Glyptotherium

The generic name Glyptotherium comes from the Greek roots glyph meaning "carved" or "grooved", after its relative Glyptodon, and therion meaning "beast", a commonly used suffix for prehistoric mammals. The species name of the type species, G. texanum, is after the holotype's discovery in Texas.[1] The species name cylindricum meaning "cylindrical" is after the cylindroid anatomy of the premolars in the holotype of G. cylindricum.[2]

Fossils attributable to Glyptotherium have been found as early as the 1870s, when civil engineers J. N. Cuatáparo and Santiago Ramírez collected a skull, nearly complete carapace, and associated postcranial skeleton of a glyptodont from a drainage canal near Tequixquiac, Mexico, the fossils coming from the Rancholabrean Pleistocene.[3][4][5] This was the first discovery of a glyptodont in North America.[4][5] Cuatáparo and Ramírez named the fossils Glyptodon mexicanum in 1875, but the fossils have been lost.[3][5] Another species of Mexican Glyptodon was described in 1889, G. nathoristi, by German paleontologists based on carapace remains from Pleistocene localities in Ejutla, Oaxaca.[5][6] Both of these species have since been synonymized with G. cylindricum.[4][5]

The first Glyptotherium fossils to be described from the United States were described in 1888 by paleontologist Edward Drinker Cope and consisted only of a single carapace osteoderm that had been collected from the Lower Pleistocene “Equus Beds” of Nueces County, Texas.[1][4] Cope named his osteoderm Glyptodon peltaliferus,[7] but Cope did not give the species a proper description that followed ICZN rules, making it a nomen nudum and it has since been synonymized with G. cylindricum .[4][5] The next year, Joseph Leidy named Glyptodon floridanus based on isolated carapace osteoderms and pieces of caudal armor, though some were also referred to G. peltaliferus,[8] that were collected from Pleistocene deposits in DeSoto County, Florida.[5][9] This species is now seen as a nomen vanum and synonymous with Glyptotherium cylindricum.[4][5]

The holotype of G. texanum next to a skeleton of the Southern naked-tailed armadillo.

Glyptotherium wasn't named until 1903 when fossils collected by an American Museum of Natural History expedition led by J. W. Gidley to the Early Pliocene strata from the Blanco Formation of Llano Estacado, Texas were described by Henry Fairfield Osborn as a new genus and species of glyptodont, Glyptotherium texanum.[1][5] The fossils were deposited at the AMNH and consist of a carapace and associated postcranial elements, one of the few G. texanum skeletons known.[1][4] The skeleton was put on display in the exhibit hall of the AMNH where it remains today.[1]

Another find came in 1910 when, while traveling in Jalisco, Mexico, fossil hunter Barnum Brown collected a complete dorsal carapace and several additional fossils, including teeth, of a single individual from the Pleistocene strata of the area.[5][2] The specimen was sent to the American Museum of Natural History as well, where it was described by Brown in 1912 as a new genus and species of glyptodont, Brachyostracon cylindricum.[2] Brown also recombined Glyptodon mexicanum into Brachyostracon mexicanum.[2] Brachyostracon is now seen synonymous with Glyptotherium on the genus level, but G. cylindricum is a valid species.[4][5] In 1923, Oliver Perry Hay named a new species of Glyptodon, G. rivipacis, based on the fossils described by Leidy from DeSoto County, Florida.[10] This species is now seen as a nomen nudum and synonymous with Glyptotherium cylindricum.[4][5] Hay also described well preserved fossils, including skull elements and teeth, that were collected from Rancholabrean strata in Wolf City and Sinton, Texas that were referred to Cope's Glyptodon peltaliferus.[5] These fossils have since been referred to G. cylindricum.[4][5]

A third genus of North American glyptodont, Boreostracon floridanum, was established by George Gaylord Simpson in 1929 based on several isolated specimens unearthed by the AMNH from Rancholabrean age localities in Florida, the holotype specimen being the rear portion of a carapace recovered from Seminole Field locality in Pinellas County, Florida.[5][11] Simpson referred all of the fossils previously described from Florida to B. floridanum and believed that all of the glyptodont fossils unearthed from North America were not of Glyptodon.[11] However, Simpson didn't designate a new genus or species for Glyptodon peltaliferus, but he still believed that they were from a separate form of glyptodont.[11] Boreostracon floridanum has been synonymized with Glyptotherium cylindricum.[4]

Research into North American glyptodonts diminished after the research of Gidley, Hay, Simpson, and others, but some paleontologists still incorrectly referred fossils from the continent to Glyptodon.[5]

Throughout the 1920s and 1930s, many Early Pleistocene age fossils were collected from a locality in Frederick, Oklahoma, including several fragmentary fossils of glyptodonts, horses, gomphotheres, and camels.[12] These glyptodont fossils were described as a new genus and species, Xenoglyptodon fredericensis, in 1953 on the basis of a partial lower jaw and several teeth.[12] The species has been synonymized with Glyptotherium texanum.[4][5] After all of these genera were named, a great reassessment wasn't conducted until 1981, David Gillette and Clayton Ray published a monograph on North American glyptodonts. In their monograph, they recombined all previously named genera and species into Glyptotherium, synonymized some species, and also researched the genus' ecology, anatomy, and distribution.[5] However, G. arizonae, G. floridanum, and G. mexicanum were kept as valid species,[5] all of which were synonymized with G. texanum and G. cylindricum after the discovery of more complete skeletons that proved their synonymy.[4][13] After later review, the former species was synonymized with G. texanum[4][13] and the latter two synonymized with G. cylindricum.[4] From the 2000s to the 2020s, hundreds of additional fossils were referred to the genus from Central America and Brazil, including fossils formerly assigned to Glyptodon.[14][15] In 2022, a host of fossils of Glyptotherium cylindricum including skulls, some preserving pathologies caused by humans, were described that had been collected from several sites in Falcon, northern Venezuela that dated to the Late Pleistocene-Early Holocene.[16]

Like its living relative, the armadillo, Glyptotherium had a shell that covered its entire torso, with smaller armor also covering the skull roof of the head, similar to a turtle. However, unlike the carapace of a turtle, the Glyptotherium shell was made up of hundreds of small hexagonal scales, with Glyptotherium preserving up to 1800 osteoderms or more in each individual.[5] The axial skeleton of glyptodonts show extensive fusion in the vertebral column and the pelvis is fused to the carapace, making the pelvis entirely immobile.[5] Glyptotherium was very graviportal and had short limbs that are very similar to those in other glyptodonts. The large tails of glyptodonts likely served as a counterbalance to the rest of the body and Glyptotherium's caudal armor ended in a blunt tube that was composed of 2-3 fused tubes,[4] in contrast to those of South American mace-tailed glyptodonts.[5] The digits of Glyptotherium are very stout and adapted for weight-bearing, though some preserve large claw sheaths that had an intermediate morphology between claws and hooves.[5] During the Pleistocene, the diversity of glyptodonts diminished but increased in size, with the largest known glyptodont, Doedicurus, evolving in the Pleistocene.[16][4] Glyptotherium weights and sizes vary, but G. texanum was smaller and lighter than the later species, G. cylindricum. One specimen of G. cylindricum was estimated at 350-380 kilograms, compared to 457 kg of its relative Glyptodon reticulatus.[16] The largest estimates of Glyptotherium's mass are based on adult specimens from Brazil and Arizona, with one estimate of a G. cylindricum specimen placing it at 710 kilograms (1,570 lb) and a G. texanum individual at 1,165 kilograms (2,568 lb).[25] One specimen of G. cylindricum, AMNH 15548, preserved a carapace length of 1690 centimeters compared to just 1400 cm for MSM P4464, a G. texanum specimen.[4]

Glyptotherium texanum differs from G. cylindricum in the configuration of the external sculpturing of symmetrical, hexagonal osteoderms in the dorsal carapace. In the osteoderms of adult G. texanum, the central figures of the osteoderms are flat or weakly convex and very large, while those of G. cylindricum are flat or concave and much smaller.[4][26] In both species, the central figures of the osteoderms get larger towards the margins of the carapace.[4][13]

Carapace and osteoderms

Osteoderm of Glyptotherium (A) and (B), displaying the "rossette" pattern.

Glyptotherium, and all other glyptodonts, had a large dorsal carapace covering much of the dorsum that was made up of interconnected osteoderms. The carapace of Glyptotherium was shorter than that of its relative Glyptodon, but much more elongated than that of Boreostemma. The high point of the carapace was at the center of the midline, while Glyptodon’s was slightly displaced. Glyptotherium's carapace was strongly arched, with a convex pre-iliac and concave post-iliac, giving it a saddle-like overhang over the tail.[4][30] In Glyptodon the top-bottom height of the carapace represents 60% of its total length, whereas in Glyptotherium it is taller at ca. 70%. The ventral margins of the carapace in Glyptotherium is much more rectangular and less convex than in Glyptodon. In Glyptotherium, the osteoderms in the antero-lateral areas of the carapace are less ankylosed than in Glyptodon, suggesting that the antero-lateral carapace regions of the former had more flexibility. The osteoderms of the caudal aperture are more conical in Glyptodon and more rounded in Glyptotherium, though in the latter the anatomy of the caudal aperture osteoderms varies by sex[13] while in Glyptodon it varies by age.[31] Although frequently used to differentiate the two taxa, Glyptotherium and Glyptodon have very similar osteoderm morphologies that differ only in several areas. Both genera have very thick osteoderms compared to those of many South American glyptodonts like Hoplophorus and Neosclerocalyptus, but Glyptotherium always preserve a "rossette" pattern, where the osteoderm's central figure is surrounded by a row of peripheral figures. Some Glyptodon specimens preserve these "rossettes", but others lack them. The central and radial sulci are deeper and broader in Glyptodon (ca. 4–6 mm) than in Glyptotherium (ca. 1–2.4 mm).[4][13] Notably, Glyptotherium osteoderms preserve small gaps for hair follicles in the sulci that indicate that Glyptotherium had a "fuzzy" carapace with fur coming out. The number of follicoles varies between ages and the area of the carapace, with juveniles having more follicles than adults and fewer follicles are known from the lateral, caudal, and rear portions of the carapace.[5][13]

Glyptotherium was primarily a grazer in forested grasslands and arboreal savannahs, though they may have preferred grasslands near water sources based on fossils from Mexico.[32] Due to their wide distribution, Glyptotherium’s paleoecology may have varied across regions and its 2 species.[15][32] Fossils from Guatemala were found from high altitudes, showing that Glyptotherium was highly adaptable and could live in a variation of environments. This is supported by the generalist diet of Glyptotherium and fossil's discoveries in tropical, subtropical, forested, and even semi-aquatic environments. In the Mexican state of Yucatan, fossils of Glyptotherium and the ground sloth Paramylodon are known from water-rich areas like riparian forests and swamps as opposed to open grasslands.[43]

In North America, Glyptotherium is known from 2 different species that lived in 3 different intervals, the Blancan, Irvingtonian, and Rancholabrean, across several US states and Mexico.[4] During the Blancan, Glyptotherium texanum coexisted with many native genera from North America, as Beringia hadn't yet formed. Because of this, the fauna of the Blancan starkly contrasted with the fauna of the Pleistocene and Holocene. The Blancan age strata of western Texas, New Mexico, and Arizona preserves the gomphothere proboscideans Stegomastodon and Cuvieronius, equids represented by grazers like Nannipus and Equus. Other xenarthrans are also known, like the megalonychid ground sloth Megalonyx and the mylodontid Paramylodon. Fossils of small mammals have also been unearthed, like the insectivores Hesperoscalops and Sorex. A giant fossil ground squirrel, Paenemarmota, is also known from the Blancan.[44] The carnivores include the unusual “bone-crushing” dog relative Borophagus and the “running hyena” Chasmaporthetes,[45] in addition to the "saber-toothed" cat Smilodon gracilis.[46] Some isolated bird fossils have also been found of vultures, falcons, and possibly corvids.[45][47][48]

Restoration of the grassland ecosystem present in the BIR.

In the Brazilian Intertropical Region in eastern Brazil, Glyptotherium was a mixed grazer in arboreal savannahs, tropical grasslands, and other grassy areas near water sources. Large, mesoherbivore mammals in the BIR were widespread and diverse, including the cow-like toxodontids Toxodon platensis and Piauhytherium, the macraucheniid litoptern Xenorhinotherium and equids such as Hippidion principale and Equus neogaeus. Toxodontids were large mixed feeders as well and lived in forested areas, while the equids were nearly entirely grazers. Other xenarthran fossils are present in the area as well from several different families, like the giant megatheriid ground sloth Eremotherium, the scelidotheriids Catonyx and Valgipes, the mylodontids Glossotherium, Ocnotherium, and Mylodonopsis. Smaller ground sloths such as the megalonychids Ahytherium and Australonyx and the nothrotheriid Nothrotherium have also been found in the area. Eremotherium was a generalist, while Nothrotherium was a specialist for trees in low density forests, and Valgipes was an intermediate of the two that lived in arboreal savannahs. Other glyptodonts and cingulates like the grazing glyptodont Panochthus and the omnivorous pampatheres Pampatherium and Holmesina were present in the open grasslands. A proboscidean species has also been found in the BIR, Notiomastodon platensis, was also present and was a mixed grazer on the open grasslands. Carnivores included some of the largest known mammalian carnivores, like the giant felid Smilodon populator and the bear Arctotherium wingei.[33][34] Several extant taxa are also known from the BIR, like guanacos, giant anteaters, collared peccaries, and striped hog-nosed skunks.[49] Two crab-eating types of extant mammals are also known from the BIR, the crab-eating raccoon and the crab-eating fox, indicating that crabs were also present in the region.[49] The environment of the BIR is unclear, as there were both several species that were grazers, but the precede of the arboreal fossil monkeys Protopithecus and Caipora in the area causes confusion over the area's paleoenvironment. Most of Brazil was thought to have been covered in open tropical cerrado vegetation during the Late Pleistocene, but if Protopithecus and Caipora were arboreal, their presence suggests that the region may have supported a dense closed forest during the Late Pleistocene.[49][50] It is possible that the region alternated between dry open savannah and closed wet forest throughout the climate change of the Late Pleistocene.[51]

G. texanum skull preserving bite marks from a Smilodon on the skull roof.

Smilodon may have occasionally preyed upon Glyptotherium, based on a skull from one Glyptotherium texanum individual recovered from Pleistocene deposits in Arizona bearing 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.[52] The Glyptotherium in question was a juvenile, with a still-developing head shield, making it far more vulnerable to the cat's attack.[53]

In 2017, a right ulna of an adult Glyptotherium that had been collected from Late Pleistocene-Early Holocene strata in Rio Grande do Norte State, Brazil was described that bore several gnawing traces from a new ichnospecies of Machichnus, M. fatimae, that may have been caused by a juvenile of the canid species Protocyon troglodytes or an adult individual of Cerdocyon thous.[54]

The first report of possible human consumption or interaction with Glyptotherium or its fossils came in 1958, where several osteoderms that were possibly consumed by humans were described from the Clovis site in Lewisville, Texas,[55][56] though there is little evidence to back up this assessment.[57] In 2022, a host of fossils of Glyptotherium cylindricum including skulls were described that had been collected from several sites in Falcon, northern Venezuela that dated to the Late Pleistocene-Early Holocene.[57] These discoveries were notable not only because they preserved skulls, but four of them exhibited breakages in the fronto-parietal region, a pattern in all of the skulls. Visual and CT evidence indicates that these were likely caused by a mechanical effect by direct percussion, most likely a blow with a stone chopper or club, which caused the bones in the region to fragment into the soft internal tissues of the skull. Despite the fact that the skulls were complete and showed no signs of taphonomic distortion or transport, they often lacked their jaws. The jaws may have been removed for "hunters" to access and consumed masticatory muscles and tongue. The coexistence of early hunter-gatherer humans and glyptodonts in South America was first hypothesized in 1881 based on fossil discoveries from the Argentine Pampas,[58] and many fossil discoveries from the Late Pleistocene to Early Holocene have been unearthed since that exhibit human predation on glyptodonts. During this period, a wide array of Xenarthrans inhabited the Pampas were hunted by humans, with evidence demonstrating that the small (300-450 kg) glyptodont Neosclerocalyptus,[59] the armadillo Eutatus, and the gigantic (2 ton) glyptodont Doedicurus, the largest glyptodont known, were hunted.[60] The only other record of human predation from outside the Pampas was a partial carapace, found also in Venezuela, that was eviscerated by humans. The discoveries in the localities in Falcón showed the first signs of human hunting on the skulls of glyptodonts, but Glyptotherium also was more defenseless than glyptodonts like Doedicurus.[57][61]

Distribution of Glyptotherium red) compared to Glyptodon's (blue).

Glyptotherium is the only known North American glyptodont and is known from several regions of the continent from different periods. During the Blancan stage of the Early Pliocene, Glyptotherium texanum inhabited only central Mexico based on the discovery of a single osteoderm of Glyptotherium texanum from the early-middle Pliocene strata of Guanajuato, central Mexico.[4] In the Blancan-Irvingtonian stages of the Early Pleistocene, G. texanum fossils are known from most of Mexico as well as the U.S. states of Arizona, Texas, Oklahoma, and Florida.[4] In the Rancholabrean of the Late Pleistocene, G. cylindricum evolved from G. texanum and its fossils have been unearthed from northern Venezuela, eastern Brazil, Central America, Mexico, and the U.S. states of Texas, Louisiana, Florida, and even as far north as South Carolina.[4][62] Fossils from Glyptotherium from the Early Pliocene have not been found in Central America, but it is likely that G. texanum inhabited the area during the Great American Biotic Interchange. Unfortunately, glyptodont fossils from the middle-late Irvingtonian are not known from North America, creating a “glyptodont gap” in the fossil record.[4]

Although commonly regarded as an exclusively North American genus, fossils of Glyptotherium from northern South America in areas like Brazil and Venezuela have been discovered.[14][16] These include fossils previously referred to Glyptodon and Hoplophorus from southeastern and northeastern Brazil, as many fossils were hastily assigned to both by 19th century paleontologists.[14][21] One of the specimens reassigned to Glyptotherium, an isolated dorsal carapace osteoderm, was collected from Pleistocene age carbonate caves in Lagoa Santa, Minas Gerais, Brazil by Friedrich von Sellow during the early 1800s.[14] It was later described as a new species of Hoplophorus, H. meyeri, in 1845 by Danish paleontologist Peter Wilhelm Lund.[63][21][14] Lund incorrectly named the taxon however, making it a nomen nudum.[14] The osteoderm was referred to Glyptotherium in 2010, making it the first known Glyptotherium specimen.[14] The fossils from South America are usually only osteoderms or caudal rings and are sometimes indeterminate on a species level, but are most likely from Glyptotherium cylindricum.[4][14][16]

Glyptotherium fossils have also been collected from Central America in Guatemala,[64] Costa Rica,[65] Honduras,[66] El Salvador,[67] and Panama.[68] The fossils from Central America are usually isolated and fragmentary, with the majority being osteoderms or isolated molariforms.[4] It is most likely that the first Glyptotherium populations started in Central America during the Great American Biotic Interchange in the Late Pliocene based on their paleobiogeography.[4][13] Glyptotherium fossils from Central America are sometimes placed as an indeterminate species,[15][14] but most are placed in Glyptotherium cylindricum or its synonyms.[64][65] This referral is also based on the age of the fossils, as fossils from the Pliocene and early Pleistocene are from G. texanum while G. cylindricum is from the late Pleistocene.[4][69]

• Glyptodon
• Boreostemma