Gee and Parker—Juvenile Metoposaurid from Arizona
Table 1. Measurement data for the intercentra associated with a juvenile specimen of K. perfectus (Case, 1922) (PEFO 35392). Numerical assignments for each element were created solely for the basis of this paper and are ordered from largest to smallest to correspond with Figure 4 with the exception of ele- ment 9, the intercentrum on the dorsal surface of the skull that is not included in Figure 4. Diameter (D) was measured in the mediolateral axis, height (H) was measured in the dorsoventral axis, and length (L) was measured in the anteroposterior axis in the same manner as measurements made by Konietzko- Meier et al. (2013). The length of element 9 could not be measured due to its position.
Specimen No. Estimated position L (mm) D (mm) H (mm) D:L 1
2 3 4 5 6 7 8 9
mid-dorsal
anterior dorsal anterior dorsal mid-dorsal precaudal precaudal precaudal presacral
anterior caudal
15.43 14.55 15.37 14.42 11.17 14.03 9.48 8.57 –
28.27 27.30 25.89 24.47 16.91 16.56 14.68 14.51 25.13
23.71
25.74 1.83 25.15 1.87 24.72 1.68 23.83 1.69 16.44 1.51 15.52 1.17 12.30 1.54 12.33 1.69 –
intercentra of that study belong to PEFO 35392 and sections of this material are briefly described here. In sagittal profile, they are characterized by a relatively wide, ventral periosteal cortex, which is triangular with an apex that terminates at or below the mid-height of the intercentra. This region is made of a parallel- layered matrix that has been partially damaged by secondary precipitation of carbonate minerals. In transverse profile, the periosteal cortex extends up the lateral margins from the ventral margin. The remainder of the internal structure consists of more randomly oriented endochondral trabeculae that are thickest at the articular faces and thinnest in the dorsal portion of the element. The intertrabecular spacing increases in the dorsal region. Calcified cartilage is abundant within the intercentra, including in the interior endochondral region; in M. krasiejo- wensis, calcified cartilage decreases in relative abundance throughout ontogeny, particularly in the interior of the intercentra (Konietzko-Meier et al., 2013). The presence of calcified cartilage in the interior of both intercentra is thus indicative of their relative immaturity (Fig. 5.2, 5.4). A weathered external cortex is preserved in both of the specimens, but it appears to still be relatively well vascularized and shows no evidence of a compact lamellar annulus or LAGs that would indicate relative maturity (Fig. 5.1, 5.3). Using the histological ontogenetic stage (HOS) designations of Konietzko-Meier et al. (2013), we designate these intercentra as HOS 3 based on the presence of a wide periosteal region (which originates in HOS 2) and an external cortex that is still relatively vascularized and lacking in LAGs (which originate in HOS 4).
Discussion
We refer PEFO 35392 to Koskinonodon perfectus based upon: (1) the presence of a relatively deep otic notch, (2) a projecting occiput, and (3) a relatively wide cultriform process, features that readily distinguish it from Apachesaurus gregorii, and the lack of documentation of “M.” bakeri west of Texas (Hunt, 1993; Long and Murry, 1995). Although these anatomical fea- tures could be affected during preservation, as may also be the case in the holotype of A. gregorii, the histological examination
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of two of the associated intercentra provides a strong second line of evidence that the individual was relatively immature at the time of death. This ontogenetic assignment is based on several anatomical and histological features, such as the absence of a compact annulus at the ventral margin and an abundance of calcified cartilage, both of which have been identified as onto- genetically variable in M. krasiejowensis by Konietzko-Meier et al. (2013) (Fig. 5). It is noteworthy that this individual is still relatively immature in spite of being nearly 50% longer than the holotype of A. gregorii, which was interpreted as a mature individual (Hunt, 1993). While PEFO 35392 does not have a clear tabular horn, as is the condition of A. gregorii, it is com- mon for the posterior margins of the skull to be weathered in taxa that normally possess a tabular horn (Fig. 2; personal observation, B.M. Gee, 2017). Therefore, the lack of a clear tabular horn in this specimen should not be automatically interpreted to mean that the structure was not present when the animal was alive, because the feature is narrow and flattened, making it susceptible to degradation and eventual loss. Both otic notch depth and variation in tabular horn shape and size have been recognized as ontogenetically variable features in other metoposaurids (Sulej, 2007), so it is also possible that the tabu- lar horn was greatly reduced at this stage in development. Although these intercentra are not as elongate as the smaller intercentra that are characteristic of A. gregorii, they are more elongate than typical intercentra of K. perfectus, and past workers have ascribed intercentra of similar proportions to those of this specimen to A. gregorii (Fig. 4; Long and Murry, 1995; Spielmann and Lucas, 2012). The ontogenetic designation that results from the histological analysis is in agreement with the intermediate degree of elongation because this specimen repre- sents an intermediate between the early post-metamorphic form and the significantly larger adult form. Accordingly, the inter- mediate proportions of the intercentra in this specimen suggest a somewhat rapid transition toward proportionately shorter inter- centra that have traditionally characterized mature individuals. The acquisition of a morphology more similar to that of large, presumably mature forms early in ontogeny is not surprising; this has been demonstrated in the plagiosaurids, another family of Mesozoic temnospondyls with similar dorsoventral flattening of the skull and a presumed bottom-dwelling ecology (Schoch, 2009; Danto et al., 2016). Their accelerated developmental tra- jectory in comparison to other stereospondyls has been sug- gested to be the result of the selective pressure of predation risk that would have resulted from the relative immobility of these large stereospondyls (Schoch, 2009). It is not implausible to hypothesize a similar trajectory for metoposaurids, considering the broad morphological and inferred ecological similarities in morphology between the two families. If large metoposaurids acquired the adult morphology relatively early in ontogeny, this would confound efforts to identify the relative maturity of spe- cimens based solely on their external features, as was the case in the original designation of A. gregorii as an adult by Hunt (1993). This possibility highlights the importance of multiple lines of analysis for determining the ontogenetic maturity of a specimen, such as in this study where we utilized both external morphology and histology. Unfortunately, informative postcrania such as the intercentra are rarely directly associated with diagnostic cranial material
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