos | 1942 | Brazil and Argentina |
| P. hipsilis | Valid | Zurita et al. | 2017 | Bolivia |
| P. intermedius | Valid | Lydekker | 1895 | |
| P. jaguaribensis | Valid | Moreira | 1965 | Brazil |
| P. lundii | Valid | Burmeister | 1874 | |
| P. morenoi | Invalid.Type specimen now serves asneotype forP. tuberculatus | Ameghino | 1881 | |
| P. oliveiraroxoi | Synonym ofB. greslebini[3] | Castellanos | 1942 | |
| P. rochai | Synonym ofB. greslebini[3] | Paula Couto | 1954 | |
| P. rusconii | Valid | Castellanos | 1942 | |
| P. subintermedius | Valid, though initially anomen nudum[3] | Castellanos | 1937 | |
| P. trouessarti | NowPhlyctaenopyga | Moreno | 1888 | |
| P. tuberculatus | Valid | Owen | 1845 | Argentina, Bolivia, Brazil, Paraguay, and Uruguay |
| P. voghti | Synonym ofP. tuberculatus[14] | Ameghino | 1889 | |
| P. vogti | Nomen nudum[3] | Roth | 1888 |
While initially believed to form a family of their own, glyptodonts are currently regarded as a subfamily of the armadillo familyChlamyphoridae, based onmtDNA analysis.[16][17] Glyptodontinae can be further divided, per Daniel Barasoain et al. (2022), into two clades: traditional glyptodontines, including genera close toGlyptodon, and the "Austral clade", containing the majority of glyptodont diversity and likely originating in South America.[15] There is some disagreement over wherePanochthus falls in the glyptodontine tree. In 2013, Martín Zamorano and Diego Brandoni recovered it as the sister genus toHoplophorus 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.[18] 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 includesHoplophorus,Panochthus, andPropanochthus (orPanochthus bullifer)[15] This contradicts the topology recovered by Zamorano and Brandoni, who recoveredP. bullifer as part of "Plohoplophorini".[18]
A genus-level cladogram of glyptodonts, based on the results of Barasoin et al. (2022), is as follows:[15]
| Glyptodonts |
| ||||||
Panochthus was a large glyptodont. The largest species,P. intermedius, is so much larger than other species that its body size is considered a diagnostic characteristic.[19] The smallest species,P. hipsilis, had a dorsal carapace roughly two-thirds the length ofP. intermedius' dorsal carapace.[13]
The skull ofPanochthus differs in size depending on the species. When measured from the front of the nasal aperatures to the upper margin of theforamen magnum,P. tuberculatus skulls were the biggest, ranging from 394–442 mm (15.5–17.4 in); the known skull ofP. frenzelianus was far smaller, measuring only 330 mm (13 in) in length. The skull was generally far deeper than long. The nasal region was inclined somewhat ventrally, sloping downwards (ventrally) at a 45° angle, and theexternal nares (nasal openings) were oriented forwards and downwards (fronto-ventrally).[19] InP. hipsilis, this was less exaggerated.[13] Thesinuses of thefrontal andnasal are highly developed. This has led to suggestions that the unusual nasal structure ofPanochthus is an adaptation forthermoregulation.[20] The presence or absence of a postorbital bar, a bony protrusion which closed the orbits (eye sockets) towards the back (posteriorly), differed between species.P. hipsilis andP. tuberculatus both had postorbital bars, while the remaining species had orbits which were open posteriorly.[13] In mostPanochthus species, the postorbital process sat between the orbital andtemporalfossae.P. tuberculatus was unique among its genus, though akin toDoedicurus andNeosclerocalyptus, in having a complete postorbital process.[19]
Like other glyptodonts,Panochthus' teeth were allmolariform, resemblingmolars. The molariforms of glyptodonts werehypselodont (high-crowned), lack roots, and grew continuously.[21] All ofPanochthus' teeth were trilobed, bearing three distinctcusps. InP. tuberculatus, the first upper molariforms were more rounded than the others, whereas in an unnamed species, they were subelliptical. The first lower molariforms ofP. tuberculatus were trilobed labially (on the outside), while those further back in the mouth were all trilobed in the typical fashion.[3]
Thehumeri ofP. tuberculatus were smaller than that ofP. subintermedius. Thedeltopectoral crest was well-developed, and in the former species took on a V-shape, whereas in the latter, it was convex and deflected outwards. In allPanochthus species, the deltopectoral crest had a smooth surface. The distalepiphysis of the humerus had an entepicondylarforamen, as in related glyptodonts.[3] Thefemora ofP. greslebini andP. tuberculatus were more gracile than those ofP. subintermedius. As in other glyptodonts, the epiphyses were more well-developedtransversely (across) thananteroposteriorly (from front-to-back). InP. tuberculatus, unlikeNeosclerocalyptus andPropalaehoplophorus, thegreater trochanter sat in a slightly higher plane compared to thefemoral head.[3]
The armour ofPanochthus, as in other glyptodonts, consisted of multiple structures. The skull was capped by a cephalic shield, a large mass of bone covered in small osteoderms;[4] the osteoderms ofP. frenzelianus' cephalic shield, as originally noted by Ameghino, were smaller than in other species.[11][22] Over the torso was the dorsal carapace, a large structure consisting of numerous transverse rows. The armour of the tail consisted of multiple structures: a set of smaller caudal ring for around the first half, and then a fused caudal tube for the distal half.[22][23] Pathologies to the caudal vertebrae suggest that the caudal tube was used in agonistic interactions both with otherPanochthus and possibly with other taxa.[24] Two different caudal tube morphologies are observed inPanochthus: a thick, cylindrical morphology; and a flatter morphology, compared to a Viking sword, minus the hilt.Panochthus with the latter caudal tube morphology could likely deliver more efficient horizontal blows.[25] InP. intermedius, the lateral margins of the caudal tube bore a series of large depressions, which may have anchored conicalspines,[25][26] similar to those proposed forDoedicurus.[27]
One specimen ofPanochthus sp. preserved threetracheal rings, C-shapedcartilaginous structures which would have supported thetrachea while allowing it to remain flexible. While tracheal rings are known from other extinct clades, includingnon-avian dinosaurs,Panochthus sp. is the first fossil mammal to preserve them.[28]
Panochthus is one of few glyptodonts (alongsideGlypotodon cf.clavipes) to preserve thehyoid apparatus,[10][29] a bony structure which would have supported the tongue, controlled air flow, and possibly modulated vocalisations. The hyoid ofPanochthus is longer and more gracile than that ofGlyptodon cf.clavipes, and had more well-developed musculature, suggesting a more flexible tongue and a different feeding method to that genus.[29]
