A case of tail mutilation in a north Texas specimen of the Pleistocene Dasypus bellus (Xenarthra: Dasypodidae).
On 10 Sept. 2009, one of us (RWS) recovered the intact, partially exposed tail tip of a large armadillo (MWSU-VP 14416) from a compact sand bank on the eastern shoreline of the Brazos River in Young County, Texas, approximately 3.0 mi E Elbert, Throckmorton County. This mineralized specimen retains the intact osteoderms of one complete tail ring and the damaged remnants of another (Fig. la, left), similar to comparable tail damage seen in a modern armadillo (Fig. la, right). The undamaged segment retains the enclosed caudal vertebra (Fig. lb), and the stub terminates in a tapering boney cap (Figure 1c). This specimen of D. bellus is unusual because of the intact nature of the specimen and the preservation of damaged tissue sustained by the animal.
Species determination was determined on several grounds. First, D. novemcinctus did not occur locally until the 1950s (Dalquest & Horner, 1984). Some degree of mineralization was readily apparent, and the sound when tapped with a metallic object was reminiscent to that of tapping porcelain. To demonstrate the extent of replacement or mineralization, two small (ca. < 2 [mm.sup.3]) fragments were removed from the interior of the fossil specimen and subjected to an energydispersive EDAX detector which was attached to a Philips XL 30 scanning electron microscope. The results were compared with two samples of bone (one endochondral, one dermal) from a modern animal. Representative component elements (and % weight) of the fossil were:
Al--7.24, 5.16; Si--14.22, 11.97; Fe--4.34, 3.88; Na--1.39, 1.21.
Elements (and % weight; dermal and endochondral, respectively) of a modern specimen were:
Al--0.50, 1.18; Si--1.03, 1.23, Fe--2.05, 0.44; Na--14.11, 5.72.
A series of 15 study skins of D. novemcinctus from the collections of Midwestern State University were assessed for comparative purposes, and included individuals ranging from subadult to old adult (for age criteria, see Stangl et al. 1995). Nearly half (n = 7) of the specimens provided examples of tail damage, ranging from superficial bite marks to the loss of the terminal two-thirds of the tail. Intact and undamaged tails were associated with subadults or young adults, whereas the tails of older animals provided examples of the most severe damage (e.g., Fig. la, right).
Initial side-by-side inspection suggested that the intact and undamaged fossil segment most closely corresponded to the 7th-9th tail segments of modern D. novemcinctus. Scute counts were taken from the distal-most row of the seventh, eight, and ninth tail segments, and the greatest width of each of the three segments were determined with digital calipers (to nearest 0.01 mm).
Dimensions of tail segments of modern armadillos decrease significantly from proximal to distal, and overlap of diameters between adjoining segments is minimal, even with inclusion of subadults (Table 1). Scute counts also decline significantly from proximal to distal, with minimal overlap (Table 1). This character is established in utero. For example, the 7th ring scute count of a neonate, MWSU 19810, was 14 and corresponded precisely to the number of underlying osteoderms. The intact segment of the fossil specimen (width of tail ring, 26.6; osteoderm/scute count, 15) corresponds most closely with the 7th segment in modern armadillos, and almost precisely matches one modern animal (MWSU 728; 7th ring width, 26.9; scute count, 15).
Table 1. Greatest tail ring widths (in mn) and scute counts from distal-most row of each ring, taken from a series of the modern nine- banded armadillo (Dasypus novemcinctus) for comparisons with a specimen of the Pleistocene beautiful armadillo (D. bellus; MWSU-VP 14416, width of 26.6, scute count of 15). Numbering system of caudal rings is proximal to distal. Descriptive statistics are: sample size of specimens examined (N); mean, standard deviation (SD), range (minimum- maximum), and confidence intervals (C.I.). Tail Mean [+ or -] SD Range 95% C.I. Tukey-Kramer segment (N) Test Width of tail ring (mm) *** Ring 7 (15) 27.4 [+ or -] 1.6 23.4 - 29.4 26.5 - 28.2 | Ring 8 (14) 24.3 [+ or -] 1.4 20.7 - 26.2 23.5 - 25.0 | Ring 9 (14) 21.2 [+ or -] 1.2 18.5 - 22.8 20.6 - 21.9 | Distal scute count *** Ring 7 (15) 14.1 [+ or -] 0.8 13 - 15 13.6 - 14.5 | Ring 8 (14) 13.1 [+ or -] 0.8 12 - 14 12.7 - 13.6 | Ring 9 (14) 11.9 [+ or -] 0.7 11 - 13 11.5 - 12.4 | *** One-way analysis of variance (ANOVA), *** = P < 0.001; Tukey-Kramer multiple comparison tests, bars indicate statistically significant subsets (at P < 0.05 level).
Possible causative factors initiating the observed damage bony regeneration in fossil and extant armadillos is speculative, but would certainly include the bite or chewing of a persistent predator. However, it is noted that D. novemcinctus (and presumably also D. bellus) is a functional heterotherm with fluctuating body temperature values reported at 30-36 [degrees]C, depending on ambient conditions (McBee & Baker, 1982, and references therein). This physiological feature would be expected to rend extremities susceptible to tissue damage from frostbite, and subsequent loss of resulting gangrenous tissues. Comparable damage to other extremities of local D. novemcinctus (e.g., toes, ear pinnae) is not noted, although the tail may simply be more vulnerable.
The fossil specimen of D. bellus may well represent an older animal whose longevity was a measure of time of exposure to such potential threats as might cause the observed damage. If the fossil specimen represents the intact 7th and damaged 8th tail segments, then the living animal would have been comparable in size to modern animals. If this scenario is correct, then perhaps the specimen dates back to earlier Irvingtonian times corresponding with both the "smallish" armadillo scute from the Slaton Quarry (see Dalquest & Schultz 1992), and reflecting the temporal size trend for D. bellus described by Klippel & Parmalee (1984).
We thank Jim Goetze and an anonymous reviewer for suggestions leading to the improvement of this manuscript.
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Frederick B. Stangl, Jr., Robert W. Stewart, and Dana R. Mills
Department of Biology, Midwestern State University Wichita Falls, Texas 76308
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|Author:||Stangl, Frederick B., Jr.; Stewart, Robert W.; Mills, Dana R.|
|Publication:||The Texas Journal of Science|
|Date:||Feb 1, 2010|
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