Sharks and sturgeons: fish with a lateral temporal fenestra

When you add the mako shark
(genus: Isurus) to the large reptile tree (LRT, 1460 taxa) it nests with the sturgeon, (genus: Pseudoscaphirhynchus, Fig. 2) close to the bottom. Both are derived from the placoderm, Entelognathus.

Long separated in prior cladograms
sharks have a skeleton of cartilage, several gill openings and are fast swimmers. Meanwhile sturgeons have bone in their skeleton, a single gill opening and tiny jaws for slow motion bottom feeding.

Now would be a good time to remember
that sharks and rays are traditional relatives and rays, like sturgeons, have a small mouth beneath a flat ultra sensitive rostrum ideal for bottom feeding.

Which came first?
Both are extant taxa and no more primitive sisters have been tested so far. Even so, the shark retains strong jaws, like those of succeeding (descendant) taxa, but the sturgeon is a flattened bottom dweller, like stem tetrapods.The last common ancestor, Entelognathus, had both of these traits.

Pseudoscaphirhychus kaufmanni (Nikolskii 1900) is the extant Amu darya sturgeon. Distinct from traditional cladograms, the LRT nests this sturgeon next to the placoderm, Entelognathus, among tested taxa. Both have weak jaws and no teeth, bony armor and (presumeably) a shark-like heterocercal tail, despite the 425 million year difference. Note the confluent orbit and lateral temporal fenestra.

Isurus oxyrinchus (Rafinesque 1810; 3.2m in length) is the extant shortfin mako shark here nesting with the sturgeon, Pseudoscaphirhynchus, contra tradition. Here the nasal is extended to form a rostrum. The lacrimal, premaxilla, maxilla, jugal, squamosal and quadrate are all fused together. The postorbital is fused to the postfrontal. A deep valley divides the skull. The orbit is confluent with the lateral temporal fenestra. The premaxillary teeth are posteriorly oriented. The jugal produced a lateral flange for increased muscle attachment. The skeleton is cartilaginous.

We have a more generalized/plesiomorphic
late-surviving Jurassic sturgeon ancestor, Chondrosteus acipenseroides (Agassiz 1843, 1 meter length; Fig. 3) and it has jaws midway in size between the two above taxa. Skull bones are reduced from the primitive state in the Late Silurian placoderm, Entelognathus. Chondrosteus has not been added to the LRT yet.

In a few days we’ll look at
‘the oldest articulated chndrichthyan,’ Doliodus, along with another Devonian shark, Cladoselache to see where they nest in the LRT. Chondrichthyans include living sharks, skates, rays and chimaeras.

References
Nikolskii AM 1900. Pseudoscaphirhynchus rossikowi, n. gen, et spec. Ann. Mus. Imp. Sci. St. Petersburg 4, 257–260 (text in Russian).
Rafinesque CS 1810. Caratteri di alcuni nuovi generi e nuove specie di animali e piante della sicilia, con varie osservazioni sopra i medisimi. Per le stampe di Sanfilippo: Palermo, Italy. pp. 105, 20 fold. Pl., online

wiki/Pseudoscaphirhychus
wiki/Sturgeon
wiki/Shortfin_mako_shark
wiki/Chondrosteus