Cretaceous Aquilolamna nests with Devonian Palaeospondylus in the LRT

Summary for those in a hurry
The authors excluded related taxa that would have helped them identify their strange, new 1.6 m shark with elongate pectoral fins. The authors also failed to identify the correct mouth, eyes, nasal capsules and gill slits.

Vullo et al. 2021 bring us a wonderful new 1.6m Turonian elasmobranch
with graceful, really long, pectoral fins, Aquilolamna milarcae (INAH 2544 P.F.17, Figs. 1, 2). The authors tentatively assigned (without a phylogenetic analysis) their fossil shark to lamniformes, like the mako shark, Isurus, which has a standard underslung mouth and overhanging rostrum. Vullo et al. thought Aquilolamna was a filter-feeder by assuming that it had a wide, ‘near-terminal mouth’ without teeth, as in the manta ray (genus: Manta). That morphology is distinct from lamniformes like Isurus.

This is a difficult fossil to interpret.
More than the fins make Aquilolamna different than most other fossil and extant sharks.

Unfortunately
Vullo et al. put little effort (Fig. 2 diagram) into their attempt to understand the many clues Aquilolamna left us. Those clues are documented here (Fig. 2) by using DGS (= color tracings) and tetrapod homologs for skull bones.

Figure 1. Aquilolamna in situ from Vullo et al. 2021. Colors added here.

Figure 1. Aquilolamna in situ from Vullo et al. 2021. Colors added here.

For proper identification, it didn’t help that Vullo et al. 

  1. imagined the mouth wide and in front, instead of small and below the occiput
  2. imagined the eyes on the sides, instead of on top
  3. imagined the gill slits on the sides, instead of ventral
  4. did not perform a phylogenetic analysis with a wide gamut of taxa
  5. did not consider Middle Devonian Palaeospondylus (Figs. 3, 4) as a taxon worthy of their time and consideration
  6. did not consider the torpedo ray, Tetronarce (Fig. 5), or the hammerhead, Sphyrna, taxa worth comparing in analysis (as in Fig. 4).
Figure 2. Skull of Aquilolamna and diagram from Vullo et al. 2021. Colors and new labels applied here. The mouth (magenta) appears under the occiput, overlooked by Vullo et al.

Figure 2. Skull of Aquilolamna and diagram from Vullo et al. 2021. Colors and new labels applied here. The mouth (magenta) appears under the occiput, overlooked by Vullo et al. White lines indicate symmetries. The hyomandibulars are small with fused quadrates at the new jaw corners and link to the intertemporals, as in all other vertebrates.

Despite these issues, Vullo et al. thought there was enough of Aquilolamna
that was strange, new and easy to understand to make it worthy of publication. And it is. And that’s okay. In science it’s okay to leave further details to other workers. Keeps us busy and feeling helpful! It’s okay to make mistakes. Others will fix those. That’s all part of the ongoing process.

From the abstract:
“Aquilolamna, tentatively assigned to Lamniformes, is characterized by hypertrophied, slender pectoral fins. This previously unknown body plan represents an unexpected evolutionary experimentation with underwater flight among sharks, more than 30 million years before the rise of manta and devil rays (Mobulidae), and shows that winglike pectoral fins have evolved independently in two distantly related clades of filter-feeding elasmobranchs.”

By contrast, in the LRT filter-feeding manta rays are more primitive than sharks that bite for a living.

Unfortunately the authors omitted important sister taxa recovered by the LRT from their comparison studies. They looked at other elamobranchs, but not the electric torpedo ray, hammerhead and Palaeospondylus (Figs. 3, 4).

By focusing on just a few traits the authors are trying to “Pull a Larry Martin.” Instead they should have performed a wide-gamut phylogenetic analysis with hundreds of traits.

Figure 1. A specimen of Palaeospondylus in situ with colors added here. This appears to be a ray in the hammerhead shark, Sphyraena family.

Figure 3. A specimen of Palaeospondylus in situ with colors added here. This appears to be a ray in the hammerhead shark, Sphyrna family, that also includes the electric torpedo ray, Tetronarce.

Figure 4. Palaeospondylus diagram from Joss and Johanson 2007 who mistakenly considered Palaeospondylus a hatchling lungfish.

Figure 4. Palaeospondylus diagram from Joss and Johanson 2007 who mistakenly considered Palaeospondylus a hatchling lungfish.

From the taphonomy section of the SuppData:
“No teeth can be observed in INAH 2544 P.F.17, possibly due to rapid post-mortem disarticulation and scattering affecting the dentition.”

Turns out the authors were looking for teeth in the wrong place. The real jaws with tiny teeth were partly hidden below the occiput, as in Middle Devonian Palaeospondylus (Fig. 4), not at the anterior skull rim of Aquilolamna.

Figure 4. Subset of the LRT focusing on the shark clades related to Aquilolamna and Palaeospondylus.

Figure 4. Subset of the LRT focusing on the elasmobranch clades related to Aquilolamna and Palaeospondylus.

The reported lack of pelvic fins in Aquilolamna
is unexpected in sharks, which otherwise always have pelvic fins. This lack of pelvic fins could turn out to be a synapomporphy of taxa descending from Palaeospondylus. We’ll have to have more taxa for that.

From the Vullo et al. 2021 diagnosis of the ‘family, genus and species’:
“Medium-sized neoselachian shark that differs from all other selachimorphs in having hypertrophied, scythe-shaped plesodic pectoral fins whose span exceeds the total length of the animal. High number (~70) of anteriorly directed pectoral radials. Head short and broad, with wide and near-terminal mouth. Caudal fin markedly heterocercal. Caudal fin skeleton showing a high hypochordal ray angle (i.e., ventrally directed hypochordal rays). Caudal tip slender with no (or strongly reduced?) terminal lobe. Squamation strongly reduced (or completely absent?).”

Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

Figure 5. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m). Note the robust caudal fin. The hyomandibular links the jaw joint to the braincase.

Aquilolamna has more vertebrae than Palaeospondylus,
but the former is much larger, an adult and geologically younger by 280 million years. We looked at Palaeospondylus just three days ago here. Very lucky timing to have Palaeospondylus for comparison prior to studying Aquilolamna.

Figure 6. Ontogenetic growth series of an electric torpedo ray. Pectoral fins in green.

Figure 6. Ontogenetic growth series of an electric torpedo ray from Madl and Yip 2000. Pectoral fins in green. Pectoral fins enlarge with maturity. Eyes migrate dorsally. Perhaps the same occurred with Aquilolamna and Palaeospondylus.

Taxon exclusion
continues to be the number one problem in paleontology. Phylogenetic analysis with a wide gamut of hundreds of taxa continues to be the number one solution to nesting all new and enigma taxa. Contra the assertions of dozens of PhDs, first-hand examination of the fossil is not required, nor is a degree or doctorate. This is the sort of profession where you learn on the job with every new taxon that comes along. This one was not in any textbooks, so everyone started like a September freshman with Aquilolamna.

And finally, if you can’t find the mouth where you think it should be,
look somewhere else.


References
Madl P and Yip M 2000. Essay about the electric organ discharge (EOD) in Colloquial meeting of Chondrichthyes head by Goldschmid A, Salzberg, January 2000. Online here.
Vullo R, Frey E, Ifrim C, Gonzalez Gonzalez MA, Stinnesbeck ES and Stinnesbeck W 2021. Manta-like planktivorous sharks in Late Cretaceous oceans. Science 371(6535): 1253-1256. DOI: 10.1126/science.abc1490
https://science.sciencemag.org/content/371/6535/1253

Online Publicity for Aquilolamna:

  1. sciencemag.org/news/2021/03/eagle-shark-once-soared-through-ancient-seas-near-mexico
  2. phys.org/news/2021-03-discovery-winged-shark-cretaceous-seas.html
  3. nationalgeographic.com/science/article/shark-like-fossil-with-manta-wings-is-unlike-anything-seen-before
  4. livescience.com/ancient-shark-flew-through-dinosaur-age-seas.html

Middle Devonian Paleospondylus nests with extant torpedo rays

Summary for those in a hurry:
a traditional enigma fish taxon, Paleospondylus (Figs. 1, 2) nests in the large reptile tree (LRT, 1815+ taxa) with the electric torpedo ray (Fig. 3) genus = Tetronarce), a taxon overlooked by all prior studies.

Before the addition of Paleospondylus,
the closest relative of the torpedo ray in the LRT was the hammerhead shark, Sphyraena. Both contributed to understanding the taxonomy and anatomy of Paleospondylus, a tiny juvenile ray with a relatively big, shark-like tail (Fig. 1). The LRT is the first wide gamut phylogenetic analysis attempt for Paleospondylus. Earlier studies compared only a few traits and few taxa, thereby “Pulling a Larry Martin” in the process.

Figure 1. A specimen of Palaeospondylus in situ with colors added here. This appears to be a ray in the hammerhead shark, Sphyraena family.

Figure 1. A specimen of Palaeospondylus in situ with colors added here. This appears to be a juvenile ray in the hammerhead shark (Sphyraena) family. The torpedo ray, Torpedo, is also a member of this clade. Shown twice life size.

According to Wikipedia:
“Palaeospondylus gunni (Gunn’s ancient vertebrae, Traquair 1890) is a mysterious, fish-like fossil vertebrate. The fossil as preserved is carbonized, and indicates an eel-shaped animal up to 6 centimetres (2 in) in length. The skull, which must have consisted of hardened cartilage, exhibits pairs of nasal and auditory capsules, with a gill apparatus below its hinder part, and ambiguous indications of ordinary jaws.”

The phylogeny of this bizarre fossil has puzzled scientists since its discovery in 1890, and many taxonomies have been suggested. In 2004, researchers proposed that Palaeospondylus was a larval lungfish. Previously, it had been classified as a larval tetrapod, unarmored placoderm, an agnathan, an early stem hagfish, and a chimeraThe most recent suggestion is that it is a stem chondrichthyan.”

Palaespondylus diagram from Joss and Johanson 2007, colorized here.

Figure 2. Palaespondylus diagram from Joss and Johanson 2007, colorized here with tetrapod homologs. The authors considered Paleospondylus a larval lungfish. Late Johanson et al. 2017 no longer supported this hypothesis of interrelationships. The lungfish, Dipterus, occurs in the same fossil beds.

From Wikipedia continued,
“Most recently, Palaeospondylus has been identified as a stem-group hagfish (Myxinoidea). However, one character questioning this assignment is the presence of three semicircular canals in the otic region of the cartilaginous skull, a feature of jawed vertebrates.”

Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

Figure 3. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

From Wikipedia continued,
“According to Johnson et al. 2017, “Previously, Palaeospondylus has been assigned to almost every major jawless and jawed vertebrate group and identified as both larval and adult. Most recently, Palaeospondylus has been identified as a stem-group hagfish (Myxinoidea). However, one character questioning this assignment is the presence of three semicircular canals in the otic region of the cartilaginous skull, a feature of jawed vertebrates.”

“Additionally, new tomographic data reveal that the following characters of crown-group gnathostomes (chondrichthyans + osteichthyans) are present in Palaeospondylus: a longer telencephalic region of the braincase, separation of otic and occipital regions by the otico-occipital fissure, and vertebral centra. As well, a precerebral fontanelle and postorbital articulation of the palatoquadrate are characteristic of certain chondrichthyans.”

Johnson et al. 2017 conclude, “the absence/non-preservation of teeth, scales and fins continues to be problematic in determination of Palaeospondylus as a jawed vertebrate. Also problematic with regards to a chondrichthyan association is the composition of the Palaeospondylus cartilaginous skeleton that includes hypertrophied chondrocyte lacunae surrounded by mineralized matrix, previously interpreted as representing an early stage in endochondral bone development, a type of bone found in bony fishes (Osteichthyes).”

Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Figure 4. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

When Palaeospondylus was added to the LRT,
it nested with the torpedo ray while retaining many traits (like a precerebral fontanelle) found in hammerhead sharks,  Palaeospondylus lived in the Middle Devonian, so transitional and primitive precursors that look like a ray with a shark tail are to be expected. Lack of fusion in the skull elements, the overall small size and the appearance of several specimens in a small area suggesting a nursery, combine to indicates a juvenile status.

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

Figure 5. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

In the most recent paper on Palaeospondylus
(Johnson et al. 2017) the following taxa were not found in the text, but at times appear in the citations: 1) shark; 2) ray; 3) torpedo. The authors reported, “The presence of
centra within the synarcual of Palaeospondylus is reminiscent of the synarcual in batoid chondrichthyans.” They did not follow up on that clue. Contra tradition, in the LRT members of the traditional batoid clade are split apart and distributed among other chondrichthyans and basal gnathostomes.

In their conclusion Johnson et al. 2017 reported,
“Palaeospondylus gunni has been a perplexing vertebrate fossil since Traquair first described it in 1890; here X-ray tomography provides new data and morphological characters demonstrating that Palaeospondylus is a jawed vertebrate. Characters that associate Palaeospondylus with chondrichthyans are a precerebral fontanelle, foramina for lateral dorsal aorta in the chondrocranium, and the articulation of the palatoquadrate to the ventral postorbital process. Palaeospondylus also lacks bone and instead manifests an entirely mineralized cartilage in the endoskeleton.”

Taxon exclusion is the number one problem affecting paleontology today
and for several prior decades. The LRT minimizes taxon exclusion by testing a wide gamut of extant and extinct taxa in a trait-based phylogenetic analysis. If only prior workers had included hammerheads and torpedos in their own phylogenetic analysis, Paleospondylus would not have been the enigma it remained until today.


References
Hirasawa T, Oisi Y and Kuratani S 2016. Palaeospondylus as a primitive hagfish. Zoological Letters. 2 (1): 20.
Joss J and Johanson Z 2007. Is Palaeospondylus gunni a fossil larval lungfish? Insights from Neoceratodus forsteri development. J Exp Zool B Mol Dev Evol. 2007 Mar 15;308(2):163-71.  https://pubmed.ncbi.nlm.nih.gov/17068776/
Johanson Z et al. 5 co-authors 2017.
Questioning hagfish affinities of the enigmatic Devonian vertebrate Palaeospondylus. Royal Society Open Science. 4 (7): 170214.
Thomson KS 2004. A Palaeontological Puzzle Solved?. American Scientist. 92 (3): 209–211.
Traquair RH 1890. On the fossil fishes at Achanarras Quarry, Caithness. Ann Mag
Nat Hist 6th Ser. 1890;6:479–86.

wiki/Palaeospondylus

Shocking news! The torpedo is a hammerhead!

This one came as a surprise
as I scored Tetronarce (= the New Zealand torpedo, an electric ray, Fig. 1), I thought:

  1. this taxon is breaking some rules, and
  2. I’ve seen that bizarre nasal before… but where?
Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m). The long red elements are tabular homologs, separated from the rest of the skull.

Tetronarce fairchildi 
(originally Torpedo fairchildi Hutton 1872, 1m) is the extant New Zealand torpedo, an electric ‘ray’ on the outside. Here it nests with Sphyrna, the hammerhead shark, based on its skeleton. So this ‘ray’ is convergent with other rays. Note the broad nasals with open medial architecture, underslung jaws with tiny, single-cusp teeth and shark-like tail. Here the eyeball stalks are preserved, distinct from most other tetrapods tested in the LRT, probably due to careful dissection to get at its cartilaginous skeleton. Two dorsal fins are preserved.

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

Skates,
like the guitarfish, Rhinobatos, and the sawfish, Pristis, have an elongate narrow rostrum and nasal. Angel sharks and eagle rays have other distinguishing traits that nest them with each other and not with the aforementioned. So do manta rays. When more rays and skates are added to the LRT that may change. Or not.

Every possibility must always be left open,
as Torpedo gently, but firmly reminds us. Do not be tempted into “Pulling a Larry Martin” here. A short list of traits don’t make a taxon. Only a nesting in a wide gamut phylogenetic analysis can do that.

Yes, outward appearances are very different.
But when you look at the skeletons and test them in phylogenetic analysis no other taxon shares as many traits with hammerheads as torpedoes. Evolutiion leaves clues. It’s up to us to find them. You won’t find a similar laterally extended nasal with a perforated medial architecture in any other tested sharks or rays. Though many skeletal traits are indeed different, taken as a suite of characters no other tested taxon comes closer.

Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Sphyrna tudes 
(orignally Zygaena tudes Valenciennes 1822; 1.3m in length) is the extant smalleye hammerhead shark. It prefers muddy habitats with poor visibility. Sphyrna has a tendency to inhabit coastal waters along the intertidal zone rather than the open ocean, as their prey item, invertebrates, fish, rays, small crustaceans and other benthic organisms hide in the sands and sediment along these zones. Gestation is 10 months. Females produce 19 pups each year. The eyes and nares are further separated by the lateral expansion of nasals, prefrontals and postfrontals creating the cephalofoil. Compare to Torpedo (above).

The largely overlooked value of the LRT 
comes from testing together taxa that have never been tested together before with a generic character list not designed specifically for sharks and rays, other fish, birds and mammals. Convergence runs rampant in the Vertebrata. Scientists need a wide gamut cladogram that minimizes taxon exclusion and character selection bias.

By the present evidence
the former clade Batoidea has now been divided into quarters. This appears to be a novel hypothesis of interrelationships. If there is a prior publication, let me know so I can promote it.


References
Hutton FW 1872. Catalogue with diagnoses of the species. Ed. Hutton, FW and Hector J (eds), Fishes of New Zealand, pp. 1-88 pls 1-12, Colonial Museum and Geological Survey Department, Wellington.

wiki/Electric_ray
wiki/Sphyrna
wiki/Torpedo_fairchildi

The traditional ‘placoderm’, Jagorina, enters the LRT with Manta.

Ray-like fish can be confusing
especially if more of the exterior than the interior is preserved (Figs. 2-4). Scoring for the large reptile tree (LRT, 1786+ taxa is based largely on bones, but also on proportions and shapes in detail and overall.

To make matters worse,
ray-like fish like to wrap and fuse their pectoral fins around their face. I mean, who else does that? This one trait is convergent across three clades. Traditional workers suffering from taxon deletion keep mantas, skates and rays in one clade, the invalidated Batoidea, which we looked at earlier here and here.

Jagorina pandora
(Fig. 1) is known from skeletal casts (hollow shapes in stone).

Figure 1. Jagorina in two views from Carroll 1988 and here colored with tetrapod homologs.

Figure 1. Jagorina in two views from Carroll 1988 and here colored with tetrapod homologs. Note the separation of the tan postparietals and red tabulars from the rest of the skull as in Manta (figure 6).

Jagorina pandora
(Jaekel 1921; 6cm snout tip to synacrural length; Mb.f 510.2; Fig. 1; Late Devonian) was originally considered a type of placoderm different from the rest, but here nests basal to the manta ray, Manta (Fig. 6). The purported dorsal nostrils are instead left and right fontanelles that merge in Manta. The toothless mouth parts were largely transverse and faced forward, as in Manta. The tooth-bearing elements: premaxilla, maxilla and dentary were not present. The post parietal and tabulars were detached from the parietal as in Manta. The purported operculum is the supratemporal. Gill openings could be ventral.

By contrast,
a traditionally related taxon, Gemuendina (Figs. 2–4), is known from exquisitely preserved dermal materials with very little of the skeleton visible beneath. Perhaps someday µCT scans will reveal the interior architecture. (PhD candidates, are you listening?)

Figure 1. Based on the scale bar, this a surprisingly small specimen, full scale on a typical computer monitor.

Figure 2. Based on the scale bar, this a surprisingly small specimen, shown full scale on a typical computer monitor of 72 dpi. Other specimens are 30cm to 1 m in length.

Gemuendina stuertzi 
(Traquair 1903; 30cm to 1m; Early Devonian; Figs. 2–4) was originally considered a placoderm close to Jagorina (Fig. 1). Here both are related to Manta (Figs. 5–6). The famous and often copied diagram above only loosely matches the in situ specimen (Figs. 3–4). The purported dorsal eyes are not dorsal, but tiny and lateral. The purported dorsal nostrils are instead left and right fontanelles that merge in Manta. The purported jutting mouth oriented upward now appears to be a pair of curled under cephalic fins, as in Manta. A mosaic of tessellated scales covers the body, obscuring the skeleton.

Figure 2. Gemuendina in situ. So much skin and ornament cover the bone, this taxon has been withdrawn from the LRT. Apparently the skull and pectoral girdle have separated from the pectoral fins during taphonomy.

Figure 3. Gemuendina in situ. So much skin and ornament cover the bone, this taxon has been withdrawn from the LRT. Apparently the skull and pectoral girdle have separated from the pectoral fins during taphonomy.

Gemuendina is an excellent specimen,
unfortunately obscuring too much of the interior architecture upon which the LRT is built.

Figure 4. Gemuendina skull in situ. So much skin makes this taxon too confusing to score, but note the apparent cephalic fins previously interpreted as a jutting mouth (as in figure 4).

Figure 4. Gemuendina skull in situ. So much skin makes this taxon too confusing to score, but note the apparent cephalic fins previously interpreted as a jutting mouth (as in figure 4).

In the above photo
(Fig. 4) I could not find the large eyes and jutting mouth illustrated by Gross 1963 (Fig. 2). But I could find soft remains of cephalic fins and tiny lateral eyes, as in Manta (Figs. 5, 6).

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 5.  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.

These taxa
(Fig. 5) form a clade of early gnathostomes lacking marginal teeth as adults. Since more primitive sturgeons have marginal teeth as hatchlings, data is needed on the embryos and hatchlings of whale sharks and mantas to see if they have marginal teeth that are ultimately lost. (PhD candidates, are you listening?)

Both extant taxa (whale sharks + manta rays)
have carpets of palatal teeth that look like patches of sharp-to-blunt scales.

Figure 6. Three views of the skeleton of Manta, colors added. Note the terminal mouth, distinct from other rays, skates and guitarfish. The cephalic fins are continuous from the large face-wrappiing pectoral fins.

Figure 6. Three views of the skeleton of Manta, colors added. Note the terminal mouth, distinct from other rays, skates and guitarfish. The cephalic fins are continuous from the large face-wrappiing pectoral fins.

This clade of marginally toothless gnathostomes
all feed on free-swimming open-water plankton, rather than the benthic (buried) prey other rays and skates prefer with their ventral mouths full of pavement-like teeth. They filter vast quantities of sea water in a large gill chamber (Fig. 7).

Figure 3. The gill chamber and digestive track of Manta shown in ventral view.

Figure 7. The gill chamber and digestive track of Manta shown in ventral view.

My earlier attempts at understanding
Gemuendina were hampered by not knowing the skin was so thick it obscured the skeleton beneath. Hopefully that mistake is repaired now. If not, further corrections will be made. The addition of Jargorina to the LRT and the deletion of Gemuendina from the LRT brings a more complete understanding of this clade and its ray-like, filter-feeding members.


References
Gross W 1963. Gemuendina stuertzi Traquair. Notizblatt des Hessischen Landesanstalt für Bodenforschung 91, 36–73.
Jaekel O 1921. Die Stellung der Pala¨ontologie zu einigen Problemen der Biologie und Phylogenie. Pal Zeit 3:213–239.
Traquair RH 1896. The extinct vertebrate animals of the Moray Firth area. Pp. 235–285 in Harvie-Brown J.A and Buckley TE (eds.): A Vertebrate Fauna of the Moray Firth Basin, Vol. II. Harvie Brown and Buckley, Edinburgh.

wiki/Turinia
wiki/Manta
wiki/Jagorina
wiki/Gemuendina

 

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.


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