2020 study on osteostracan swimming omits 1999 study on sturgeon swimming

Osteostracans are primitive and extinct jawless fish
that evolved a variety of head shields (Fig. 1). Some of these head shield shapes were tested for their hydrodynamic properties by Ferrón et al. 2020.

Figure 1. Hemicyclaspis and a variety of other osteostracans in dorsal view created by Ferrón et al. 2020 to test their hydrodynamics.

Figure 1. Hemicyclaspis and a variety of other osteostracans in dorsal view digitally created by Ferrón et al. 2020 to test their hydrodynamics.

In brief:
“Ferron et al. show, using computational fluid dynamics, that early jawless vertebrates were ecologically diversified and had complex hydrodynamic adaptations. These findings challenge the traditional scenario of jaws as the key evolutionary innovation that precipitated the ecological diversification of our ancestors.”

Not according to the large reptile tree (LRT, 1741+ taxa; subset Fig. x). That diversification followed the genesis of jaws and pelvic fins (Chondrosteus (Fig. 4) and descendants, (Fig. 1) and accelerated with the genesis of marginal teeth (Polyodon  and descendants, Figs. 5).

Figure x. Subset of the LRT focusing on fish.

Figure x. Subset of the LRT focusing on fish.

Ferron et al. report, 
“the extinct jawless ‘‘ostracoderms’’ are interpreted as cumbersome deposit feeders lacking key apomorphies of jawed vertebrates including multiple pairs of appendages and an epicercal tail, as well as jaws. This popular scenario belies the challenge of constraining the biology of ostracoderms that lack living analogs, traditionally compromising attempts to derive functional interpretations of their morphology.”

Accoding to the LRT (Fig. x) ostracoderms have living homologs (and analogs), sturgeons (Figs. 2, 3), a clade of extant fish omitted by Ferrón et al. 2020. Traditionally sturgeons are considered much more derived ray-fin fish, not living osteostracans. Taxon exclusion, like this, is a common problem in paleontology.

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 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, a sturgeon with a protrusible tube for a mouth and reduced armor homologous with that of the osteostracan, Hemicyclaspis.

Ferron et al. report,
“We analyzed the functional morphological diversity of the Silurian-Devonian Osteostraci, the jawless sister group to all jawed vertebrates, which cover all the morphological grades exhibited by ostracoderms, including forms that entirely lack paired appendages and others that possess just a single (pectoral) pair.”

Due to taxon exclusion,
Ferrón et al. wrote: “These results suggest different hydrofoil functions among osteostracan headshield morphologies, compatible with ecological diversification and undermining the traditional view that jawless stem-gnathostomes were ecologically constrained with the origin of jaws as the key innovation that precipitated the ecological diversification of the group.”

Ferron et al. did not realize that 
living sturgeons are derived ostracoderms and stem gnathostomes, according to the LRT. Sturgeons also come in a variety of sizes and shapes. Rather than creating computer graphics of osteostracoderms alone, the Ferron team could have started with actual sturgeons in actual water over actual lake beds… or cited Wilga and Lauder 1999 (Fig. 3).

Figure 2. Sturgeon swimming in a test tank from Wilga and Lauder 1999.

Figure 3. Sturgeon swimming in a test tank from Wilga and Lauder 1999. Note pectoral fin in middle image has posterior edge elevated.

According to the LRT,
(subset Fig. x) ostracoderms are sturgeon ancestors. Sturgeons gave rise to jawed fish (gnathostomes) like Chondrosteus (Fig. 4) and THAT’s where the present diversity of fishes arises.

Wilga and Lauder 1999 reported,
“In the plesiomorphic pectoral fin condition, exemplified by sturgeon, pectoral fins extend laterally from the body in a generally horizontalorientation, have been assumed to generate lift to balance lift forces and moments produced by the heterocercal tail, and are not oscillated to generate propulsive force. Three-dimensional kinematic analysis showed that during steady horizontal swimming the pectoral fins are oriented with a negative angle of attack predicted to generate no significant lift.”

But note (Fig. 3) the entire sturgeon is a lifting body with a flat underside and convex dorsal surface. So the negative angle of attack of the pectoral fins balances the natural hydrodynamic lifting forces. Note the similar overall  body shapes of sturgeons and the osteostracn, Hemicyclaspis (Figs. 1, 2), along with similar pectoral fins capable of rising posteriorly, like airplane elevators (fgi. 3).

Wilga and Lauder 1999 continue:
“The orientation of the pectoral fins estimated by a two-dimensional analysis alone is greatly in error and may have contributed to previous suggestions that the pectoral fins are oriented to generate lift.”

See:But note” above. Same answer here.

Re: osteostracoderms Ferrón et al. reported:
“Combined electromyographic and kinematic data showed that the posterior half of the pectoral fin is actively moved as a flap to reorient the head and body to initiate rising and sinking movements.”

As shown by Wilga and Lauder (Fig. 3).

Ferrón et al. continue:
“During steady locomotion, the pectoral fins generate no lift and the positive body angle to the flow is used both to generate lift and to balance moments around the center of mass.”

As shown by Wilga and Lauder (Fig. 3).

Ferrón et al. continue:
“To initiate rising or sinking, the posterior portion of the pectoral fins is actively moved ventrally or dorsally, respectively, initiating a starting vortex that, in turn, induces a pitching moment reorienting the body in the flow to body angle initiated by the pectoral fins serve as the primary means by which moments are balanced.”

As shown earlier by Wilga and Lauder (Fig. 3). Wilga and Laude 1999 were not cited by Ferrón et al. 2020. Chondrosteus is also missing,

Not only did Chondrosteus have real jaws (still without teeth),
the rostrum was raised from ventral to middle, making it more of a bullet shape with a more circular cross-section with a narrower transversely rounder belly). These traits helped make Chondrosteus a better open water swimmer, like many living sharks and paddlefish, freeing it from seeking buried prey. Chondrosteus (Fig. 4) also increases the size of the pelvic fins for added hydrodynamic control and the lower lobe of the caudal fin was larger, more like the heterocercal upper lobe (Fig. 2).  These traits continue in the paddlefish, Polyodon (Fig. 5).

Figure 1. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

Figure 4. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

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

Figure 5. Skull of Polyodon from a diagram published in Gregory 1938, plus a dorsal view and lateral photo. Note the large size of the pelvic fins.

Chondrosteus descendants
like Loganellia, Manta and Rhincodon (Fig. 6), returned to a flattend skull and body shape, but with an anterior toothless mouth and an open-water swimming pattern.

Figure 11.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish. Note the lack of teeth. 

Figure 6.  Manta compared to Thelodus (Loganellia) and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes, distinct from most fish. Note the lack of teeth.

Other Chondrosteus descendants
include free-swimming, circular, cross-section, predatory Gregorius and its descendants, the bony fish. This clade splits from similar free-swimming, circular, cross-section Hybodus and its descendants, the cartilaginous fish (sharks + ratfish). Thereafter both clades produced some bottom dweller feeders with flattened ventral surfaces ( e.g. sawfish, Panderichthys). These taxa can be considered reversals.


References
Ferrón HG, Martinez-Perez C, Rahman IA, Selles de Lucas V, Botella H and Donoghue PCJ 2020. Computational Fluid Dynamics Suggests Ecological Diversification among Stem-Gnathostomes. Current Biology 30:1–6.
Wilga CD and Lauder GV 1999. Locomotion in sturgeon: Function of the pectoral fins. The Journal of Experimental Biology 202, 2413–2432.

wiki/Sturgeon
wiki/Osteostraci

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