Sallen 2016 presents a fascinating flawed look at fish tails

Sallen 2016 reports,
“The symmetrical, flexible teleost fish ‘tail’ has been a prime example of recapitulation — evolutionary change(phylogeny) mirrored in development (ontogeny).”

Sallan’s cladogram (Fig. 1) lays out the traditional cladogram of fish. Note the position of the bichir (Polypterus), at a basal node and the sturgeon + paddlefish (Acipcenser + Polyodon) near the middle.

Figure 1. Cladogram from Sallan 2016 (above) and young fish tails (below).

Figure 1. Cladogram from Sallan 2016 (above) and young fish tails (below).

Unfortunately,
taxon exclusion mars the cladogram of Sallan 2016 according to the the large reptile tree (LRT, 1704+ taxa; Figs. 2, 5). Due to tradition Sallan has chosen the wrong outgroup. Jawless sturgeons and shark-like paddlefish should be the outgroups here, not lungfish-like bichirs (Polypterus), which are highly derived taxa close to lungfish and tetrapods.

Figure 2. Same taxa as above, but rearranged to fit the LRT tree topology.

Figure 2. Same taxa as above, but rearranged to fit the LRT tree topology. Remember, sturgeons, paddlefish and sharks are basal taxa in the LRT. Esox is a catfish related to placoderms.

Salan reports,
“Paleozoic ray-finned fishes (Actinopterygii), relatives of teleosts, exhibited ancestral scale-coveredtails curved over their caudal fins. For over 150 years, this arrangement was thought to be retained in teleost larva and overgrown, mirroring an ancestral transformation series. New ontogenetic data for the 350-million-year-old teleost relative Aetheretmon overturns this long-held hypothesis.”

By contrast,
in the LRT Aetheretmon nests with Pteronsculus (Figs. 5–7)) far from the base of all bony fish, much closer to lobefin fish and tetrapods.

The Sallan point is still made:
Many fish tails do have two parts, especially when hatchlings.

Unfortunately, Sallan does not understand
the topology of the family tree of fish due to taxon exclusion. This is something the LRT minimizes by testing a wider gamut of taxa. As readers know, we see this same taxon exclusion problem all the time in paleontology.

Figure 2. Muskie (Esox) tail ontogeny from Sallan 2016 (middle row). Top row and photo added here.

Figure 3. Muskie (Esox) tail ontogeny from Sallan 2016 (middle row). Top row (to scale) and photo (below) added here. You might remember, Esox is a derived catfish without barbels.

Salan writes,
These two tails appear at a shared developmental stage in Aetheretmon, (Fig. 4) teleosts and all living actinopterygians. Ontogeny does not recapitulate phylogeny; instead, differential outgrowth determines final morphology.”

That appears to be so, but it still needs a valid tree topology.

Figure 3. Fish tail ontogeny in extinct Aetheretmon and extant Monotrete. Note the upper and lower lobes.

Figure 4. Fish tail ontogeny in extinct Aetheretmon and extant Monotrete. Note the upper and lower lobes. In the LRT these two fish are not closely related. Aetheretmon is basal to lobefins. Monotrete is a puffer fish.

Salan speculates:
“The double tail likely reflects the ancestral state for bony fishes.”

No, the ancestral state for bony fish is the heterocercal tail documented by sturgeons and whale sharks, and this goes back to armored osteostracans according to the LRT (Fig. 5).

Figure x. Subset of the LRT, focusing on fish for July 2020.

Figure x. Subset of the LRT, focusing on fish for July 2020.

Salan speculates,
“Many tetrapods and non-teleost actinopterygians have undergone body elongation through tail outgrowth extension, by mechanisms likely shared with distal limbs.”

Not sure what those ‘mechanisms’ would be, but basal tetrapods and stem tetrapods in the LRT have relatively short, straight tails and elongated bodies with great distances between the fore and hind limbs. Look at Panderichthys.

Figure 5. Aetheretmon is known from the oldest complete growth series for vertebrates.

Figure 6. Aetheretmon is known from the oldest complete growth series for vertebrates.

Figure 6. Pteronisculus, a sister to Aetheretmon in the LRT.

Figure 7. Pteronisculus, a Triassic sister to Early Carboniferous Aetheretmon in the LRT and it is easy to see why.

Sallan is ‘Pulling a Larry Martin’
by putting too much emphasis on one trait without testing all the traits on many more taxa. Only after a valid phylogenetic context is established can one begin to figure out if trait A came before trait B or not.

Sallan goes into great detail describing
the successive stages of growth in Aetheretmon, but this is problematic because the cladogram is invalid. “First things first” is a motto all paleontologists should ascribe to. First get the phylogeny correct. Fish workers are relying on an invalid family tree. The LRT is here to fix that.

Its worth remembering,
many fish on the other branch of bony fish (perch, anglers, etc., Fig. 5, orange right column) bring the pelvic fins beneath the pectoral fins, shortening the gut cavity and elongating the tail to extremes in some cases (oarfish). This is all distinct from the longer torso, shorter tail trend in the stem tetrapod branch of bony fishes (Fig. 5, yellow left column).


References
Sallan 2016. Fish ‘tails’ result from outgrowth and reduction of two separate ancestral
tails. Current Biology 26, R1205–R1225.
White EI 1927. The fish fauna of the Cementstones of Foulden, Berwickshire. Transactions of the Royal Society Edinburgh 55:255–287.

https://www.the-scientist.com/the-nutshell/a-tale-of-two-tails-32394

Another Use for Pterosaur Tale Vanes

Where are Pterosaur Tail Vanes Found?
Basal pterosaurs are often illustrated with tail vanes, but they are not found on many basal pterosaurs. Soft tissue preservation is rare. The Campylognathoides/Rhamphorhynchus clade had the most prominent tail vanes. Various Dorygnathus may have had something like a vane. It’s never clear. Sordes had some sort of tail expansion and Pterorhynchus did not have a single vane, per se, but several very short ones along the length. The tail vane seems to have coalesced from several smaller vanes, which, in turn, may have developed from specialized ptero-hairs seen on the tail of Cosesaurus.

Tail vane animation on the C5 specimen of Campylognathoides.

Figure 1. Click to animate. Tail vane animation on the C5 specimen of Campylognathoides zitteli.

Tail Vane Usage
The tail vane has typically been considered a steering mechanism, but airplanes don’t steer with their tail (vertical stabilizer). That just produces a skid and lots of drag. To initiate a turn airplanes, birds and bats roll into a bank.  Presumably pterosaurs did likewise. The tail vane would have worked like feathers on an arrow shaft, keeping the back of the tail in line with the line of least drag, in line with the body at all times, and all without effort.

Correlations
You might note that the most prominent tail vanes are also found in the clade with the longest wings in relation to their body size. In Campylognathoides and Rhamphorhynchus the wing tips extend far above their head. The tail itself was stiff, able to rise and fall tilting at its base. The same was true of the wings. They were stiff and able to fold and unfold at the metatarsophalangeal joint. Those are similar actions as seen from the side. That got me thinking.

The Metronome Hypothesis
What if the tail rose and fell like a metronome? The wings could open and fold in counterpoint. Together the three elements might have produced a secondary sexual behavior that attracted mates… or was just a way to relax.

Pure speculation.  Enjoy the animation.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.