Dire Wolf Bacula of Rancho La Brea

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Dire Wolf Bacula of Rancho La Brea

The Rancho La Brea (RLB) fossil collection at the La Brea Tar Pits and Museum represents one of the largest assemblages of carnivoran fossils in the world.
This is certainly true of the bacula; the collection houses several hundred bacula of Canis dirus.

In the current study, we compare the RLB fossil bacula to those of all of the modern analogues curated at the American Museum of Natural History and the Smithsonian.

Nine measurements were taken on each complete specimen, and three of these were combined to create an overall volumetric size proxy.

Of the large La Brea Tar Pits and Museum collection of C. dirus bacula, 159 are complete. They have been recovered from nearly all of the fossiliferous RLB deposits and span the entire Late Pleistocene accumulations represented at the site. The C. dirus bacula are almost five times as large as those of C. lupus (N 5 10) in terms of a volumetric proxy, and the C. dirus mean total bacular length exceeds that in C. lupus by 44%. The C. dirus bacular width and height variables are even more dramatic in comparison, ranging up to 236% the size of those of C. lupus.

CARNIVORAN AND CANID BACULA
Though the baculum has often been ignored in carnivoran morphometric studies, the few studies
that have focused on this bone have found it to be highly variable between closely related species and
yet fairly morphologically uniform within them; thus, the baculum is a valuable bone for species
differentiation.

THE COMPARATIVE ANATOMY OF CANIS DIRUS
A detailed morphological and ontogenetic analysis has not been done, nor has this bone been thoroughly
compared with that of other canids. However, qualitatively, it is clear that the dire wolf baculum
is substantially different from that of Canis lupus (gray wolf)—the most obvious comparative taxon
and the largest modern canid—and thus a review of the literature on phylogenetic and comparative
postcrania of C. dirus is warranted.

While the postcranial studies of dire wolf indicate some intraspecific variation, and interspecific
variation when compared with C. lupus, the relatively modest variation is in stark contrast to
the results of this study when comparing C. dirus and C. lupus bacula, suggesting that the distinct
C. dirus baculum is an important taxonomic tool for species identification.

MATERIALS AND METHODS
SAMPLE

The RLB specimens were compared with all of the modern relatives in the collections of the
AMNH and USNM. These specimens are uncommonly curated in these collections but the
comparative sample includes all 10 C. lupus bacula available in the two collections and bacula
rom one C. rufus. These were all instrumentally determined maximum or minimum measurements
(see Fig. 2 caption for description).

MEASUREMENTS
Nine measurements (Fig. 2) were taken on each specimen using digital Mitutoyo calipers to the
nearest 0.01 mm connected by cable directly to a spreadsheet. These were all instrumentally
determined maximum or minimum measurements (see Fig. 2 caption for description).

In addition to these raw measurements, we created a volumetric size proxy modeling each
baculum as an elliptical cylinder using the midshaft height (a) and width (b) along with
total length (l), as follows: V 5 p * 0.5a * 0.5b * l.

The morphology of the canid baculum is sufficiently different from the other carnivoran taxa for
which ontogenetic studies have been published that we are reluctant to assign anything but
relative maturation stages within this sample. To that end, we were able to divide the sample into
four clear morphostages (and fairly apparent semistages between these) that almost certainly
correspond to ontogenetic development from youngest (stages 0.5 and 1) to oldest (stages 3.5
and 4) individuals (Fig. 3; Table 1).

Figure 3 Canis dirus bacular ontogenetic stage scores compared with the baculum of C. lupus. Lateral (left) and
dorsal (right) views. From top (stage 0.5) to bottom (C. lupus), these specimens are LACMRLP R30149, LACMHC
PP8866, PP8649, PP9141, PP9009, PP8650, PP8854, PP8638, and AMNH 98230. Canis lupus is stage 3. Some
images are reversed to align all distal ends to the left.

Table 1 Ontogenetic stage score definitions.
Stage Definition
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1. 0.5 Length is up to 50% of maximum length, and bacula are not fully ossified.
2. 1.0 Length is 50%–60% of maximum length, and they are the smallest ossified bacula with porous ends,
a broad distal end, and a proximal end that is not yet fully defined. A substantial urethral groove runs
the entire length of the baculum.
3. 2.0 Length is 70%–80% of maximum length. A deep urethral groove extends up to 90% of the total length,
and the baculum is fairly gracile.
4. 3.0 Length is 80%–90% of maximum length. The more defined distal end is pointed or bulbous, and the
proximal end is more substantial with increased ossification and rugosity. The urethral groove is deep
extends up to 80% of total length, and has encroaching dorsal edges (i.e., the dorsal edges of the
groove are closer in approximation than the maximum mediolateral diameter of the canal)
5. 4.0 Length is 90%–100% of maximum length. The well-defined distal ends are pointed or bulbous and the
proximal ends are very substantially defined with substantial ossification and rugosity. The urethral
groove extends approximately 70% of total length, appearing more constrained due to the lateral
walls encroaching deeply at the dorsal edges. Dorsal edges of the urethral groove are highly rugose,
forming a noticeable edge on the lateral sides of the baculum that extends approximately 50% of the
way toward the ventral side, while the ventral surface appears as a smooth rounded crest.
This appears to be a tendinous origin, possibly of a tissue covering the urethral canal. Thus the
presumably more mature animals would appear to have a stronger urethral cover.

Table 2 Size comparison between C. dirus and C. lupus bacular variables.

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C. dirus (N = 137a) mean (SD) C. lupus (N = 10) mean (SD) Wilcoxon p-value
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Volumetric proxy (mm3) 25,748.1 (9,670.9) 5,637.0 (1,413.8) ,0.0001
Total length (mm) 159.4 (19.7) 109.8 (8.1) ,0.0001
Proximal end length (mm) 29.9 (5.5) 16.73 (5.7) ,0.0001
Proximal end width (mm) 15.4 (2.0) 7.72 (1.6) ,0.0001
Proximal end height (mm) 20.7 (3.4) 8.9 (1.6) ,0.0001
Midshaft width (mm) 14.3 (2.6) 8.2 (1.3) ,0.0001
Midshaft height (mm) 13.9 (2.0) 7.9 (0.7) ,0.0001
Urethral canal width (mm) 4.35 (0.8) 4.2 (0.8) 0.7192
Distal end height (mm) 6.36 (1.6) 4.41 (0.3) ,0.0001
Distal end width (mm) 7.44 (1.5) 4.42 (1.3) ,0.0001


(creating tables in wordpad is a bitch) and all that time to make it look correct and this place rearranges the damned things and removes all formatting no matter how i try to make the table
 
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