is the upper tooth-bearing jaw bone found in sharks, ratfish and skates. Sometimes marginal teeth are not illustrated on the bone (Figs. 1, 2). Sometimes marginal teeth are indeed absent.
According to Wikipedia
“In some fishes, the palatoquadrate is the dorsal component of the mandibular arch, the ventral one being Meckel’s cartilage. The palatoquadrate forms from splanchnocranium (the portion of the cranium that is derived from pharyngeal arches) in various chordates including placoderms and acanthodians.”
I don’t see a palatoquadrate in any tested placoderm, but then I have no data on the palate in any placoderm. All the dermal jaw bones are homologous with those of tetrapods and their more fish-like ancestors.
Likewise, I don’t see a palatoquadrate in any tested acanthodian. In Ischnacanthus the maxilla + squamosal + jugal look like a palatoquadrate. Perhaps that is the source of this misconception: misidentification.
Previous workers did not realize the jugal, squamosal, lacrimal and quadrate were fused together to form the palatoquadrate in Elasmobranchii (the shark-ish clade, Fig. 1). That you learn by adding taxa and identifying bones based on phylogenetic bracketing. Palatal bones are difficult to ascertain in most fossil taxa, but appear as thin struts in jawless Birkenia (Fig. 1) and the sturgeon Pseudoscaphorhynchus.
On a side note, before we continue:
Rocek 1993 described a putative palatoquadrate in the osteolepiform, Eusthenopteron, but that bone is a large fused quadrate and pterygoid plus a separate palatine and ectopterygoid (according to the diagram in Fig. 4), as in many other bony fish and tetrapods.
Figure 1. Traditional diagram from Richter and Underwood 2018 demonstrating their views on palatoquadrate evolution.
This palatoquadrate is not found in tetrapods.
Tetrapods, like basal fish, separate the lacrimal, jugal, squamosal, palatine, ectopterygoid, pterygoid, and quadrate. Only in sharks, ratfish and skates do these bones fuse to create a palatoquadrate.
Richter and Underwood 2018
produced a diagram (Fig. 1) intended to demonstrate the origin of the palatoquadrate from a hypothetical autostylistic taxon (ghosted out here) and its purported evolution to known taxa with other mandibular arch supports. In their diagram some of the proximal relatives do not look similar to one another.
Some terms in figure 1
were new to me, and they may be new to you, so here are the definitions:
- Autostyly: The mandibular arch is not supported by a hyomandibula (typical lungfish + tetrapods)
- Holostyly: Palatoquadrate fused with the cartilaginous cranium and the second visceral arch entire and free from the cranium.
- Hyostyly: Ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. (typical Chondrichthyes).
- Euhyostyly: The hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.
- Amphistyly: The palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. (typical Actinopterygia).
- Orbitostyly: The orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support.
From the Richter and Unwood summary:
“Much progress has been made in the understanding of the vertebrate skull since pioneering anatomical descriptions made last century. There is still much uncertainty about precise homologies between parts of the skull of distinct groups of fishes, due to the fact that the vertebrate skull shows a remarkable morphological and anatomical plasticity.”
After testing, their ‘uncertainty’ seems to be due to taxon exclusion (Fig. 1). After further testing, adding taxa (Fig. 2) changes the tree topology and sheds new light on the origin of the palatoquadrate.
Figure 2. Modified from Richter and Underwood 2018 with more taxa, and bone colors adjusted to match those in tetrapod taxa in the LRT. Compare to figure 1.
Only when marginal teeth appear
(in Falcatus (Fig. 2) for sharks and Polyodon (Fig. 2) for bony fish) do we find the premaxilla and maxilla in chordates. In shark-like taxa with a palatoquadrate the new premaxilla and maxilla that create, support and shed teeth have their genesis as dermal layers fused to the large, supporting lacrimal. Teeth also appear on various palatal bones in various patterns in an assortment of tetrapods.
the large reptile tree (LRT, 1631+ taxa) the last common ancestor of all fish is Birkenia (Fig. 2). For a skeletal system, it has many small dermal splinters that typically fuse together in derived / descendant taxa. The jaw elements remain as distinct bones in tetrapods and bony fish, including Falcatus (Fig. 3). By contrast, in Elasmobranchii (sharks, rays and ratfish) the upper jaw elements fuse to produce a single element, the palatoquadrate.
The LRT nests finless, jawless Arandaspis, Poraspsis and kin not as traditional fish, but as massively armored lancelets, derived from Branchiostoma.
In short and simple fashion
here are the broad strokes of jaw, gill and palatoquadrate evolution.
- Birkenia: mouth anterior and ventral, several gill openings low, eyes dorsal
- Osteostraci: mouth anterior and ventral, gill openings ventral, eyes dorsal
- Sturgeons: mouth posterior and ventral, single gill openings lateral, eyes lateral… origin #1 of the palatoquadrate
- Loganiella (derived from 1. Birkenia): mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
- Rhincodon: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral… second origin of the palatoquadrate
- Falcatus (derived from 4. Loganiella): mouth anterior and subterminal, several gill openings lateral and covered, eyes lateral…second break up of palatoquadrate
- Heterodontiformes: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral… continuation of palatoquadrate
- Chimaera: mouth anterior and terminal, single gill opening lateral and covered, eyes lateral
- Cladoselache: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
- Hexanchiformes / Chlamydoselache: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
- Squatina: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral
- Rays: mouth ventral and subterminal, several gill openings ventral and covered, eyes lateral
- Squaliformes: mouth ventral and subterminal, several gill openings lateral and covered, eyes lateral
- Skates: mouth posterior and ventral, several gill openings ventral and covered, eyes dorsal
- Lamniformes, Lamnidae, Carcarhiniformes: mouth anterior and subterminal, several gill openings lateral and covered, eyes lateral
- Polyodon: (derived from 4. Loganiella) mouth anterior and ventral, single gill openings lateral and covered, eyes lateral
- Hybodus: mouth anterior and terminal, several gill openings lateral and covered, eyes lateral…break up of palatoquadrate
Figure 3. Falcatus skull. This taxon is close to Polyodon in the LRT. The palatoquadrate is a single unit in the above diagram, but divide into separate elements in the DGS colored image.
Falcatus (Fig. 3) is the last common ancestor in the diagram (Fig. 2) based on the LRT to have separate jaw elements. Descendant taxa all have a palatoquadrate. Falcatus and more primitive taxa are missing from the Richter and Underwood diagram (Fig. 1).
Figure 4. The purported palatoquadrate in Eusthenopteron (Fig. 5) is actually a fused pterygoid + quadrate. The palatine and ectopterygoid are separated by sutures. The dermal skull bones are shown in figure 5.
While overall similar to the shark palatoquadrate (Figs. 1, 2), the fused pterygoid + quadrate of Eusthenopteron (Fig. 4) is not homologous, except that both include the quadrate. This diagram, which indicates a fused quadrate + pterygoid, may in error, or an exception. Most tetrapods and their fish ancestors don’t fuse these bones.
Figure 5. Eusthenopteron. The interior of the skull, including the palatal bones are shown in figure 4.
Figure F. Basal tetrapods 2020.
Coates M, Gess R, Finarelli J, Criswell, K and Tietjen K 2016. A symmoriiform chondrichthyan braincase and the origin of chimaeroid fishes. Nature. doi: 10.1038/nature20806
Janvier P 1996. Early Vertebrates. Oxford: Claredon Press.
Richter M and Underwood C 2018, 2109. Origin, Development and Evolution of the Fish Skull, pp 144-159 in Johanson Z, Underwood C and Richter M Eds. Evolution and Development of Fishes. Cambridge University Press.
Rocek Z 1993. Palatoquadrate in a Devonian fish Eusthenopteron Evidence of its dual origin. Journal of Zoological Systematics and Evolutionary Research 31(1):38–46.. https://doi.org/10.1111/j.1439-0469.1993.tb00177.x