Unique growth pattern of Metoposaurus diagnosticus krasiejowensis (Amphibia, Temnospondyli) from the Upper Triassic of Krasiejów, Poland

Abstract

Morphology and bone histology of femora, tibiae, and fibulae of the temnospondyl Metoposaurus diagnosticus krasiejowensis from the Upper Triassic locality of Krasiejów (Poland) are studied for the first time. The growth pattern of Metoposaurus as preserved in a small growth series of femora, shows a regular alternating sequence of fast and slow growth phases, which are interpreted as representing zones and annuli. The slow growth phases (annuli) of the inner and outer cortex as well as those of different specimens are of a regular broad thickness. Such broad annuli are so far unknown for any vertebrate and make the growth pattern for Metoposaurus unique. These slow growth phases always contain several resting lines but do not end in lines of arrested growth. Interestingly, all hind limb bones show a phase of slow growth in their outer cortex, implying that the individuals died during an unfavorable season, possibly during the same catastrophic event. The growth pattern of Metoposaurus is compared with published data of the closely related and similar sized temnospondyl Dutuitosaurus from Agarna locality (Upper Triassic, Morocco). Dutuitosaurus growth differs from that of Metoposaurus by the absence of extensive (broad) phases of slow growth (annuli). Instead Dutuitosaurus shows zones, which regularly alternate with thin annuli and/or annual lines of arrested growth. This implies one regular cessation in growth per anno for Dutuitosaurus, most likely reflected by an aestivation period. Metoposaurus on the other hand, stayed active during most of the unfavorable season, except for several short periods where growth had to cease temporarily, which is indicated by several resting lines. Differences between the intensity of wet and dry seasons and a possible lower persistence of water bodies in Argana during the unfavorable season compared to Krasiejów may explain differences in the recorded growth pattern between both taxa. This would make growth more dependent on functional constrains than on phylogenetic ones. The interpretation of the alternating growth phases in Metoposaurus revealed ages between two and five years. In comparison to Dutuitosaurus, the Metoposaurus sample consists of ontogenetically younger individuals, which fits to femur length.