Embryos inside placoderms: Austroptyctodus and Materpiscis

Updated June 17, 2022
by moving placoderms down to arandaspids, separating them from ptyctodontids, which remained with catfish and related discoidal chirodontids (Fig 2).

Figure 1. Austroptyctodus and Materpiscis to scale.

Figure 1. Austroptyctodus and Materpiscis to scale.

According to Wikipedia,
Materpiscis (Latin for mother fish) is a genus of ptyctodontid placoderm from the Late Devonian located at the Gogo Formation of Western Australia. Known from only one specimen, it is unique in having an unborn embryo present inside the mother, with remarkable preservation of a mineralised placental feeding structure (umbilical cord). This makes Materpiscis the oldest known vertebrate to show viviparity, or giving birth to live young. The juvenile Materpiscis was about 25 percent of its adult size. Materpiscis would have been about 11 inches (28 cm) long and had powerful crushing tooth plates to grind up its prey, possibly hard shelled invertebrates like clams or corals.”

When added to
the large reptile tree (LRT, 1615+ taxa then, 2116 taxa in 2022) Austroptyctodus (Long, Trinajstic, Young and Senden 2008; Devonian 380 mya, 11cm estimated length) nests with another mild-mannered placoderm with a tall head, Bothriolepis (Fig. 2). Note the use of tetrapod homologs in the renaming of several skull bones. This facilitates the comparison of all vertebrates with other vertebrates.

Figure 1. The evolution of Ptyctodontida in the LRT illustrated to scale. Here Robustichthys is basal to a Cheirodus clade and a Materpiscis clade.

Figure 2. The evolution of Ptyctodontida in the LRT illustrated to scale. Here Robustichthys is basal to a Cheirodus clade and a Materpiscis clade.

Wikipedia reports,
“The ptyctodontid fishes are the only group of placoderms to display sexual dimorphism, where males have clasping organs and females have smooth pelvic fin bases. It had long been suspected that they reproduced using internal fertilisation, but finding fossilised embryos inside both Materpiscis and in a similar form also from Gogo, Austroptyctodus, proved the deduction was true.”

Some videos about Materpiscis attenboroughi

Austroptyctodus gardineri (originally Ctenurella (Miles and Young 1977; Long 1997; Late Devonian) appears to be toothless in the illustration above, but had tooth plates. Bones are relabeled here with tetrapod homologs. Distinct from relatives, Austroptyctodus had a conjoined upper and lateral temporal fenestra, an antorbital fenestra, and fused temporal bones. One specimen is pregnant with three embryos inside, indicating another example of internal fertilization.

Materpiscis attenboroughi (Long, Trinjstic, Young and Senden 2008; Late Devonian; 28cm long est.) is similar to Austroptyctodus and includes a single embryo one-fourth the size of the adult, likely indicating viviparity. Note the ratfish (Chimaera)-like appearance of this placoderm, by convergence.


References
Long JA 1997. Ptyctodontid fishes from the Late Devonian Gogo Formation, Western Australia, with a revision of the German genus Ctenurella Orvig 1960. Geodiversitas 19: 515-555.
Long JA, Trinajstic K, Young GC and Senden T 2008. Live birth in the Devonian period. Nature. 453 (7195): 650–652. doi:10.1038/nature06966

wiki/Materpiscis
wiki/Austroptyctodus