Caught in the act: the first record of copulating fossil vertebrates

Abstract

The behaviour of fossil organisms can typically be inferred only indirectly, but rare fossil finds can provide surprising insights. Here, we report from the Eocene Messel Pit Fossil Site between Darmstadt and Frankfurt, Germany numerous pairs of the fossil carettochelyid turtle Allaeochelys crassesculpta that represent for the first time among fossil vertebrates couples that perished during copulation. Females of this taxon can be distinguished from males by their relatively shorter tails and development of plastral kinesis. The preservation of mating pairs has important taphonomic implications for the Messel Pit Fossil Site, as it is unlikely that the turtles would mate in poisonous surface waters. Instead, the turtles initiated copulation in habitable surface waters, but perished when their skin absorbed poisons while sinking into toxic layers. The mating pairs from Messel are therefore more consistent with a stratified, volcanic maar lake with inhabitable surface waters and a deadly abyss.

Figure 1.

SMF ME 2449, one of nine mating pairs of the carettochelyid turtle Allaeochelys crassesculpta found at Messel Pit Fossil Site. The male (to the right) is about 20% smaller than the female and can be recognized by its relatively longer tail and lack of plastral kinesis. Scale bar, 5 cm.

Figure 2.

Sexual dimorphism in the carettochelyid turtle A. crassesculpta. SMF ME 3566, ventral view of the tail of (a) a female and (b) a male individual. Note that the caudal vertebrae of the male are notably long and that the tail would have terminated beyond the posterior margin of the shell, if extended. The tail of the female, by contrast, consists of shorter vertebrae and terminates near the margin of the shell. (c) Quantitative differences between the relative tail length of male and female individuals (see electronic supplementary material). The general trend is already apparent starting with the most terminal caudal vertebra. SMF ME 3566, detailed view of the xiphiplastron of (d) a female and (e) a male individual. Note that the hyoplastral/xiphiplastral suture is straight in females. This allows for plastral kinesis, which is useful in oviposition. The same suture is sinuous in male individuals and therefore locked. (f) WDC CMG 69, detailed view of the tails of male and female individuals. The elongate tail of the male to the right is wrapped below the carapace of the female and both tails are aligned.