524 THE FUNCTIONAL MORPHOLOGY OF DERMAL BONE ORNAMENTATION IN TEMNOSPONDYL AMPHIBIANS LARRY F. RINEHART AND SPENCER G. LUCAS New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, NM 87104; email: larry.rinehart@state.nm.us Abstract—The dermal bones of the skulls, jaws, and shoulder girdles of temnospondyl amphibians are orna- mented with patterns of raised, reticulate, polygonal texture often referred to as "honeycomb-" or "waffle iron- like" and/or ridge-and-groove patterns. The function of this texture has been the subject of inquiry in several previous studies. The most prominent of the competing hypotheses propose that the texture: (1) increases the surface area of the skin to support cutaneous respiration and/or thermal regulation, (2) strengthens the bone against stresses incurred during feeding, (3) protects blood vessels supplying the skin, (4) results from the growth of nutrient channels through the bone and/or (5) provides a large surface for tightly anchoring the overlying skin. These interesting hypotheses have been largely qualitative in nature. Instead, we take a quantitative approach, particularly towards testing the skin-breathing, thermal regulation, and structural strength hypotheses. To do so, we studied dermal bones of the Middle Triassic cyclotosaurid Eocyclotosaurus, and the Late Triassic metoposaurid Koskinonodon. Allometric analysis of the surface area increase due to ornamentation shows very strong negative allometry. The rate of surface area increase is more than an order of magnitude too low to maintain support of skin breathing or thermal regulation throughout growth. Thus, while some skin breathing is possible in these animals, particularly the smaller forms, the surface area increase due to dermal bone texture cannot be a significant contrib- uting factor. In contrast, our calculations show that the dermal bone texture provides a substantial increase in strength and stiffness accompanied by a relatively small increase in mass. The ridge-and-groove texture requires a smaller investment to construct and is lighter weight than the reticulate texture, but its strength is unidirectional, whereas the reticulate texture provides omnidirectional strength. Thus, our quantitative results reinforce the argument for structural significance of the dermal bone texture, and weaken the support for the idea of its contributing to skin breathing and/or thermal regulation. INTRODUCTION Temnospondyl amphibians were a long-lived and diverse group of tetrapods. Their fossil remains have been found on every continent. They first appeared in the Carboniferous and reached their ecological zenith during the early Mesozoic (Triassic) when they were represented by a wide variety of animals, from < 250 mm-long trematosaurs to the five-meter-long Mastodonsaurus. Most became extinct at the end of the Triassic, but a few persisted until the Early Cretaceous (Schoch and Milner, 2000). Temnospondyls were quadrupedal aquatic to semi-aquatic preda- tors that typically had relatively large, dorsoventrally flattened skulls with numerous sharply pointed teeth, relatively small limbs, and long mediolaterally compressed tails. Their four-fingered hands and five-toed feet, together with their palatal vacuities, short straight ribs, bilateral occipital condyles, and numerous other anatomical features place them in the Amphibia. One of the most characteristic features of temnospondyl amphib- ians is the heavily textured, or "ornamented" dermal bone that covered their skulls, lower jaws, and shoulder girdles (Fig. 1). Schoch and Milner (2000) show many very high quality illustrations of temnospondyl skulls, lower jaws, and shoulder girdles that clearly define the dermal bone ornamentation of numerous taxa. Clearly, the animals invested consider- able energy to grow and to carry around this ornamentation, which we will show increases the mass of dermal bones by 10% to as much as 50%. Both the energy investment required to produce the ornament and its unusual appearance beg the question, what was its function? Excellent, detailed summaries of the various hypotheses and pre- vious work regarding the purpose of dermal bone ornamentation in temnospondyls have been provided by Coldiron (1974) and more com- prehensively by Witzmann et al. (2010). These various studies propose that the dermal bone ornament (1) increases the surface area of the skin to support cutaneous respiration and/or thermal regulation, (2) strengthens the bone against stresses incurred during feeding, (3) protects blood vessels supplying the skin, (4) results from the growth of nutrient chan- nels through the bone and/or (5) provides a large surface for tightly anchoring the overlying skin. We will elaborate on these various hypoth- eses below. There is general disagreement regarding the answer to this interesting question, and in some cases, there seems to be an either-or attitude that rejects all but one purpose for the ornamentation. We do not necessarily expect an either-or solution; rather, given the propensity of natural selection to co-opt existing structures for new uses, we expect that the dermal ornamentation of temnospondyls contributes in some way to several, if not all, of the proposed purposes. What we offer that is different from the previous work is a quantitative approach. The functional morphology of the ornamentation, especially with respect to skin-breathing, thermal regulation, and structural strength readily lends itself to such analysis. The scope of this study is limited, and most of the calculations are first approximations, but nonetheless, some definitive conclusions are evident. PREVIOUS WORK Bystrow (1935) described the polygonal reticulate and ridge-and- groove structure of temnospondyl dermal bone ornamentation and be- lieved that the ridges and grooves form as a result of, and show the direction and extent of growth from a center of ossification (e.g., Bystrow, 1944). Later, he proposed that the ornamentation supported cutaneous respiration by increasing surface area (Bystrow, 1947). Romer (1947) hypothesized that the texture of "labyrinthodont" amphibian dermal bone served for tightly anchoring the skin, but Coldiron (1974) pointed out that in extant crocodylians the skin is just as tightly Tanner, L.H., Spielmann, J.A. and Lucas, S.G., eds., 2013, The Triassic System. New Mexico Museum of Natural History and Science, Bulletin 61.