Glyptodon

Georges Cuvier (1769-1832), describer of Megatherium

The history and taxonomy of Glyptodon is storied and convoluted, as it involved confusion with other genera and dubious species, as well as a lack of detailed data. The first recorded discovery of Glyptodon was as early as 1814 when Uruguayan priest, scientist, soldier, and later politician Dámaso Antonio Larrañaga (1771-1848) wrote about the discovery of several unusual fossils in his Diario de Historia Natural, which included his descriptions of many new species of ants, birds, mammals, and even one of the first figures of the extinct Megatherium, a genus of giant ground sloth that was named in 1796 by French scientist Georges Cuvier (1769-1832).[3][4] This was the first recorded discovery of a glyptodontine or fossil cingulate.[4] The unusual fossils consisted of a femur, carapace fragments, and a caudal tube (an armored tail covering found in glyptodontines) that he collected from the Pleistocene aged (ca. 2.5-0.011 mya) strata on the banks of the Solís Grande Creek, Uruguay.[4][5] Larrañaga identified the fossils as those of Dasypus (Megatherium), believing that Megatherium was a subgenus of Dasypus based on the incorrect referral of glyptodontine osteoderms to Megatherium years earlier by Spanish scientist Juan Bautista Bru de Ramón, which misled other scientists to believe that glyptodontine fossils were actually those of armored megatheres.[6][4]

Larrañaga wrote to French scientist Auguste Saint Hilaire about the discovery, and the letter was reproduced by Cuvier in 1823 in the second volume of his landmark book Recherches sur les ossemens fossiles.[7] Larrañaga also noted that similar fossils had been found in "analogous strata near Lake Merrim, on the frontier of the Portuguese colonies (southern Brazil)."[7][6] These fossils were also likely those of glyptodontines, possibly the closely related Hoplophorus.[8] The armored Megatherium hypothesis was further supported later in 1827 when portions of a Glyptodon carapace, as well as a partial femur and some caudal armor, were found by a Prussian traveler to Montevideo, Uruguay named Mr. Sellow, who sent the carapace to Berlin where it was described by Professor von Weiss, who referred it to Megatherium.[9] The femur and caudal armor were recovered from the Quegnay in northern Uruguay, while the carapace had been found in the Arapey River.[9][5] Weiss and other paleontologists noted that the osteoderms closely resembled those of armadillos like Dasypus, but Cuvier's hypothesis was popularized based on the incorrect referral of glyptodontine osteoderms Megatherium.[6][7]

Another work on the armored Megatherium hypothesis was published in 1833 by Berlin scientist E. D'Alton, who described more of the material sent by Sellow, including portions of the limbs, manus, and shoulder girdle. D'Alton recognized the great similarities of the fossils to Dasypus and speculated that it was a giant armadillo, contrary to the notion that they were from Megatherium. Despite this, D'Alton did not erect a new name for the fossils and instead wrote that additional material was necessary to distinguish it from other armadillos. D'Alton did not mention Megatherium or its osteoderms in the paper, but he implied that all of the "Megatherium armor" was instead from his armadillo. This hypothesis was supported by Laurillard in 1836, who mentioned that a plaster cast of a large armadillo carapace represented a distinct taxon from Megatherium and that the armor referred to the sloth was instead from an armadillo.[5]

1837 saw the naming of the first glyptodontine, Hoplophorus euphractus, when Danish paleontologist Peter Wilhelm Lund published a series of memoirs on the fossils of Lagoa Santa in Brazil, dating to the Pleistocene.[10][8] The fossils included osteoderms comparable to those described earlier by Larrañaga, as well as teeth, skull fragments, limb bones, and other elements.[5][8] After 1837, several new genera and species of glyptodontines were named in quick succession by European paleontologists: Chlamydotherium based on Sellow's carapace and Orycterotherium based on Sellow's femur were named by German scientist H. G. Bronn 1838,[11] Pachypus by Eduard D'Alton in 1839 based on Sellow's 1833 material,[5] Neothoracophorus (originally Thoracophorus but the name was preoccupied by a beetle) in 1889 by Argentine paleontologist Florentino Ameghino[12] based on isolated osteoderms now identified as those of a juvenile Glyptodon from Patagonia,[13] and Lepitherium in 1839 by Geoffroy Saint-Hilaire based on Sellow's osteoderms.[14][15] Saint-Hillaire considered the osteoderms found by Sellow to not even be mammal, but instead of a relative of Teleosaurus, a crocodile-like reptile known from Jurassic deposits in France.[15][14]

Richard Owen's 1838 reconstruction of a Glyptodon skeleton; with the namesake tooth on the right

In 1838, British diplomat Sir Woodbine Parish (1796–1882) was sent an isolated molariform and a letter about the discovery of several large fossils from the Matanza River in Buenos Aires, Argentina that dated to the Pleistocene.[16][17] Parish later collected several more fossils from localities in Las Averias and Villanueva; the latter preserved the most complete skeleton which included a mandible fragment, partial limbs, and unguals of a single individual. They were deposited in Parish's collection at the Royal College of Surgeons in the United Kingdom that year. Some of these fossils were cast at the Natural History Museum, London, but the original fossils were destroyed after German aerial bombing raids hit the college during World War II from 1940 to 1941.[13][17] Glyptodon was named by Richard Owen (1804-1892), one of the most influential British naturalists of the Victorian Era, writing a chapter about the animal and publishing a reconstruction of its skeleton in the book Buenos Ayres, and the provinces of the Rio de La Plata: their present state, trade, and debt in 1839.[18][19] Within this book, Owen erroneously believed they were all from the same specimen, the name Glyptodon ("grooved tooth") based on the anatomy of the molariform.[19][17] A later study found the molariform to actually be from another glyptodontine, Panochthus, and the Villanueva individual was designated the lectotype by Robert Hoffstetter in 1955.[20] The Las Averias individual consists of a carapace that was only mentioned in Owen's description, but was used in later reconstructions of the animal and has since been lost. An issue with the lectotype of G. clavipes is that the material is undiagnostic and indistinguishable from other Glyptodon species and even Glyptotherium, making it dubious.

Cuadrelli et al (2018) designated the species a species inquirenda due to this issue and commented that more analyses are necessary.[17] In 1860, Signor Maximo Terrero collected a partial skeleton, including a skull and carapace, of G. clavipes from the River Salado in southern Buenos Aires and dated to the Pleistocene. These fossils were also sent to the Royal College of Surgeons, where they were described in detail by British paleontologist Thomas Henry Huxley (1825–1895) in 1865 during a comprehensive review of the taxon.[5] This skeleton was also destroyed during WWII, but Huxley published several illustrations that presented great amounts of new information on the taxon.[5][21]

Later in 1845, many more fossils found by Parish from Pleistocene layers in Argentina were named as new species of Glyptodon by Owen: G. ornatus, G. reticulatus, G. tuberculatus, and G. clavicaudatus in 1847. Of these additional species, only G. reticulatus is still considered a valid species of Glyptodon as G. ornatus was reassigned to the genus Neosclerocalyptus,[22] G. tuberculatus to Panochthus,[23] and G. clavicaudatus to Doedicurus.[24] G. reticulatus was named on the basis of several carapace fragments that had also been recovered from the Matanza River, but they lack detailed locality information and the fossils too were destroyed during WWII. The fragments were cast by the NHMUK as well, being used to diagnose the species.[25]

Other paleontologists also started erecting names for Glyptodon species after the 1840s, but many of them are now seen as dubious, species inquirenda, or synonymous with previously named species.[26][17] Par L. Nodot described a new genus and species of glyptodontine in 1857, Schistopleurum typus, on the basis of a caudal tube found in the Pampas of Argentina, but it has since been synonymized with G. reticulatus.[27] Another species now seen as valid, G. munizi, was described in 1881 by Argentine paleontologist Florentino Ameghino (1853–1911) on the basis of several osteoderms found in the Ensenadan of Arroyo del Medio, San Nicolás, Argentina.[28][29] For many years the taxon was only known from the fragmentary holotype, but skull and complete carapace material of the species was later described in detail in 2006 that cemented its validity.[28][17] German zoologist Hermann Burmeister described several Glyptodon fossils in the 1860s, many of them he named as new species of Glyptodon itself or the synonym Schistopleurum, all of which are now synonyms of Glyptodon and its species.[30][17] In 1908, Florentino Ameghino named another species of Glyptodon, G. chapalmalensis, based on a carapace fragment that he had collected from the Atlantic Coast of Buenos Aires Province that dated to the Chapadmalalan. In 1932, A. Castellanos made a new genus for G. chapalmalensis, Paraglyptodon, which later included another species, P. uquiensis, that was based on more complete specimens that had been collected from Uquía, Argentina between 1909 and 1912.[31][32] The former species is dubious, but likely not Glyptodon based on its age.[33] P. uquiensis has been synonymized with Glyptodon and is possibly a valid species, though further analysis is necessary to settle its status.[25][17]

The Hunterian Museum at the Royal College of Surgeons c. 1842 with a mounted skeleton of Glyptodon in the bottom right

In the 1950s, Argentine paleontologist Alfredo Castellanos (1893-1975) erected new generic names for several species of Glyptodon, the first being Glyptocoileus and second of these being Glyptopedius in 1953 that was made for the species G. elongatus that had been named by Robert Burmeister in 1866 on the basis of a single carapace,[30] though its validity is disputed.[26][17] Castellanos also referred the species G. reticulatus to the genus, but this unsupported.[17] Yet another genus was erected in 1976 named Heteroglyptodon genuarioi by F. L. Roselli based on an incomplete skeleton that had been collected from the Pleistocene aged Libertad Formation in Nueva Palmira, Uruguay,[34][35] but it has since been found to be an indeterminate specimen of Glyptodon.[35] Several Glyptodon fossils from Pleistocene deposits in Colombia were described in 2012, extending the known range of the genus north greatly.[36]

Another Glyptodon species was described in 2020 called G. jatunkhirkhi by several authors led by Argentine zoologist Francisco Cuadrelli on the basis of an individual preserving a nearly complete carapace, several caudal rings, and a pelvis that had been collected from Yamparaez, 24 kilometres (15 mi) southeast of the Bolivian city of Sucre. The strata they were found in was made up of fluvial, sandy sediments that dated to the Late Pleistocene from elevations as high as 4,100–2,500 metres (13,500–8,200 ft) above sea level.[26] Several additional paratypes were referred to the species from other Late Pleistocene sites in Eastern Cordillera, Bolivia including a nearly complete skull and several osteoderms.[26] In a phylogenetic analysis conducted by Cuadrelli et al., 2020, G. jatunkhirki was recovered as the most basal Glyptodon species, despite being the same age as the more derived species G. clavipes. Reassesment of Glyptodon species began in the late 20th and early 21st centuries, with various hypotheses developing on the number of valid species. Numbers varied, with some authors considering up to 4 species valid, while phylogenetic analyses in 2018 and 2020 only found the species G. reticulatus, G. munizi, and G. jatunkhirkhi definitively valid; G. clavipes and G. uquiensis as species inquirendas.[17] However a 2016 review of G. uquiensis determined that G. uquiensis was actually a juvenile specimen of Glyptodon, though the species could not be determined.[33]

Fossils from North America were also described and referred to Glyptodon starting in 1875, when civil engineers J. N. Cuatáparo and Santiago Ramírez collected a skull, nearly complete carapace, and associated postcranial skeleton of a glyptodontine from a drainage canal near Tequixquiac, Mexico, the fossils coming from the Rancholabrean stage of the Pleistocene.[37][25] These fossils were the first found of glyptodontines in North America and were named as a new species of Glyptodon, G. mexicanum, but the fossils have since been lost and the species was synonymized with Glyptotherium cylindricum.[25][38] Several other North American glyptodontine species were named throughout the late 19th-early 20th century, typically based on fragmentary osteoderms. All North American and Central American fossils of glyptodontines have since been referred to the closely related genus Glyptotherium, which was named in 1903 by American paleontologist Henry Fairfield Osborn.[39]

Skull of Glyptodon in side view.

Glyptodont dentition contains entirely hypsodont molariforms, which have one of the most extreme examples of hypsodonty known from terrestrial mammals.[57] The dentition is typical of other armadillos, but is fluted on each side by deep grooves. The anterior teeth were compressed, while the posterior teeth were cylindrical.[58] Glyptodont skulls have several unique features; the maxilla and palatine are enlarged vertically to make space for the molariforms, while the braincase is brachycephalic, short and flat.[59] In Glyptodon and many other glyptodontines, the roof of the skull was covered by a shield composed of polygonal, irregular osteoderms that were variable in size and ankylosed together to form a robust cephalic shield that had a smoothly convex exterior surface without ornamentation.[28] Each osteoderm has a rugose and slightly convex dorsal surface, with ornamentation pattern defined by a central figure, slightly elevated and surrounded by an area without peripheral figures or foramina. Sutures separating osteoderms are well marked, as in Panochthus.[28][60] Other Pleistocene glyptodontines are known by complete/sub-complete skulls, allowing for comparisons to Glyptodon.[13][43] Glyptotherium's zygoma are narrow, slender, almost parallel, and close to the sagittal plane in frontal view; in Glyptodon, this structure is broader, robust, divergent rather than parallel and more laterally placed.[51][25]

The nasal passage was reduced with heavy muscle attachments for some unknown purpose. Some have speculated that the muscle attachments were for a proboscis, or trunk, much like that of a tapir or elephant. The lower jaws were very deep and helped support massive chewing muscles to help chew coarse fibrous plants. Some paleontologists have proposed that Glyptodon and some glyptodontines also had a proboscis or large snout similar to those in proboscideans and tapirs,[61] but few have accepted this hypothesis.[43][62] Another suggestion, made by A.E. Zurita and colleagues, is that the large nasal sinuses could be correlated with the cold arid climate of Pleistocene South America.[62][63] A distinctive bar of bone projects downwards on the cheek, extending over the lower jaw, perhaps providing an anchor for powerful snout muscles. In turn, the infraorbital foramina are narrow and not visible in anterior view in Glyptotherium, but in Glyptodon they are broad and clearly visible in anterior view. In lateral view, the dorso-ventral height between the skull roof and the palatal plane in Glyptodon decreases anteriorly, contrary to Glyptotherium; the nasal tip is in a lower plane with respect to the zygomatic arch in Glyptodon, but in Glyptotherium is higher than the zygomatic arch plane. The 1st molariform (molaiform is abbreviated as mf#) of Glyptodon is distinctly trilobate (three-lobed) both lingually and labially, nearly as trilobate as the mf2; on the contrary, Glyptotherium shows a very low trilobation of mf1, which is elliptical in cross-section, the mf2 is weakly trilobate, and the mf3 is trilobate. In both genera, the mf4 to mf8 are fully trilobate and serially identical.[25] These traits separate the two genera.[25] Within the genus Glyptodon this trait varies as well, with G. reticulatus having triloby to a greater degree than G. munizi.[17]

Glyptodon skeleton and shell in Museum für Naturkunde, Berlin

The mandibles of Glyptotherium and Glyptodon are very similar, but Glyptotherium's mandible is smaller by about 10% in total size. The angle between the occlusal plane (part of the jaw where upper and lower teeth contact) and the anterior margin of the ascending ramus is approximately 60 in Glyptotherium, while it is 65° in Glyptodon. The ventral margin of the horizontal ramus is more concave in Glyptodon than in Glyptotherium. The symphysis area is extended greatly in Glyptotherium antero-posteriorily compared to Glyptodon. The mf1 is ellipsoidal in Glyptotherium and the mf2 is "submolariform", while in Glyptodon both teeth are trilobate.[25]

A mounted skeleton of G. reticulatus (Schistopleurum asperus), demonstrating its size.

Glyptodon has 7 cervical vertebrae, of which the first 3 cervicals were fused together while the rest of the cervicals were free except for the 7th.[5] The 7th cervical and the first 2 dorsal vertebrae were fused together into a trivertebral, a broad, flat bone with very small spinous processes (projections from a vertebra) and large articular surfaces that held ribs.[5] All of the other 13 vertebrae in the dorsal column were fused into one long continuous tunnel that is not seen in mammals outside of glyptodontines, some of these vertebrae were so tightly fused that the segments of them cannot be discerned. The centra of these vertebrae were curved, thin bony plates that created a cylinder to support the carapace and the shape of the animal.[5] Spinous processes in these vertebrae are also heavily reduced, with some being only a thin blade of bone ankylosed with other vertebrae.[5] Sacral vertebrae in Glyptodon are also fused and 13 in number, which preserve very unusual oval-shaped, thin, and slightly concave ends on the centra.[5] The pelves are also unusual, as they preserve giant ilia and are fused to the rest of the skeleton.[5]

Close-up view of carapace

Glyptodon carapace in Hungarian Natural History Museum

Glyptodon's osteoderms were attached by synotoses (bony connections) and were found in double or triple rows on the front and sides of the carapace's edges, as well as in the tail armor and cephalic shield. The carapace's osteoderms were conical with a rounded point, while the ones on the tail were just conical. The sulci between these raised structures were deep and wide with parallel lines.[64] The carapace of Glyptodon was strongly elongated compared to those of Boreostemma and Glyptotherium, with the carapace being relatively 65% longer than the former and 14% than the latter. In Glyptodon, the top-bottom height of the carapace represents 60% of its total length, whereas in Glyptotherium it is taller at circa 70%. The antero-posterior dorsal profile of the carapace was convex and its posterior half was higher than the anterior. The apex of the carapace was slightly displaced posteriorly in most Glyptodon species, while in Glyptotherium and Glyptodon jatunkhirkhi it was at the center of the midline. The carapace of most species of Glyptodon is arched subtly, while Glyptotherium and Glyptodon jatunkhirkhi's has a very arched back and convex pre-iliac and concave post-iliac, giving it a saddle-like overhang over the tail. Glyptodon osteoderms in the antero-lateral regions of the carapace are strongly ankylosed, giving them little flexibility, while in Glyptotherium they are less ankylosed and more flexible.[25] The osteoderms of the caudal aperture (large conical osteoderms that protect the base of the tail) are more conical in Glyptodon and more rounded in Glyptotherium, though in the latter the anatomy of the caudal aperture osteoderms varies by sex while in Glyptodon it varies by age.[25][65] The caudal aperture is more vertically oriented in the latter genus, while in Glyptotheirum it is angled posteriorily.[25] Although frequently used to differentiate the two taxa, Glyptodon and Glyptotherium have similar osteoderm morphologies that differ only in several areas. Both genera have tall, thick osteoderms compared to those of many other glyptodontines such as Hoplophorus and Neosclerocalyptus. Glyptodon sometimes preserves a "rosette" pattern, where the osteoderm's central figure is surrounded by a row of peripheral figures, while other specimens lack them completely. G. reticulatus varies from a complete rosette pattern to a reticular surface, which has convex central and peripheral figures.[65][25] Glyptotherium however always preserves rosettes.[65][25] The central and radial sulci are deeper and broader in Glyptodon (ca. 4–6 mm) than in Glyptotherium (ca. 1–2.4 mm). The osteoderms in Glyptodon and Glyptotherium have 5-11 peripheral figures, rugose exposed surfaces, and heights up to 47 millimetres (1.9 in).[25]

Osteoderms on the ventral side of the body were first mentioned by paleontologist Hermann Burmeister in 1866, postulating that there was a ventral plastron like in turtles based on evidence of small armor in the dermis.[30] This hypothesis has since been disproven, but in the early 2000s, the presence of osteoderms on Glyptodon's face, hind legs, and underside was confirmed in several species. The fossils with these characteristics were from the Pleistocene, evolving in younger species like G. reticulatus . These small to medium-sized ossicles were actually embedded in the dermis and did not connect in a pattern.[64]

Armor at end of tail

Glyptodon had very primitive tail anatomy for a glyptodont, possessing eight or nine mobile caudal rings of fused, large, conical osteoderms. These enclosed the base of the tail, which terminated in a short caudal tube composed of two fused caudal rings. Caudal rings were composed of two or three rows of pentagonal osteoderms that transitioned from flat, slightly convex in the posterior rings to conical tubercles by the third caudal ring. The more posterior the rings were, the larger they were, with the exception of the 2nd ring which was the largest and 1st complete ring in the series, creating a cone-shaped tail. The distal scutes are larger, and their free margins are rounded producing a fan-like shape. Most of the osteoderms of the distal row (some individuals preserving up to 12) bear prominent conical outlines, in stark contrast to more advanced glyptodontines like Doedicurus and Panochthus, which had completely fused tails that formed an inflexible mace or club.[66] The caudal tube at the distalmost end of the tail is cylinder-shaped with smaller conical osteoderms and is stubbier proportionally in Glyptodon. In Glyptotherium, this caudal tube represents ca. 20% of the total length of the caudal armor, whereas in Glyptodon, this structure represents 13% of the total length.[25] In Glyptodon, the caudal armor length represents circa 30-40% of the carapace's total length in contrast to Glyptotherium, where this value is greater at around 50%.[25] For example, in specimen MCA 2015 of Glyptodon reticulatus, the terminal tube measured only 73.23 millimetres (2.883 in) long in comparison to Glyptotherium texanum specimen UMMP 34 826's 210 millimetres (8.3 in) long tube.[25]

Glyptodon head restoration, Munich

Many armadillo species have digging capabilities, with large claws adapted for scraping dirt in order to make burrows or forage for food underground.[67][68] Much of armadillo diets consist of insects and other invertebrates that live underground,[69] in contrast to the herbivorous diets of Glyptodon and related genera.[70] Being a large armadillo, Glyptodon's fossorial capabilities have been researched on several occasions. Owen (1841) opposed this idea,[16] though pushback came from Nodot (1856) and Sénéchal (1865) who believed digging was possible for the genus.[27][71] However, the evolution of a rigid carapace as opposed to a flexible one in extant armadillos as well as a weakly developed deltoid crest on the humerus (upper arm bone) provided evidence against fossorial hypotheses. The elbow had a great range of movement, as with digging cingulates, but this is more likely to be due to size adaptations.[55][72]

Endocasts of Glyptodon (left) and Doedicurus (right).

Several complete skulls of Glyptodon enable the endocranial anatomy to be analyzed, as well as compared to other well-preserved taxa like Doedicurus and Panochthus. The brain cavities of the larger glyptodontines Glyptodon, Doedicurus, and Panochthus had a braincase volume of 213 to 234 cubic centimeters (7.2 to 7.9 U.S. fl oz). The encephalization quotient of these taxa are 0.12 to 0.4, lower than most modern armadillos (0.44-1.06) and corresponds to those of pampatheres. The brain of the glyptodontines had an extensive olfactory bulb that took up between 4.8 and 9.7% of the entire brain, while around two thirds of it were occupied by the cerebrum and the rest by the cerebellum. Overall, this is akin to that of other armadillos, but in the latter the cerebrum is smaller relative to the cerebellum and the braincase's total volume. Deviating from the armadillos with their wide olfactory bulb, glyptodontines and pampatheres have elongated and triangular olfactory systems. Several other neuroanatomical characteristics differ between glyptodontines and armadillos, such as the presence of a pronounced sulcus praesylvianus.[73][74]

In general, living cingulates have smaller brains than anteaters and sloths for reasons unknown. Several theories have been made as to why, such as a shorter rearing phase of offspring, dedication of resources to the development of the carapace, and other biological and functional handicaps. Members of Cingulata also tend to have extremely low metabolisms, causing less energy flow to the development of the brain's neurons. The pattern of large bodies bearing adequate protection and a reduction of intelligence is found in several other groups such as ankylosaurs and stegosaurs, two types of armored dinosaur. However, the carapace itself is considered as a restrictive functional component as it prohibited much neck movement and forced a reduced brain size. This reduction thus resulted in weight loss in the skull, which had a great affect on the skulls of large-headed glyptodontines like Glyptodon.[73][74]

Teeth, with the distinct hypdodonty of glyptodontines.

Two main groups of glyptodontines can be distinguished by their feeding habits: narrow-muzzled Miocene propalaehoplophorids and wide-muzzled post-Miocene glyptodontines.[75] The propalaehoplophorids were selective feeders, while the post-Miocene glyptodontines were bulk feeders (obtain nutrients by consuming an entire plant). However, because of their body form and fusion of the cervical vertebrae glyptodontines would have needed to forage near the ground. Their craniomandibular joint limited their jaw to side-to-side movement.[76] Glyptodon's jaws had large ridges of osteodentine which could effectively be used to grind food particles before shearing and pushing them via the constant motion of the mandible.[76] They had a well-developed snout musculature, along with a mobile neck region that helped them secure food.[77] The hyoid shows a robust design that suggests Glyptodon had a large and robust tongue, which may have aided in food intake and processing.[78][79]

Like most other xenarthrans, glyptodontines had lower energy requirements than contemporary mammal groups.[80] The stomachs of glyptodontids are mysterious due to being entirely herbivorous, in contrast to modern, omnivorous armadillos which have simple stomachs instead of the chambered ones of sloths.[81] This in conjugation with the proposed idea that aquatic grazing may have caused the isotopes strongly associated with herbivory observed in Glyptodon fossils.[81] However, aquatic grazing in Glyptodon is little supported[82] though more backing for this hypothesis has been found in the related Glyptotherium.[83][84][38] A carbon isotopic analysis of Glyptodon bones by França et al (2015) found that it consumed a variety of both C3 plants and C4 grasses at lower latitudes while it ate exclusively C3 grasses at higher ones, implying an ecological shift based on the climate. A 2012 analysis of isotopes supports this, but the isotopic results are not backed by morphological evidence.[76] The isotopic conclusion would place Glyptodon as a mixed browser in most environments, similar to some other glyptodontines.[85][86] The 2012 paper also noted that Glyptodon may have had a more flexible diet than previously imagined,[87] with a mix of slightly wooded and slightly open habitats as implied by the consumption of C3 and C4 material.[86][56] The C4 plants include groups like Poaceae, Cyperaceae, Asteraceae, and Amaranthaceae based on palynological evidence, meaning that Glyptodon likely ate C4 flowering plants in addition to C3 grasses.[88][85] A mesowear analysis supported their conclusion, however, finding that mixed-feeding causing blunt wear that suggests a more abrasion-dominated diet. This is similar to that of Neosclerocalyptus, but in contrast to Hoplophorus which had sharper wear ends. Neosclerocalyptus favored more open environments despite this, as found by isotopic studies.[43][56] The mesowear angles of Glyptodon were noted to possess a bimodal distribution, implying a difference between populations, sexes, or species in diet.[70]

Tail of Glyptodon, which may have been used in combat.

Glyptodonts are believed to have taken part in intraspecific fighting. It was presumed that since the tail of Glyptodon was very flexible and had rings of bony plates, it was used as a weapon in fights. Although its tail could be used for defense against predators, evidence suggests that the tail of Glyptodon was primarily for attacks on its own kind. A G. reticulatus fossil displays damage done on the surface of its carapace. A study based on this specimen calculated that Glyptodon tails would have been able to generate enough force to break the carapace of another Glyptodon. This suggests that they likely fought each other to settle territorial or mating disputes through the use of their tails, much like male-to-male fighting among deer using their antlers. .[89]

In 2009, a partial skeleton of a prenatal individual of Glyptodon was described that had been found inside of the pelvic region of a carapace of an adult.[90] The skeleton had been collected from the Pleistocene-aged deposits in the Tarija Valley of Bolivia and included a partial skull, partial mandible, and fragments from the scapulae and femora. The skeleton is the only known prenatal specimen of a glyptodontine and is one of the most complete specimens of an immature Glyptodon known, though dozens of isolated osteoderms from juveniles are known.[90] The preserved skull measures only 51 mm long, but still bears many characteristics of Glyptodon such as a subtriangular naris, a lateral margin on the naris that forms an acute angle of 30-degrees, oval infraorbital foramina, and several other traits.[90] However, the mandible differs in that the ascending ramus is at a 90-degree angle in contrast to the 60-70 degree angles preserved in adults. Interestingly, this mandibular morphology is alike to that in some specimens of Glyptotherium cylindricum.[60][90]

In the osteoderms of juvenile Glyptodon reticulatus, the central figures are larger than the peripheral osteoderms. These central figures are planar, sometimes even concave, and elevated compared to the peripherals. The peripherals in younger individuals are also less distinct and bear weakly marked or absent furrows (grooves that separate osteoderms). On the other hand, peripherals and central figures of adults are similarly sized, distinct, and of similar heights.[17][25]

Several interpretations of glyptodontine posture have been made,[91] starting with those by Richard Owen in 1841 using comparative anatomy.[16] Owen theorized that the phalanges were weight-bearing due to their short and broad physiology, in addition to evidence provided in the postcranial skeleton.[16] It was also proposed that an upright posture was possible for Glyptodon, first by Sénéchal (1865) who stated that the tail could be an equilibrium for the front half of the body as well as a method of supporting the legs.[71] Linear measurements were later taken which provided insight into this hypothesis, finding that bipedalism would be possible.[92][93] The patellar articulation with the femur suggests rotation of the lower leg during knee extension and potentially even knee-locking were feasible.[94]

No evidence of sexual dimorphism in Glyptodon has been described, but it has been observed in the close relative Glyptotherium based on fossils found in Pliocene deposits in Arizona. In the genus, the caudal aperture of males and females differ in that the marginal osteoderms of males are much more conical and convex than those of females. Even in the carapaces of newborn Glyptotherium, the marginal osteoderms are either conical or flat, which enables their sex to be determined.[65] No direct evidence of glyptodontine group behavior has been described, though some localities preserving juveniles, subadults, and adults of Glyptotherium together are known.[65][51] Living armadillos are loners and only come together during mating season, with the number of offspring varying between one and even twelve babies depending on the species.[95]

Glyptodon coexisted with a variety of large predators including the cat Smilodon, jaguar, and canid Protocyon.[107][108] This belief is furthered by the discovery of fractured dorsal armor, which implies that Glyptodon had been in physical conflict with other animals.[64] However, isotope analyses of the collagen from Glyptodon and other mammals of the Pampas region by Bocherens et al. (2015) discovered little evidence to support the idea of predators feeding on Glyptodon.[107] Instead, it was found that Glyptodon as well as herbivorous mammals living in denser forests made up a smaller portion of carnivore diets, whereas open grazers such as Lestodon and Macrauchenia were consumed more often.[107] Furthermore, the appearance of secondary armor in the dermis of Glyptodon coincides with the arrival of North American predators in South America during the Great American Interchange.[2] For this reason, it was hypothesized that the osteoderms developed as a defensive/offensive mechanism to combat the new arrivals of the area.[64]

Smilodon may have occasionally preyed upon glyptodontines, based on a skull of Glyptotherium texanum which bears the distinctive elliptical puncture marks that best match those of the machairodont cat, indicating that the predator successfully bit into the skull through the armored cephalic shield.[109] The Glyptotherium in question was a juvenile, with a still-developing head shield, making it far more vulnerable to the cat's attack.[110] Although originally theorized by George Brandes to be possible in 1900,[111] Smilodon canines could not pierce the thick carapace osteoderms of glyptodontines.[112] Brandes imagined that the evolution of thick glyptodontine armor and long machairodont canines was an example of coevolution,[111] but Birger Bohlin argued in 1940 that the teeth were far too fragile to do damage against glyptodontine armor.[112][64]

Humans hunting Glyptodon, by Heinrich Harder

The coexistence of early hunter-gatherer humans and glyptodontines in South America was first hypothesized in 1881 based on fossil discoveries from the Pampas,[113] and many fossil discoveries from the Late Pleistocene to Early Holocene have been unearthed since that exhibit human predation on glyptodontines. No fossils of Glyptodon preserving direct interactions have been unearthed, but it did inhabit this region alongside humans. At the site of Pay Paso 1, an archaeological site in northwestern Uruguay preserving human-made spear points and other signs of culture were found associated with fossils of Glyptodon and the horse Equus. These were used for radiocarbon dating using collagen, supposedly dating to around 9,000 to 9,500 BP but these dates cannot be verified.[114][25] During this period, a wide array of Xenarthrans inhabited the Pampas were hunted by humans, with evidence demonstrating that the small (300–450 kg, 660–990 lb) glyptodontine Neosclerocalyptus,[115] the armadillo Eutatus, and the gigantic (2 ton) glyptodontine Doedicurus, the largest glyptodontine known, were hunted.[116] The only other records of human predation from outside the Pampas area a partial carapace, which was eviscerated by humans, and several skulls preserving signs that they were dispatched by human tools. All were found in Venezuela.[117][51] The discoveries there showed the first signs of human hunting on the skulls of glyptodontines.[118] Hunters may have used the shells of dead animals as shelters in inclement weather.[119][120]