Acipenser enters the LRT as a basal sturgeon

This one could have been predicted
based on the short rostrum of Acipenser brevisrostrum (Figs. 1, 2), the long rostrum of the other two tested sturgeons (e.g. Pseudoscaphorhynchus Fig. 2) and the short rostrum of ancestral and descendant relatives of sturgeons like Hemcyclaspis and Chondrosteus (Fig. 2). The LRT tests a long list of 234 other traits, too, of course.

Figure 1. Acipenser skulls from Hilton 2005 and colored here.

Figure 1. Acipenser skulls from Hilton 2005 and colored here.

Acipenser brevistrostrum
(Lesueur 1818 non Heckel 1836, Hilton 2005) is the extant short-nosed sturgeon, with fossils going back to the Late Cretaceous (70mya). In the LRT it is the most primitive tested sturgeon.

The short rostrum inherited from Hemicyclaspis (Fig. 2) is retained in Chondrosteus and Isurus. The orbit is low and the oral cavity is set back from the rostral tip, as in Thelodus (Fig. 2), not Hemicyclaspis. Barbels are present, as in Thelodus, not Hemicyclaspis. The dorsal scutes are low and scattered, as in Thelodus, not Hemicyclaspis.

Younger shortnose sturgeons tend to have longer snouts compared to their older counterparts, but that is not reflected in the skull. After losing hatchling teeth, adult sturgeons have bony plates along their esophagus that help them crush hard items.

Figure 2. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser brevirostrum, a short-listed sturgeon with a protrusible tube for a mouth and reduced armor. Pseudoscaphorhynchus, a derived sturgeon. Chondrosteus, a fish with jaws, but no marginal teeth.

Figure 2. Top to bottom: Thelodus a soft jawless fish with a ventral oral opening and gill slits, perhaps a hint of diamond-shaped armor laterally. Hemicyclaspis, adds extensive armor. Acipenser brevirostrum, a short-listed sturgeon with a protrusible tube for a mouth and reduced armor. Pseudoscaphorhynchus, a derived sturgeon. Chondrosteus, a fish with jaws, but no marginal teeth.

Shortnose sturgeons
are cartilaginous overall with bones in the skull, jaw and pectoral girdle. Bone is further reduced in descendant sharks, then reversed to more bone in basal bony fish following Hybodus. The sturgeon swim bladder is connected to the intestinal tract by a narrow duct, representing the origin of lungs. This is lost in sharks, then a reversal returns this to certain bony fish, including lungfish and crossopterygians.

Acipenser gives us our clearest clue into the soft tissue
of ancestral thelodonts and osteostracans. At present, Acipenser is the oldest surviving vertebrate, a living fossil, with phylogenetic origins deep into the Silurian based on Thelodus (Fig. 2) from the Early Silurian.


References
Hilton EJ 2005. Observations on the skulls of sturgeons (Acipenseridae): shared similarities of Pseudoscaphirhynchus kaufmanni and juvenile specimens of Acipenser stellatus. Environmental Biology of Fishes 72:135–144.
Le Sueur CA 1818. Description of several species of chondropterygious fishes of North America, with their varieties. Trans. Amer. Phil. Soc. 1: 383–394.

wiki/Shortnose_sturgeon_Acipenser

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