This study confirms the work of Etheridge (1964) in suggesting the peculiarity of the genus Phrynosoma within the family Iguanidae. The anterior osteology and myology of Phrynosoma platyrhinos platyrhinos Girard and Phrynosoma douglassi hernandesi Girard indicate Phrynosoma to be highly specialized and to differ significantlv from the Iguanid structure as portrayed by Oelrich (1956), Robison and Tanner (1962), and Avery and Tanner (1964). They also differ because of: ( 1 ) absence of lacrimal and postfrontal bones; (2) occurrence of occipital (also mandibular and temporal) spines; (3) posterodorsal shift of the skull elements; (4) divided nature of the M. sternohyoideus, M. subscapularis II and episternocleidomastoideus; (5) reduced nature of the M. serratus. M. trapezius, M. obliquus abdominis extemus; and (6) greater mass of the M. branchiohyoideus.
The absence of lacrimal and postfrontal bones, occurrence of occipital (also mandibular and temporal) spines, and posterodorsal shift in the shape of the skull indicate Phrynosoma to be a highly specialized iguanid genus.
The divided nature of the M. sternohyoideus, M. subscapularis II, and M. episternocleidomastoideus, and the reduced condition of the M. serratus, M. trapezius, and M. obliquus abdominis extemus, and the expanded nature of the M. branchiohyoideus are also suggestive of specialization of the genus Phrynosoma from the iguanid structure as portrayed by other authors (loc. cit).
The structure of the M. biceps in Phrynosoma indicates that this is more closely related to Sceloporus and Crotaphytus than to Sauromalus, in that a small, anterior, muscular body of the M. biceps is lacking in the latter.
Comparison of the anterior anatomy of Phrynosoma platyrhinos and Phrynosoma douglassi shows platyrhinos to be more specialized than douglassi in that platyrhinos shows a greater departure from the basic iguanid structure. In platyrhinos the tendency for a reduction in size and a loss of a sternal articulation for the third sternal rib, segmentation of the M. latissimus dorsi, and a reduction in size of the M. obliquus abdominis externus and the M. pseudo temporalis superficialis when compared with douglassi, are indicative of the greater specialization of platyrhinos.
There appears to be an inverse relation between the relative sizes of the occipital spines and the size of the lower jaw ( Table 1 ) in Phrynosoma which may be indicative of intrageneric relationships and phylogeny.
Observation of the type and paratype (USNM 36013, 36022) of Phrynosoma ditmarsi leaves no doubt that it is a unique species belonging to the douglassi group. The almost complete absence of spines, the pronounced supraoccipital notch, the massive lower jaw, and the pronounced anteriorly convex occipital structure are obviously relating characters in these two species. An x-ray of the skull of ditmarsi also indicates a close, if not identical, series of bones and bone patterns to douglassi.
There is no evidence to support a conclusion that the douglassi group, even though more primitive, is the ancestral stock of the platyrhinos phylogenetic line. In fact both lines show structural specializations that are peculiar to each. In platyrhinos the large spines, reduced number of sternal ribs, and the muscular specializations are unique. Species in the douglassi group are peculiar in that the spines are short or absent, the skull is shorter, wider, and with a noticeable convexity in the occipital region, and the lower jaw is larger.
Comparative osteological and myological studies on genera believed to be closely related to Phrynosoma are needed in order that phylogenetic relationships between Phrynosoma and other genera can be correctly determined.
Jenkins, Richard L. and Tanner, Wilmer W.
"Osteology and myology of Phrynosoma p. platyrhinos Girard and Phrynosoma d. hernandesi Girard,"
Brigham Young University Science Bulletin, Biological Series: Vol. 9
, Article 1.
Available at: http://scholarsarchive.byu.edu/byuscib/vol9/iss4/1