Except for mammals, most vertebrates have a cesspool – an all-purpose opening that is used for reproduction and for excreting feces and urine. In dinosaurs, this opening has now been described for the first time using a well-preserved fossil. Although the reconstruction does not reveal anything about the gender or age of the individual, it reveals indications of the reproductive behavior of the dinosaurs: Presumably, like today’s crocodiles, they had scent glands next to the cloaca, with which they could secrete sexual attractants. The heavy pigmentation around the cloacal opening also suggests that they sent visual cues, as some birds do today.
Much is already known about dinosaurs: Using fossils, researchers have been able to reconstruct the physique and appearance of many different species, draw conclusions about their diet and way of life, and even use pigment analyzes to find out how they were colored. However, one area has so far been left out: the reproductive and excretory opening. For most known dinosaur fossils, this region is not sufficiently well preserved, and even when it did, the scientific focus was often on other features.
Comparison with today’s cesspools
A team led by Jakob Vinther from the University of Bristol has now reconstructed the dinosaur’s sewer opening for the first time. For this purpose, the researchers used the fossil of a Psittacosaurus. This is a herbivorous dinosaur that was about the size of a Labrador. “I noticed the cesspool a few years ago after we reconstructed the color patterns of this dinosaur using a remarkable fossil that is on display in the Senckenberg Museum in Frankfurt and whose skin and color patterns are clearly preserved,” says Vinther. “It took a long time before we were able to work on this area, because nobody has ever bothered to compare the appearance of cloacal openings in living animals. So it was largely new territory. “
The cesspool of the Frankfurt fossil is exceptionally well preserved so that the researchers were able to depict the individual parts precisely. In order to have a comparison, they also dealt in detail with various cloaca forms that occur in animals living today. Co-author Diane Kelly says, “We found that the cloacal opening looks different in many different groups of terrestrial vertebrates, but in most cases it doesn’t say much about the sex of an animal. These distinguishing features are hidden inside the cloaca and unfortunately they are not preserved in this fossil. “
Visual and olfactory signals
Nevertheless, the reconstruction provided the authors with some interesting information: “The lateral lips are heavily pigmented and covered with smaller, interlocking scales, which make them stand out from the adjacent skin on the tail,” the researchers report. They suspect that this pigmentation was used during courtship to showcase the sexual opening and thus to attract potential partners. This behavior is known today from baboons, for example, but also from some birds. “Given a sample size of only one specimen, we can at best guess whether the signaling function was sexual, but this is a possibility,” the researchers say.
The appearance of the dinosaur cloaca is unique, but has features that are reminiscent of living crocodiles. “The lateral swellings correspond to the position where the crocodiles have musk glands that secrete a fragrant secretion as part of the mating behavior,” explain Vinther and colleagues. So it is possible that the dinosaurs also attracted their reproductive partners with odor signals.
Paleo-artist Robert Nicholls, who also contributed to the publication, says, “As a paleo-artist, it was absolutely amazing to have the ability to reconstruct one of the last remaining features that we knew nothing about in dinosaurs. Knowing that at least some dinosaurs were signaling each other gives an exciting artistic freedom to speculate on a whole range of now plausible interactions during the dinosaur courtship. “
Source: Jakob Vinther (University of Bristol) et al., Current Biology, 2021