Traditional batoids (skates + rays): taxon exclusion hampers prior phylogenetic results

McEachran and Aschliman 2004 reported,
“all authors agree that batoids constitute a monophyletic group.”

Underwood et al. 2015 reported, 
“While the monophyly of the Batoidea is not in doubt, phylogenetic relationships within the group are uncertain.”

By including a wider gamut of taxa,
the large reptile tree (LRT, 1785+ taxa, subset Fig. 1) recovers rays apart from skates and mantas apart from both. So the monophyly of the Batoidea is in doubt when more taxa are added. It is also surprising that a character list with no batoid characters is able to lump and split them, indicating the primacy and necessity of adding taxa.

Figure 1. Subset of the LRT focusing on basal gnathostomes. Traditional rays and skats are highlighted.
Figure 1. Subset of the LRT focusing on basal gnathostomes. Traditional rays and skates are highlighted along with Squaloraja, a traditional chimaerid with a sawshark appearance and Tristychius, a flattened nurse shark relative with large fins.

Franklin et al. 2014 wrote:
“A database of 253 specimens, encompassing 60 of the 72 batoid genera, reveals that the majority of morphological variation across Batoidea is attributable to fin aspect-ratio and the chordwise location of fin apexes. Both aspect-ratio and apex location exhibit significant phylogenetic signal.”

Figure 2. Four 'batoid' cladograms published in Underwood et al. 2015 with citations listed.
Figure 2. Four ‘batoid’ cladograms published in Underwood et al. 2015 with citations listed. They don’t agree with each other largely due to taxon exclusion and inappropriate outgroup taxa.

For those who want evidence of evolution
the four cladograms offered in Underwood et al. 2015 (Fig. 2) offer little.

  1. They employ suprageneric taxa for outgroup taxa
  2. They exclude pertinent taxa (see Fig. 2) from both the in-group and out-group.
Figure 3. Batoid cladogram frrom Sasko et al. 2006 with notes on swimming motions.
Figure 3. Batoid cladogram frrom Sasko et al. 2006 with notes on swimming motions. Note the differences compared to those in figure 2. 

Sasko et al. 2006 published a batoid phylogeny
that included notes on swimming styles. Taxon exclusion also mars this study. As a result convergence is ignored. These authors didn’t think they were cherry-picking taxa… but they were doing exactly that. They thought they were covering ‘all the bases’. The editors and referees agreed. That’s why the LRT tests a wider gamut of taxa to minimize the possibility of this sort of taxon exclusion. Outgroups are important. Omit pertinent outgroups and nothing else goes right.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1. Note the sturgeon-like reversal in the guitarfish, Rhinobatos.

By contrast
in the LRT (subset Fig. 1, diagram Fig. 4) Holocephali (=ratfish) is a derived clade, not a basal bauplan upon which rays and skates evolved. While more rays and skates are listed in the four Underwood et al. cladograms, the LRT includes outgroup taxa back to headless chordates. Long nosed sawfish and guitarfish nest together in the LRT. Marginally toothless and filter-feeding mantas nest with similar whale sharks and kin (not found in Underwood et al. cladograms). Bottom line: prior authors assumed too much. More taxa would have helped, as shown in Fig. 2.

Figure 2. The spotted eagle ray, Aetobatus in vivo.
Figure 5. The spotted eagle ray, Aetobatus in vivo.

A key to understanding evolution
is to understand that most of the time (tunicates, starfish and kin a clear exception), simpler taxa evolve into more complex taxa by the gradual accumulation of derived traits. In vertebrates, jawless chordates appear first. Then pre-jaws appear ventrally in sturgeons. In mantas and whale sharks marginally toothless jaws migrate anteriorly. In the rest, the sensitive rostrum continues to overhang the now tooth-lined jaws. Starting with this scenario, the rest of the chondrichthyes evolves wither a shorter or longer rostrum, pectoral fins might take over propulsion (convergent with mantas), and teeth might turn into pavement stone analogs.

Figure 5. Sturgeon mouth animated from images in Bemis et al. 1997. This similar to ostracoderms, basal to sharks.
Figure 6. Sturgeon mouth animated from images in Bemis et al. 1997. This returns in guitarfish (Fig. 7).
Figure 3. Rhinobatus jaw mechanism animation. This is how skates and rays eat, distinct from the Thelodus/ whale shark/ manta ray method of ram feeding.
Figure 7. Rhinobatus jaw mechanism animation. This is how skates and rays eat, distinct from the Thelodus/ whale shark/ manta ray method of ram feeding. Compare to the sturgeon in figure 6.

While we’re at it,
please note the overlooked sturgeon-like reversal displayed by the guitarfish, Rhinobatos (Figs. 4, 7), basal to skates. That tiny-extending mouth morphology (Figs. 6, 7) didn’t appear de novo. It was waiting in the sturgeon-shark-skate gene pool to return.


References
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Franklin O, Palmer C and Dyke G 2014. Pectoral fin morphology of batoid fishes (Chondrichthyes: Batoidea): explaining phylogenetic variation with geometric morphometrics. J Morphol 275(10):1173-86. doi: 10.1002/jmor.20294. Epub 2014 May 5.
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Undersood CJ et al. (6 co-authors) 2015. Development and Evolution of Dentition Pattern and Tooth Order in the Skates And Rays (Batoidea; Chondrichthyes). PLoS ONE 10(4): e0122553. doi:10.1371/journal.pone.0122553