The nesting of Chondrosteus with sturgeons
is not controversial.
the nesting the traditional ‘bony fish’ Chondrosteus and sturgeons with sharks and ratfish is heretical and documents only part of the evidence for a multiple genesis of bony fish. In the large reptile tree (LRT, 1499 taxa) Chondrosteus (Fig. 1) nests as a shark-like sturgeon from the Early Jurassic, at which time it was a late survivor of a much earlier (likely Silurian), genesis and radiation.
Chondrosteus and sturgeons share a last common ancestor
in the shark-like Cladoselache, which we looked at earlier here.
Figure 1. Chondrosteus skull reconstructed, skull in ventral view in situ and skeleton.
Chondrosteus acipenseroides (Agassiz 1848; Egerton 1858; Hilton and Forey 2010; Early Jurassic) is a prehistoric sturgeon without dorsal armor. It is traditionally considered a ray fin fish but nests in the LRT with ratfish and Cladoselache. Most of the skull is poorly ossified.
FIgure 2. The small sturgeon Pseudoscahirhynchus skull in several views. Note the perforated rostrum (nasal) sensitive to prey hiding in mud. The mouth is reduced to a tiny sucking tube disconnected from the quadrate. Even so, this sturgeon nests with sharks in the LRT.
The spiral intestine question
According to this website, “The digestive system of the lamprey, posterior to the pharynx, is a simple tube. The first portion is called the esophagus, although it is not greatly different in its external appearance from the intestine which follows. Inside the intestine is found a modified fold of the intestinal lining called a typhlosole, or spiral valve. It forms a long curtain-like partition which is suspended into the lumen of the intestine through most of its length. This long typhlosole’s attachment to the intestine wall is mid dorsal at the anterior end, then spirals slightly as it progresses toward the posterior end.”
“In sharks, other chondrichthians, polypterids, sarcopterygians, sturgeons, paddlefish, bowfins, and gars, the intestine retains the spiral partition. Its spiral is more pronounced in these fish than in the lamprey.”
“Higher teleost fish loose the spiral valve modification of the intestine in favor of a lengthened system which coils about within the body cavity.”
“Intestines, urinary ducts, and reproductive passages terminate at a common space known as a cloaca in most fishes and tetrapods. Exceptions include lampreys, holocephalans, coelacanths, and therian mammals.”
I cannot make any more sense of this than what was just presented. Evidently the spiral valve comes and goes. Sturgeons, in this case, do not have a spiral valve because holocephalans (ratfish) do not have a spiral valve.
Figure x. Subset of the LRT focusing on fish and other basal vertebrates.
Agassiz L 1848. Nomenclatoris zoologici index universalis: continens nomina systematica classium, ordinum, familiarum et generum animalium omnium, tam viventium quam fossilium, secundum ordinem alphabeticum unicum disposita, adjectis homonymiis plantarum, nec non variis adnotationibus et emendationibus.
Egerton PdMG 1858. On Chondrosteus, an Extinct Genus of the Sturionidaek, Found in the Lias Formation at Lyme Regis. Philosophial Transactions of The Royal Society of London 148:871–885.
Hilton EJ and Forey PL 2010. Redescription of Chondrosteus acipenseroides Egerton, 1858 (Acipenseriformes, †Chondrosteidae) from the lower Lias of Lyme Regis (Dorset, England), with comments on the early evolution of sturgeons and paddlefishes. Journal of Systematic Palaeontology. 7(4):427–453.