Gnathostomata and Placodermi

John Merck


Radinsky, Leonard, 1987 Evolution of Vertebrate Design

Engima: - the origin of jaws

Gnathostomata - the jawed vertebrates combine a great range of evolutionary novelties. They appear suddenly, with a large suite of fully-blown derived features.

Synapomorphies:

This list invokes some challenging problems in gnathostome evolution. These are take up below, after an anatomy review.

Jaw anatomy: Up until now, when we have spoken of a "skull" we have meant only a plating of dermal bone around the head, or, in the case of Galeaspids and Osteostraci, that plus in endochondrally ossified neurocranium or braincase. With the appearance of the gnathostome jaws and branchial skeleton, the skull becomes a complex composite structure. The following illustrations show its components using the fossil bony fish Eusthenopteron as an example.


Five Gnathostome Issues:

1. The Origin of Jaws:

Many gnathostome synapomorphies are continuations of longstanding vertebrate trends toward increased skeletal ossification, brain enlargement, and improvement of swimming. Big exceptions are the changes to visceral arches.

Oddly, fossil intermediaries between jawed and jawless vertebrates just don't seem to exist. What's up?

Three historic hypotheses:


2. New Branchial Arches:

The velar skeleton of the ammocoetes larvae of lampreys has both internal and external components that bear on the issue of gnathostome branchial arches.

Note:


Lamprey velar skeleton in dorsal view
from Janvier, 1993 in Hanken and Hall. The Skull. Vol 2
For fun, the lamprey velar skeleton (right) actually has both medial and lateral components.

As with jaws, Mallatt and Janvier square off:



Metaspriggina from Conway-Morris and Caron, 2014
Ontogeny: Developmentally, lampreys and gnathostomes are working with the same material: The gills form from blocks of mesoderm that are surrounded top, bottom, medially, and laterally by a layer of migrating neural crest ectoderm. In gnathostomes, the neural crest cells develop into branchial arches medially because of an inductive relationship with adjacent gut-tube endoderm. In lampreys, neural crest apparently is triggered to form branchial arches by a similar relationship with regular ectoderm. Thus, the evolution of medial gill arches really reflects the evolution of a change in the way in which neural crest is signaled to make cartilage.

Overall, the observed development of branchial arches is consistent with Janvier's argument, but the 2014 description of gnathostome-like branchial arches in Metaspriggina (right), if correct, supports Mallatt.


Kathemacanthus rosulentus from Royal BC Museum

3. The Pelvic Fins:

The evolution of distinct pectoral and pelvic fins is prefigured in the fin-folds of anaspids and thelodonts. In some cases, both pairs appear in the same animal. When we next encounter these structures in gnathostomes, both sets are proper fins with internal skeletons, however these differ: In fact, the pelvic fin is often in line with various other structures also derived from the postbranchial fin-fold, such as the series of spines in Kathemacanthus (right. Other examples below). Indeed, the pectoral fin is also part of an array of spines arranged perpendicular to the spines of the postbranchial fin-fold.


Gnathostome membranous labyrinth
from Eastern Kentucky University - BIO342

4. The Horizontal Semicircular Canal:

A true enigma. We see no intermediate forms. However:


Gnathostome braincase development
from Palaeos

5. Braincase Development:

The braincase of adult gnathostomes ossifies in a variety of patterns, however the early development of the cartilages that give rise to it is relatively uniform. The first cartilages to appear give rise to the floor of the braincase or form the special sense capsules. From front to back we see: These define the basic landmarks of the braincase. For example, the hypophysis forms on the midline between parachordals and trabeculae. During growth, these cartilages grow together to form a trough-shaped braincase.

All craniates have the special sense capsules (albeit only one nasal capsule for some basal vertebrates) and parachordals. Only gnathostomes definitely have the remainder (maybe galeaspids and osteostracans). The gnathostome braincase encloses the exits of the twelve cranial nerves. The occipital arch is believed to have originated from a short series of arcualia or vertebrae that became incorporated into the rear of the skull. Note that the otic capsules and occipital arches do not fuse well, resulting in a persistent lateral otic fissure.

Gnathostome diversity:

Traditionally, three major groups of unknown relationships were recognized:


Heterosteus ingens by Dmitri Bogdanov from Palaeos

Placodermi:

(Silurian - Devonian) Lovely and entertaining armored gnathostomes and poster-children for the Devonian. Experienced a rapid worldwide diversification at the beginning of the Devonian and abrupt decline and extinction at its end.



From Janvier, Philippe. 1996. Early Vertebrates
Placoderms were very diverse and occupied many ecological roles. During the Devonian, they greatly outnumbered other marine fish (right). Their specializations included:

Fossil record: Fragmentary record begins in the Middle Silurian, followed by a rapid diversification. During the Devonian, placoderms were the dominant vertebrate group. Both marine and fresh water forms are recorded with a worldwide distribution except for puzzling absence in South America. (Perhaps, like galeaspids and osteostracans, they were unable to disperse across open ocean, or maybe South America, then at high latitudes, was inhospitable.) Placoderm diversity was greatly reduced by an extinction event in the Late Devonian. They were completely extinguished by the mass extinction event at the end of the Devonian. Thus, entire radiation took up only about 50 million years, but while it lasted, it was spectacular. But maybe -


Placoderm diversity:

Placoderms look different and distinct, and for over a century, were assumed to be monophyletic, so it came as a surprise when Brazeau, 2009 and others began recovering them as paraphyletic. Today we regard them as basal gnathostomes. Recent phylogenies differ in detail but generally agree on which placoderm groups are closer to the base of Gnathostomata and which are closer to the crown. (Though King et al., 2016 find paraphyletic and monophyletic Placodermi to be equally parsimonious.) A sampling of some placoderm high-points:


Stensioella heintzi from
Wikipedia
Stensioella: (Early Devonian)Among the most basal gnathostomes. Although poorly known, gnathostome synapomorphies including paired nares and pelvic fins are apparent. Features: Janvier has suggested that Stensioella is actually a chondrichthyan close to ratfish - a minority opinion.


Bothriolepis sp. by Outlier from DeviantArt
Antiarchi: (Silurian - Devonian) Diverse and successful weirdos (second only to Arthrodire) common both in sea and fresh-water environments. Key features: Plesiomorphies: While definitely basal gnathostomes, Antiarchi show suggestive similarities to Osteostraci: Have antiarchi convergently evolved these traits or simply inherited them?


Gemuendina stuertzi from Wikipedia
Rhenanida: (Devonian) Secondarily flattened bottom dwellers with large pectoral fins. We have seen the "bottom-dweller" ecomorph before and will see it again. Rhenanids are the first gnathostomes to take up that life style through the enlargement of their pectoral fins. Features: Brazeau, 2009 recovered Rhenanida as the sister-taxon of the next group.


Materpiscis attenboroughi from Tsjok's Blog
Ptyctodontida: (Devonian) Strongly similar to living ratfish. Features:


Coccosteus cuspidatus from Wikipedia
Arthrodira: (Devonian) The most diverse and speciose placoderm group, including both deposit feeders and predators, including giant predators like Dunkleosteus. Features:

Potential synapomorphy of Arthrodira and Osteichthyes:



Entelognathus primordialis by Brian Choo from Sci-News.com Scale bar = 1 cm.
Entelognathus primordialis: (Silurian) Zhu et al., 2013. described this unremarkable creature with a extensive thoracic shield. Entelognathus looks like an arthrodire without the hinged articulation of head and thoracic armor or nuchal gap.

What makes Entelognathus interesting is its possession of potential synapomorphies with Osteichthyes:



Qilinyu rostrata from Daily Mail
Zhu et al, 2016 now reveal Qiliyu rostrata, a second "maxillae" placoderm-like fish. Based on Qilinyu, Zhu et al conclude that the premaxilla, maxilla, and dentary are homologous to placoderm gnathal plates. This identification facilitates the further identification of the jugal and lacrimal in typical placoderms.

Entelognathus and Qilinyu, with their extensive armor, is not what we expect of a close relative of living gnathostomes, whose ancestral body covering, we presumed, consisted of small scales and plates. We take them up next.

Additional reading: