This small fish that lived 300 million years ago had a unique way of reproduction

When it comes to the reproduction of fish, what comes to mind is often the scenes of ornamental fish in fish tanks or edible fish in fish ponds "throwing seeds": most of the common fish around us belong to the superclass of teleost fish, which is the most prosperous group of existing fish and even vertebrates. 99% of the more than 30,000 existing fish species belong to the class of ray-finned fish, accounting for about 50% of the existing vertebrate species. Most ray-finned fish reproduce by in vitro fertilization: sexually mature male and female fish release sperm and egg cells into the water, allowing them to combine in the water environment to form fertilized eggs.

However, have all fish from ancient times to the present reproduced in this primitive way? A strange little fish that lived more than 300 million years ago may give us the answer.

A father clownfish guards his newly hatched child (caution for trypophobics) | Silke Baron / Wikimedia Commons

The little guy in full armor

Microbrachius, meaning "small limbs", lived in freshwater from the Amslangian stage of the Early Devonian period 393.3 million years ago to the Givetian stage of the Middle Devonian period 382.7 million years ago. Its fossils have been found in Scotland, Belarus, Estonia and Yunnan, my country. Microbrachius belongs to the Placoderm class: an extinct ancient group whose body surface was covered with thick bony armor for protection. Placoderms were the first vertebrates on Earth to evolve upper and lower jaws. They flourished from the Middle Silurian to the Late Devonian period, and evolved into the top predator Dinichthys terrelli, which was over 6 meters long, and the giant filter feeder Titanichthys, which was similar to the extant whale shark.

Compared to these large relatives, the body of the microbrachius is much smaller, with the front half of the body covered with armor (the part preserved in the fossil) only 2-4 cm long. Don't look down on these "little guys" because of this: the order of armored fish to which the microbrachius belongs is the master of armor stacking among the placoderms: their heads and the front half of the torso are tightly wrapped in thick bone armor, like a bone box, and even the pectoral fins are covered with hard armor plates. It is not easy for freshwater predators to break through the tight defense of the microbrachius.

Reconstruction of Microbrachius dicki | E. Mark-Kurik et al. / Estonian Journal of Earth Sciences (2018)

In 2013, paleontologists discovered two peculiar little bone rods with grooves on them in a fossil of a small-limbed fish excavated in Scotland. These are "things" with extraordinary significance.

Part of the carapace of Microbrachius kedoae | E. Mark-Kurik et al. / Estonian Journal of Earth Sciences (2018)

Organ of historical significance

Although the vast majority of existing fish reproduce through external fertilization, there are exceptions: some species of ray-finned fish such as the family Cyprinidae (including common ornamental fish such as guppies and mollies and the notorious invasive species mosquitofish) and the family Carcharhiniformes reproduce through internal fertilization, while all members of the higher classes of cartilaginous fish (including various sharks, rays, stingrays and chimaeras) use internal fertilization.

The grooved bone rod of the microbrachius is very similar to the copulatory organs of male elasmobranch cartilaginous fish such as existing sharks, which indicates that the microbrachius, and perhaps other placoderms, also mate in a similar way to cartilaginous fish. When existing sharks and rays mate, the male will bite the female's pectoral fin with his teeth to fix himself, while the mouth of the microbrachius is very small, just a small hole on the head tightly wrapped by bone armor. The body of the microbrachius, which is fully armed with helmets and armor, is very stiff, just like a European medieval knight wearing a full set of plate armor, and it is completely unable to perform flexible and difficult movements.

Male genitalia of a microbrachium | John Long et al. / Nature (2014)

So how do male microbrachiids mate with females? The pair of copulatory organs formed by bone rods of microbrachiids extend to the left and right sides, forming a 90-degree angle with the body. At the same time, paleontologists have also found two small bone plates in the cloaca of some microbrachiid fossils that do not have copulatory organs, which may be females. These features show that male microbrachiids may approach females from the side and mate with them in a "shoulder-to-shoulder" position.

Bony plates found on the cloaca of microbrachius may have been a device to assist in mating | John Long et al. / Nature (2014)

An artist's impression of a microbrachium mating pose | John Long et al. / Nature (2014)

Devonian Mother

The existence of the copulation organ in microbrachians shows that at least some placoderms can carry out internal fertilization. So how do they produce offspring?

Internally fertilized ray-finned fishes usually reproduce by giving birth to live young, with the fertilized eggs hatching in the mother's body and developing into independent young before being released. Cartilaginous fishes reproduce in three different ways: some members give birth to live young, some members give birth to large, hard-shelled eggs in small numbers, and more advanced members (such as Carcharhinus and Sphyrna) give birth to live young like mammals, with the mother providing nutrition to the developing embryo directly after the embryo has absorbed all the yolk.

Genitalia of fish on different branches of the evolutionary tree | John Long et al. / Nature (2014)

Another type of placoderm discovered in Western Australia in the late Devonian period 380 million years ago: the Attenborough fish mother Materpiscis attenboroughi. Its abdominal cavity not only preserves fossils of unborn embryos, but also shows traces of pseudoplacental structures. This is strong evidence that as early as the Devonian period more than 300 million years ago, the ancient placoderms were not only able to mate through internal fertilization, but also reproduced offspring through viviparity like existing higher cartilaginous fish.

In the abdominal cavity of the Attenborough fish mother, you can still see the skeleton of the embryo | Sularko Museum Victoria Derivative work MagentaGreen

Placoderms, the first generation of vertebrates on Earth, became extinct in the mass extinction at the end of the Devonian period and disappeared forever. However, there are still many secrets about their special body structure and unique living habits buried deep in the strata, waiting for our discovery.

This article comes from the Species Calendar, welcome to forward

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