Placodermi - Taxonomy and Phylogeny

Taxonomy and Phylogeny

Currently, Placodermi is divided into 9 or 10 recognized orders.

One order is the monotypic Stensioellida, comprising of the enigmatic Stensioella. Another order is the equally enigmatic Pseudopetalichthyida, which comprises three, poorly preserved and poorly studied genera. Both Stensioellida and Pseudopetalichthyida are considered to be basal or primitive groups within Placodermi, though, their precise placement within the class remains unsure. Fossils of both are currently only known from the Hunsruck lagerstatten.

The Petalichthyida are a group of flattened placoderms, known primarily from skulls dating from the Lower to Middle Devonian. It is assumed that they were all benthic predators, though, no mouth parts have ever been found or observed. They have a superficial resemblance to the Phyllolepida.

Brindabellaspida is typified by Brindabellaspis, though, various Early to Middle Devonian placoderm incertae sedis have been inserted in the order, too. Brindabellaspida is considered to be either a stem group, or closest to the stem placoderm, as the cranial anatomy of Brindabellaspis displays many similarities with the cranial anatomies of osteostracan and galaeaspid agnathans. The gross anatomy of Brindabellaspis resembles that of the Weejasperaspid Acanthothoracids.

The Acanthothoraci are a group of generalized placoderms with a stout spine emanating from the median dorsal plate, and (with most), an elongated head. One family, Weejasperaspidae, is considered monophyletic, being the sister-group of Brindabellaspida. Another family, Palaeacanthaspidae, however, is considered paraphyletic, as various genera have been demonstrated to have several anatomical similarities with Rhenanida, Arthrodira, and Antiarchi, thus implying that these genera are the sister-groups of these orders. Brindabellaspid fossils are found mostly in Early and some Middle Devonian strata. The fossils of Acanthothoraci are found in Early and Middle Devonian strata, though, if the fossil Grazosteus is really an acanthothoracid, and not an arthrodire, their range is extended into the Late Devonian.

The Rhenanida are a group of distinctive, superficially ray-like fishes with large, upturned mouths. Their fossils range from near the beginning of the Devonian until the Late Devonian. As with Stensioellida, the armor of Rhenanids are not solid plates, but a mosaic of scales and small plates arranged in patterns corresponding to the various plates typical to placoderms. Upon death, these mosaics come apart, and it is suggested that the rhenanids' rarity in the fossil record reflects postmortem disassociation, and not an actual rarity of species.

The Arthrodira are the most successful of the placoderm groups, their fossils being found from the Late Silurian until the very end of the Devonian (though, no Silurian arthrodires have been officially described or named). The Arthrodires inhabited both fresh- and saltwater environments, and had a wide array of body-shapes, generally ranging from flattened to bullet-shaped. Many arthrodires were benthic-dwelling, though many were also actively swimming nekton, as well. The Arthrodires include many giant members, including the earliest known "giant vertebrate," the 2.5 meter long Tityosteus of Early Devonian Germany, and the first vertebrate superpredators, including Dunkleosteus. According to fossils of Incisoscutum containing unborn fetuses, it is strongly implied that arthrodires gave birth to live young.

The Phyllolepida are a group of freshwater bottom-dwellers that superficially resemble the Petalichthyids. The anatomy of the Phyllolepids, however, suggest that they are actually either a sister-group of the Arthrodires, or are a sub-group within Arthrodira, closely related to the Wuttagoonaspids.

The Ptyctodontida are a group of lightly armored placoderms that bear a strong, albeit superficial likeness to chimaeras. Because only their heads were armored, most ptyctodontids are known from their beak-like toothplates, though, many genera are known from whole-body fossils. These whole-body fossils also show that ptyctodontids had sexual dimorphism, with the males having pelvic claspers. The articulation of headplates in Campbellodus and Ctenurella also suggest that the males had head-claspers, as well.

After Arthrodira, the second-most successful placoderm group are the Antiarchi, a group of bizarre, box-like marine and freshwater benthic creatures whose pectoral fins were modified into caliper-like appendages. The earliest known antiarch is Silurolepis, from the Ludlow epoch of Yunnan. Antiarchs would persist in the fossil record for the entirety of the Devonian.

Below is a cladogram showing the interrelationships of placoderms from Carr et al. (2009):