Placodermi

The placoderms (Placodermi, from the Greek, πλάξ=plate, δέρμα=skin, "plate-skinned") are an extinct class of primitive fish. late Devonian (about 359 million years ago), although they may have survived into the Lower Carboniferous.

Placoderms were armored fish, and their origin as well as their relationship to the evolution of chondrichthyans (the group of cartilaginous fish to which sharks, rays, and chimaeras belong) remain an enigma to scientists. They inhabited a great diversity of environments: marine, deltaic, lagoon and freshwater. The Gogo paleontological site in Australia, dating from the Upper Devonian, is where the greatest diversity of these organisms is found, since they have found remains of more than 20 species, most of them belonging to monospecific genera.

Anatomical features

Cranial plates Dunkleosteus (Arthrodira). You can appreciate the characteristics of the teeth, and the plates that surround the eye.

fossil BothriolepisAn antiarchi order plaster. You can appreciate the central pit.

fossil Lunaspis broilia Petalichthyida order.

Reconstruction of PhyllolepisA placoderm of the order Phyllolepida, buried in the sediment.

Placoderms were characterized by the presence of bony plates on the front of their bodies, as well as having an even number of limbs and mandibles, which originated from the first modified gill arch. The rest of the body The body was covered with scales, or could even lack them. The plates could appear fused, or they could be articulated, presenting joints. The teeth (two or three pairs) did not resemble those of modern vertebrates, but rather they were projections of the bony plates that formed the jaw, although some scientists affirm that more evolved placoderms of the order Arthrodira could have developed teeth with dentin and cavities to house the dental pulp. This fact could imply that the teeth have appeared twice throughout the biological evolution of vertebrates, producing a convergence phenomenon.

These fishes were also characterized by their flattened morphology in their dorsoventral plane, with a heterocercal caudal fin. As a rule they were not large, around 10-15 centimeters, but some members of the Dinichthyidae family reached lengths of up to 9 metres. There is evidence indicating that, like modern sharks, the eyeballs were attached to the skull by cartilage, and the arrangement of the eye muscle was similar to that of agnathus. In addition, a ring of plates Bony bones surrounded the eyes, serving as protection. They had a notochord with spines, usually cartilaginous, shaped like a Y vertebrae.

The characteristic apomorphies of placoderms that make them considered a monophyletic group are:

• Simple jaws with two or three pairs of dental plates.
• A layer of semidentine covering the dermal bones.
• A particular pattern in the disposition of the dermal bones of the upper part of the skull.
• The operculum consists of a dermal plaque that connects to the skull using a hioid cartilage.
• An exoskeleton that surrounds the trunk and is articulated with the skull by a joint formed by two plates.

Taxonomy

There are several orders of placoderms, and there is no consensus in the scientific community about the number, which can be nine or more. The most important is the order Arthrodira, which represents 60% of the placoderm finds. The most studied are Antiarchi, Arthrodira, Petalichthyida, Phyllolepida, Ptyctodontida, Rhenanida and Acanthothoraci. Paleontological sites are continuously discovered, with new genera and species, which allow taxonomic revisions of the placoderm orders.

Antiarch

The record of these placoderms began in the Lower Silurian and ended in the Upper Devonian. They had a short head, with the eyes, olfactory system, and parietal eye housed in a central fossa on the dorsal part. between 20 and 30 centimeters in length, reaching a maximum of 1 meter. The pectoral fins were covered with bone.

They were adapted to live on the seabed, feeding on the organic matter present in the mud. The most successful genus within this group was Bothriolepis, with worldwide distribution and more than 100 species, probably capable of breathing air in stagnant water, supporting semi-terrestrial life.

Arthrodira

The arthrodires, whose name means "jointed neck", is an order of placoderms, whose range of distribution extends from the Lower Devonian to the Upper Devonian, and which were characterized by having a division between the plate cephalad and thoracic plate. The most primitive forms had scales on the tail, which the most modern forms lost. They used to measure about 20-30 centimeters, but the genus Dunkleosteus could reach 9 meters It seems that most of them were predators of marine environments, with small placoderms having been found in the abdominal cavities of the largest arthrodir fossil remains from the Gogo site.

Petalichthyida

Like the order Antiarchi, members of this order fed on organic matter present on the ocean floor. The best-studied genus is Lunaspis. Dorsoventrally flattened, the plates were ornamented with tubers. They appeared and disappeared in the Devonian.

Phyllolepida

The name means "leaf scale," and they were flattened forms of placoderms up to 60 centimeters in length. They appeared on record in the Middle Devonian, and became extinct in the Late Devonian. eyes underdeveloped (perhaps absent), and except for the genus Yurammia, the plates were ornamented with concentric rings. The genus Phyllolepis had fins that seemed adapted to make it easier for the animal will bury itself to the bottom. Only freshwater forms are known. Some classifications include this group within the order Arthrodira.

Homostius and Pterichthys.

Diandongpetalichthys.

Ptyctodontidae

These placoderms, which normally inhabited marine environments, did not exceed 20 centimeters in length. In this group there was a reduction in bone plates. Some forms had claspers, which are elongated anatomical elements, which are associated with to the pelvic fins, and whose function is internal fertilization. Its record spans from the Middle Devonian to the Upper Devonian.

Rhenanida

Probably inhabiting marine environments, these Devonian forms had a ray-like appearance. The pectoral fins were very broad and the tail very narrow, as can be seen in fossil specimens of the genus Gemuendina .

Acanthothoraci

The head plates were similar to those of the most primitive specimens of the order Arthrodira. One of its most characteristic features is that the juvenile specimens had separate plates, and these joined when the specimen was an adult. There are record of these forms in the Lower Devonian (Lochkovian).

Phylogeny

The phylogenetic relationships of placoderms within gnathostomes are not very clear. The most accepted hypotheses are:

• Placodermi is a brother taxon of the rest of the gnathostomados, when presenting the palato square (dorsal part of the mandibular arch) in the form of omega.

• Placodermi is more closely related to the Chondrichthyes class than to the rest of gnathostomados. The class Elasmobranchiomorphi, which included placodermos and condrictios, was created. The similarities presented by these two groups are the presence of a clasper in the pelvic fin and the presence of a cartilage that unites the eyeball with the skull.

• Placodermi is more closely related to the Osteichthyes class than to the Chondthyriches class, because they share the presence of dermal and parasphenoid plates (a skull bone).

In the past, phylogenetic relationships were also established between placoderms and dipnoans, or acipenserids.

Cladogram

The following cladogram shows the interrelationship of placoderms according to Carr et al. (2009):

Playback

Placoderms are the first viviparous organisms to appear in the fossil record. Fossil remains of two species of ptyctodontids (Materpiscis attenboroughi) were found at the Gogo site in Australia, dating back 380 million years. and Austroptyctodus gardineri), with embryos inside. A year later, the discovery of embryos inside fossil specimens of the species Incisoscutum ritchiei (Arthrodira) was published in the same site, and further evidence was found indicating that they used the pelvic fin for copulation, like the Chondrichthyes. Other placoderms that used the pelvic fin for copulation were those of the genus Austrophyllolepis. Placoderms were previously thought to be oviparous, with the eggs being fertilized outside the mother's body.

fossil Materpiscis attenboroughi with the embryo inside. The viteline sac, umbilical cord and the bones and jaw of the embryo can be seen.

Louis Agassiz (1807-1873) was the first naturalist to study placoderms.

Extinction

At the end of the Devonian, on the Frasnian-Famenian border, a mass extinction occurred, in which 75% of the families of fish and 70% of the marine invertebrates disappeared. It is not known what they are the causes that caused it, although several hypotheses have been formulated: glaciation, volcanism, changes in the chemistry of the water, decrease in the amount of oxygen and meteorite impact. There was a decrease in the circulation of water between the ocean basins, which generated a state of anoxia in the coastal seas. Freshwater species were also affected by this event.

The constituent organisms of the reefs of the time: stromatoporids and rugose corals, also disappeared in large numbers. Many species of trilobites and brachiopods also disappeared. Within vertebrates, conodonts and agnaths almost completely disappeared, and No species survived from the class Placodermi. It is striking that neither plants nor terrestrial animals were affected by this episode.

History of the study of placoderms

The first naturalist to publish studies on placoderms was Louis Agassiz, in his five-volume work on fish fossils (1833-1843). However, it was the Swedish paleontologist Erik Stensiö who first studied their anatomy in depth. At the beginning of the s. XX realized that to determine the relationships between fossil fishes he had to focus on the internal anatomy of such organisms. In 1969 he proposed that certain extinct sharks, rays and chimaeras were related to three different groups of placoderms: Arthrodira, Rhenanida and Ptyctodontida. The order Stensioellida, a group formerly considered to belong to the class Placodermi but now included within the Holocephalans, is named in his honor.

In the 1930s, D. M. S. Watson, a Scottish anatomist, observed plate-covered cartilaginous appendages arising from the bony girdle in several male pictodontids, but did not associate them with fertilization. The discovery of embryos in Fossils of pictodontids and arthrodiers in the first decade of the 21st century confirmed that certain species of placoderms reproduced by copulation, probably being this class of fish where this form of reproduction originated.

Pop Culture

The first documentaries featuring this family of fish include Sea Monsters, broadcast in 2003. Additionally, a greater diversity of placoderms is shown in the documentary Animal Armageddon, broadcast in 2010 In the documentary Curiosity: From the Cell to the Human Being, from 2011.