Variation in vertebral number and identities in fossil sloths from Hispaniola
Location
Suwanee, GA
Start Date
17-4-2026 12:00 PM
End Date
17-4-2026 1:00 PM
Description
Introduction
Xenarthrans (armadillos, anteaters, and sloths) have unusual vertebral numbers and proportions relative to other mammalian orders. Xenarthrans are also unique in possessing a synsacrum, wherein the sacrum is fused with the pelvic bones. The synsacrum also attaches to the ischium, whereas in most mammals the sacrum only articulates with the ilium. Variations in the number and morphology of synsacral vertebrae were studied in three genera of fossil sloths from Hispaniola: Acratocnus, Neocnus, and Parocnus as recent explorations have produced a great number of fossil remains exhibiting excellent preservation and completeness.
Methods
Fossil sloth pelvises from the Museo Nacional de Historia Natural in Santo Domingo, Dominican Republic were surveyed for their vertebral counts. The definition of a synsacral vertebra was fusion of the vertebral body and/or alae to adjacent vertebrae and/or the presence of a sacral foramen. The morphology of these vertebrae, particularly the cranial- and caudal-most, were compared to lumbar and caudal vertebrae from these taxa.
Results
Overall, 20 individual pelvic specimens were surveyed and synsacral counts were taken. Parocnus (n = 2) individuals both possessed eight total vertebrae, Acratocnus ye (n = 7) had 3 specimens with seven, and 4 had eight. Neocnus sp. (n = 7) had 1 with five and 6 with six, whereas in the Neocnus n. sp. (n = 3), 2 specimens have six and 1 has seven synsacral vertebrae. For each taxon, despite being part of the synsacrum, individual vertebrae may have the morphology of either a lumbar or a caudal. The “lumbar” vertebrae typically possess transverse processes that contributed to the sacral ala, fusing both to adjacent ala and the ilium. The “lumbar” vertebral bodies were also variably fused to the true “sacral” vertebrae. Often, the last vertebra was a “caudal” wherein the transverse processes were fused to the ischium to form a sacroischial fenestra.
Discussion
Variations in vertebral number is a common theme in living sloths but the paucity of complete vertebral remains in extinct sloths makes evaluation of vertebral count differences difficult. That the numbers of sacral vertebrae and the incorporation of caudals and lumbars differs within species suggests vertebral variation is an ancestral trait for sloths. The incorporation of caudal vertebrae, with the transverse processes fusing with the ischium is seen in other Xenarthra, such as armadillos. The incorporation of a “caudal” vertebra into the synsacrum is present in all but three pelvises surveyed. There is at least one Neocnus with a “caudal” fused to the vertebra above, but not to the ischium. There are multiple Acratocnus individuals that have “lumbar” vertebrae where the transverse processes are fused to the ilium and adjacent vertebrae, but there is partial or no fusion of the vertebral bodies. The variable number of sacralized “lumbar” vertebrae largely accounts for the difference in sacral vertebral numbers. In extant sloths, incorporation of “lumbar” elements into the synsacrum are correlated with variations in the presacral vertebral counts, suggesting that vertebral number variability may be a trait associated with this lineage, unlike most mammals.
Embargo Period
5-15-2026
Variation in vertebral number and identities in fossil sloths from Hispaniola
Suwanee, GA
Introduction
Xenarthrans (armadillos, anteaters, and sloths) have unusual vertebral numbers and proportions relative to other mammalian orders. Xenarthrans are also unique in possessing a synsacrum, wherein the sacrum is fused with the pelvic bones. The synsacrum also attaches to the ischium, whereas in most mammals the sacrum only articulates with the ilium. Variations in the number and morphology of synsacral vertebrae were studied in three genera of fossil sloths from Hispaniola: Acratocnus, Neocnus, and Parocnus as recent explorations have produced a great number of fossil remains exhibiting excellent preservation and completeness.
Methods
Fossil sloth pelvises from the Museo Nacional de Historia Natural in Santo Domingo, Dominican Republic were surveyed for their vertebral counts. The definition of a synsacral vertebra was fusion of the vertebral body and/or alae to adjacent vertebrae and/or the presence of a sacral foramen. The morphology of these vertebrae, particularly the cranial- and caudal-most, were compared to lumbar and caudal vertebrae from these taxa.
Results
Overall, 20 individual pelvic specimens were surveyed and synsacral counts were taken. Parocnus (n = 2) individuals both possessed eight total vertebrae, Acratocnus ye (n = 7) had 3 specimens with seven, and 4 had eight. Neocnus sp. (n = 7) had 1 with five and 6 with six, whereas in the Neocnus n. sp. (n = 3), 2 specimens have six and 1 has seven synsacral vertebrae. For each taxon, despite being part of the synsacrum, individual vertebrae may have the morphology of either a lumbar or a caudal. The “lumbar” vertebrae typically possess transverse processes that contributed to the sacral ala, fusing both to adjacent ala and the ilium. The “lumbar” vertebral bodies were also variably fused to the true “sacral” vertebrae. Often, the last vertebra was a “caudal” wherein the transverse processes were fused to the ischium to form a sacroischial fenestra.
Discussion
Variations in vertebral number is a common theme in living sloths but the paucity of complete vertebral remains in extinct sloths makes evaluation of vertebral count differences difficult. That the numbers of sacral vertebrae and the incorporation of caudals and lumbars differs within species suggests vertebral variation is an ancestral trait for sloths. The incorporation of caudal vertebrae, with the transverse processes fusing with the ischium is seen in other Xenarthra, such as armadillos. The incorporation of a “caudal” vertebra into the synsacrum is present in all but three pelvises surveyed. There is at least one Neocnus with a “caudal” fused to the vertebra above, but not to the ischium. There are multiple Acratocnus individuals that have “lumbar” vertebrae where the transverse processes are fused to the ilium and adjacent vertebrae, but there is partial or no fusion of the vertebral bodies. The variable number of sacralized “lumbar” vertebrae largely accounts for the difference in sacral vertebral numbers. In extant sloths, incorporation of “lumbar” elements into the synsacrum are correlated with variations in the presacral vertebral counts, suggesting that vertebral number variability may be a trait associated with this lineage, unlike most mammals.