In previous parts of this series, I explained how finding the first embryos in ancient placoderm fishes led us to the realization that they were copulating. But how reliable are such inferences about reproductive behavior using fossil evidence?
The males of all living sharks, rays, and holocephalans (Chondrichthyes) have cartilaginous claspers that they insert into the female cloaca during copulation to pass a package of sperm. The similarity of the clasper cartilages supporting the claspers from the pelvic girdle is remarkable between sharks and placoderms, so the behavior of these two types of aquatic animals is confidently assumed to be similar. Yet when we look at modern ray-fined fishes (actinopterygians) – the most diverse group of backboned animals on the planet (approximately 29,000 species) – some have extraordinary mating behaviors that are not reflected at all by their skeletal features. Most of these fish spawn in water, but a few lineages have developed internal fertilization in strange ways.
Take the bronze catfish Corydoras anaeus for example. A paper published by Japanese researcher Masanori Kohda and his colleagues has the intriguing title “Sperm drinking by female catfishes: a novel mode of insemination.” This is a novel way to get pregnant indeed, even if somewhat “hard to swallow” (pun intended)! This popular aquarium species has evolved a mating ritual whereby the female aligns its body at right angles to the middle of the male, with her mouth close to the male’s genital opening. In a split second, the male ejects his sperm, which is sucked inside the female’s mouth. Almost immediately, she sheds her now-fertilized eggs out of her cloaca. At first, scientists couldn’t figure out how this system worked so quickly, but by placing blue dye in the water, they were able to watch the flow of the male’s sperm. Once in the female’s mouth, the sperm was transferred by various ducts into the body cavity of the female, where the eggs had been previously deposited from the ovaries. A perfect system – bearing in mind that these tiny fishes live in fast-flowing streams where sperm intended for eggs shed in water might be quickly washed away with the currents.
We can therefore infer very little about the sexual habits of extinct fishes and other animals except in cases when embryos or distinct sexual organs are preserved. Some extinct groups, like the water-dwelling plesiosaur reptiles, have only recently opened their sexual secrets to us. A superb new specimen described by Robyn O’Keefe and Luis Chiappe for the Natural History Museum of Los Angeles county revealed an unborn embryo inside its mother, confirming that plesiosaurs most likely reproduced like dolphins, mating in water, and giving birth to a well-developed large baby, almost 40% as long as the mother.
The superb specimen of the Cretaceous marine reptile Polycotylus, on display at the Natural History
Museum of Los Angeles County, is the only known example that shows an unborn embryo, evincing that they
reproduced by copulation and gave birth to love young. Image by Stephanie Abramowicz (NHM LA County).
Full artist’s rendition of the live birth of the plesiosaur Polycotylus, by Stephanie Abramowicz (NHM LA County).
Dinosaurs have always been thought to have reproduced much like primitive reptiles, like the New Zealand endemic Tuatara, a relict form of living reptile. The Tuartara has no penis and sheds its sperm directly into the female’s cloaca. The now not-so-recent epiphany that birds are descendents of theropod dinosaurs forces us to reconsider this assumption. It is most likely that, because living archosaurian reptiles (such as crocodilians) have penes, and many squamates (snakes and lizards) have a pair of hemipenes, copulation was the norm in dinosaurs, including the giant species. However, until we find that one exceptional dinosaur fossil with soft tissues preserved or mineralized, our theory about this primeval act will remain speculative.
All male mammals have a penis, so we assume that fossil mammals reproduced in much the same way as their nearest living counterparts. But how does our own sexual act relate back to placoderm mating habits? See the fourth and final post next week in the ETVOL exclusive series “Dawn of the Deed.”
John A. Long is Vice President of Research and Collections at the Natural History Museum of Los Angeles County. His most recent book is Dawn of the Deed: The Prehistoric Origins of Sex, published by the University of Chicago Press.