An overview of sturgeons: Bemis et al. 1997

Bemis et al. 1997 looked at sturgeons,
those strange semi-armored, lake-dwelling fish without a proper set of jaws (Fig. 1).

From the abstract:
“Acipenseriformes [= sturgeons and kin] has existed at least since the Lower Jurassic (approximately 200 MYBP), and all fossil and recent taxa are from the Holarctic. Phylogenetic relationships among Paleozoic and Early Mesozoic actinopterygians are problematic, but most workers agree that Acipenseriformes is monophyletic and derived from some component of ‘paleonisciform’ fishes.”

In the large reptile tree (LRT, 1684+ taxa; subset Fig. 2) sturgeons, like Pseudoscaphirhynchus, do not nest with bony fish, or sharks, but prior to the shark-bony fish split. Sturgeons can be thought of as less-armored osteostracans and more-armored thelodonts.

Figure 1. 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, a sturgeon with a protrusible tube for a mouth and reduced armor.

Figure 1. 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, a sturgeon with a protrusible tube for a mouth and reduced armor.

Again from the abstract:
“We discuss five features fundamental to the biology of acipenseriforms that benefit from the availability of our new phylogenetic hypothesis:

  1. specializations of jaws and operculum relevant to jaw protrusion, feeding, and ram ventilation;
  2. anadromy or potamodromy and demersal spawning;
  3. paedomorphosis and evolution of the group;
  4. the biogeography of Asian and North American polyodontids and scaphirhynchines; 
  5. and the great abundance of electroreceptive organs in the rostral and opercular regions.”

The LRT (Fig. 2) recovers sturgeons prior to the invention of terminal jaws and teeth, so what sturgeons have is not a specialization, but a primitive state.

Figure x. Newly revised fish subset of the LRT

Figure 2. Newly revised fish subset of the LRT. Sturgeons appear in the small gray box above.

Bemis et al. do not discuss
possible relationships with osteostracans and their thelodont ancestors. They do remark on the interest in sturgeon systematics, “Acipenseriforms also are noteworthy because of their unusual mixture of characters, which caused early debate about their classification. Two aspects of living Acipenseriformes were especially problematic for early ichthyologists: (1) reduced ossification of the endoskeleton combined with presence of an extensive dermal skeleton; and (2) the presence of a hyostylic jaw suspension and protrusible palatoquadrate recalling the jaws of sharks.”

In the LRT sturgeons are less armored osteostracans, arising from even less armored thelodonts. The ossification is not ‘reduced’. Rather the ossification is at a precursor stage.

From Bemis et al.
“Linneaus 1758, Lacépede 1797 and Heckel 1836 the classical idea was sturgeons must be closely related to sharks based on a similar endoskeleton and jaw suspension.”

Polyodon, the paddlefish (Fig. 3), has been lumped with sturgeons, but that is not supported by the LRT. Polyodon has real jaws.

Figure 4. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo.

Figure 3. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo. Traditionally lumped with sturgeons, the LRT does not support that interrelationship.

The fossil taxon, Chrondrosteus, has been lumped with sturgeons. In the LRT this taxon was added, then deleted due to a lack of good reconstruction due to a lack of skull bones and my lack of experience with fish. Putative relatives were studied yesterday, and do not nest with sturgeons. I will discuss this toothless taxon soon.

From Bemis et al.
“Müller 1846 defined the three grades of bony fish: Chondrostei, Holostei and Teleostei base on increasing degrees of ossification. Müller accepted sturgeons as osteichthyans.”

Those divisions are not supported by the LRT.

The Bemis et al. cladogram
nests sturgeons with paddlefish, both derived from Polypterus, the bichir (a lungfish in the LRT, and a sister to lungfish in Bemis et al). More taxa split three apart from one another in the LRT (Fig. 2). Taxon exclusion is the problem with Bemis et al. as no osteostracans, and dozens of other pertinent taxa, are included in their cladogram.

As mentioned earlier,
this is a novel hypothesis of interrelationships. If there is an earlier version of this hypothesis, send the citation so I can promote it.


References
Bemis WE, Findeis EK and Grande L 1997. An overview of Acipenseriformes. Environmental Biology of Fishes 48: 25–71.

https://en.wikipedia.org/wiki/Chondrostei

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