Research Article: Ontogenetic changes in the long bone microstructure in the nine-banded armadillo (Dasypus novemcinctus)

Date Published: April 25, 2019

Publisher: Public Library of Science

Author(s): Christian Thomas Heck, David J. Varricchio, Timothy J. Gaudin, Holly N. Woodward, John R. Horner, Anthony Fiorillo.


Analysis of ontogenetic changes in long bone microstructure aid in vertebrate life history reconstructions. Specifically, osteohistological examination of common fauna can be used to infer growth strategies of biologically uncommon, threatened, or extinct vertebrates. Although nine-banded armadillo biology has been studied extensively, work on growth history is limited. Here we describe long bone microstructure in tibiae and femora of a limited ontogenetic series of nine- banded armadillos (Dasypus novemcinctus) to elucidate patterns of bone growth. The cortex of the smallest individual is composed of compacted coarse cancellous bone (CCCB) and woven tissue. Extensive cortical drift is driven by periosteal erosion and further compaction of trabeculae resulting in an increase in the amount of CCCB. The cortex of the largest specimens is primarily CCCB with thickened endosteal bone and thin outer cortices of lamellar and parallel-fibered tissue. The outer cortices of the largest individuals are interpreted as an external fundamental system (EFS) indicating a cessation of appositional bone growth corresponding to skeletal maturity (i.e. asymptotic or adult size). The EFS forms in femora prior to tibiae, indicating femoral growth rates begin decreasing earlier than tibial in D. novemcinctus. Growth trends in common fauna like the nine-banded armadillo can be used as a foundation for understanding life histories of related, but uncommon or extinct, species of cingulates.

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Recent molecular work suggests that Xenarthra is one of the four original clades of placental mammals [1–6]. Xenarthra is composed of sloths (Folivora), anteaters (Vermilingua), and armadillos (Cingulata). Cingulata contains all modern armadillos (Dasypodidae and Chlamyphoridae) and two extinct sub-families nested within Chlamyphoridae: Glyptodontinae and Pampatheriinae [7,8]. Currently, only eight of the 20 extant species of armadillo, including the nine-banded armadillo, are listed as Least Concern in the International Union for Conservation of Nature Red List of Threatened Species [9], and an additional five species lack sufficient data for assessment [10,11]. General biological, ecological, and physiological characteristics of a species must be understood in order to successfully implement conservation strategies [11,12]. Despite a recent increase in armadillo studies, gaps in armadillo research continue to inhibit conservation efforts due to difficulties in observing and tracking individuals in the wild and maintaining individuals in captivity [11,13–16]. Here, we describe growth patterns in wild Dasypus novemcinctus through analysis of long bone microstructure for their potential utility in future ecological, biological, and paleontological studies of cingulates.

This study marks the first ontogenetic analysis, albeit a limited ontogenetic series, of bone microstructure changes in the long bones of the nine-banded armadillo, D. novemcinctus. Our smallest sampled femur, UTCM 802 (femoral length = 77.20 mm), has endosteal resorption lines within the cortex indicating moments of cortical drift. Endosteal resorption lines are likewise found throughout our entire sample of femora. Resorption cavities within the cortex are present in all individuals sampled, decreasing in frequency in larger individuals. Resorption cavities are more numerous throughout the cortex in the lateral corner of the femur, where resorption and deposition occur more rapidly due to the close proximity of the third trochanter. The mid-cortex of D. novemcinctus is composed mostly of CCCB with regions of primary woven tissue. Primary bone is resorbed both endosteally and periosteally throughout the femur, with the exception of the lateral corner, where only endosteal resorption occurred. All individuals have woven bone with longitudinal vascular canals in the lateral region and the number of secondary osteons in the lateral region increases through ontogeny. Vascularity in the lateral corner decreases in larger individuals and shifts to more longitudinally oriented canals within primary and secondary osteons. CCCB is formed from the infilling of cancellous bone, such as trabeculae, as the bone undergoes cortical drift. The extent of CCCB within the cortex increases throughout ontogeny in the femora of D. novemcinctus. An external fundamental system composed of poorly vascularized lamellar and parallel-fibered bone is deposited intermittently on the periosteally eroded surface of the femur in the larger individuals sampled. The EFS appears to be deposited first along the postero-medial and antero-medial corners. Deposition of the EFS then continues on the medial side and antero-medial side before being deposited lastly along the posterior and lateral sides. The EFS is continuous around the entire circumference of femora from UTCM 1557 (femur length = 95.59mm) and OMNH 40175 (femur length = 99.7mm), but not OMNH 40173 (femur length = 97mm). The incomplete nature of the EFS in OMNH 40173 indicates appositional growth was still occurring at the time of death. Lack of an EFS in OMNH 40173, despite a femoral length comparable to UTCM 1557 and OMNH 40175, could be due to individual variation or sexual dimorphism (see discussion below). The thickness of the cortex in the femur increases with size primarily as a result of thickening of the endosteal bone and deposition of the outer cortex of lamellar and parallel-fibered bone.

We examined changes in bone microstructure in tibiae and femora in an ontogenetic series (n = 6) of the nine-banded armadillo (D. novemcinctus), with the intent that nine-banded armadillo osteohistology could be used as a comparative tool for reconstructing the life histories of biologically rare, threatened, or extinct cingulates. In ontogenetically younger individuals, femora and tibiae of nine-banded armadillos are generally composed of compacted coarse cancellous bone with regions of primary woven tissue. Periosteal and endosteal resorption and differential deposition occurs extensively as cortical drift takes place in older individuals. Drift and elongation also result in further compaction of cancellous bone previously located in the metaphysis [33]. Two specimens (UTCM 1557 and OMNH 40175) had a complete external fundamental system in both the tibia and femur, indicating cessation of linear growth and achievement of skeletal maturity. However, the presence of the EFS did not appear consistently in larger individuals, indicating individual variation in growth and timing of skeletal maturity. The three largest specimens had either a double-LAG (UTCM 1557 and OMNH 40175) or two distinct LAGs (OMNH 40173) in the outer cortex that did not continue around the entire circumference of the bone.




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