The phylogenetic comings and goings of lobe fins

The distribution of lobe fins, spiny fins and ray fins 
in the large reptile tree (LRT, 1756+ taxa, subset Fig. 1) indicate that each type of fin came and went and sometimes came back again.

So, contra traditional paleontology,
each type of fin does not represent a monophyletic clade. That would be “Pulling a Larry Martin” by setting up clades based on just a few characters.

The LRT provides a more holistic approach,
looking at 238 character traits from nose to toes and letting the software decide without tradition or bias. The LRT documents the multiple evolution of ray fins by convergence.

Figure 1. Subset of the LRT focusing on basal vertebrates and highlighting ray fins, spiny sharks and lobe fins. Catfish retain spines on their ray-like pectoral fins.

Figure 1. Subset of the LRT focusing on basal vertebrates and highlighting ray fins, spiny sharks and lobe fins. Catfish retain spines on their ray-like pectoral fins.

The basalmost lobefin in the LRT has gone unrecognized until now,
perhaps because Ticinolepis longaeva (Fig. 2) has such a little lobe on its pectoral fin and it is only known from Middle Triassic fossils. Ticinolepis longaeva nests at the base of all lobefins in the LRT (subset Fig. 1) so it would have had a Silurian genesis.

Figure 2. Ticinolepis longaeva in situ and reconstructed. Note the pectoral fin has a small lobe and this taxon nests at the base of all lobefins in the LRT.

Figure 2. Ticinolepis longaeva in situ and reconstructed. Note the pectoral fin has a small lobe and this taxon nests at the base of all lobefins in the LRT.

The resemblance of Ticinolepis longaeva to the next most basal lobefin,
Miguashaia (Middle Devonian; Fig. 3) is also instructive. (As a side note, Ticinolepis crassidens nests with Perleidus, not with Ticinolepis longaeva in the LRT, contra López-Arbarello and Sferco 2018).

Figure 2. The lobefin Miguashaia. Compare to the spiny shark, Diplacanthus, figure 1.

Figure 3. The lobefin Miguashaia. Compare to the spiny shark, Diplacanthus, figure 1.

Ticinolepis longaeva
(López-Arbarello and Sferco 2018; 12cm; Middle Triassic; MCSN 8072) nests at the base of the lobefin fishes. Note the tiny lobe in the middle of the ray fin. Compare that pectoral fin to the one in figure 3.

Miguashaia bureaui
(Schultze 1973, Cloutier 1996; Middle Devonian; 45cm) was considered the sister group (outgroup) of the Actinista (coelocanths). Notably Miguashaia reverses to a heterocercal tail. That’s why it looks a little odd. The dentary is short and the teeth are small.

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

Figure 4. Sturgeon swimming in a test tank from Wilga and Lauder 1999.

Final notes to be covered in more detail later:
Basal pectoral fins are rather inflexible and extend horizontally (Fig. 4). Ratfish hold their pectoral fins vertically, against the torso. Iniopterygians raise the pectoral fin to the dorsal margin. Moray eels lose their fins. So there is more variety here yet to explore.


References
López-Arbarello A and Sferco E 2018. Neopterygian phylogeny: the merger assay. Royal Society open sci. 5: 172337. http://dx.doi.org/10.1098/rsos.172337
Schultze H-P 1973.
 Crossopterygier mi heterozerker Schwanzfloss aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-Resten aus dem Middledevon Spaniens und aus dem Karbon der USA. Palaeontograhica A 143:188–208.

the large reptile tree

wiki/Miguashaia

A solution to the Acanthodes problem

The namesake for the clade of spiny sharks,
Acanthodes (Figs. 1, 2), is often touted as the last and best preserved member of the Acanthodii. The braincase and hyomandibular arch are well preserved in 3D, but the cheek and rostral bones are entirely absent. Here (Fig. 1) those elements are restored based on phylogenetic bracketing.

Figure 1. Acanthodes skull with elements restored.

Figure 1. Acanthodes skull with elements restored. The fragility of those elements is why we don’t have them.

Acanthodes bronni (Anonymous 1880; Early Permian 290 mya; 20cm) is the latest occurring acanthodian, the largest and has the best ossified braincase. Davis et al. mislabeled the hyomandibular as a giant quadrate and the preopercular as the mislabeled hyomandibular. Acanthodes is toothless and presumed to be a filter feeder. No extra spines are present. Other species can reach 41cm.

Figure 2. Acanthodes in situ.

Figure 2. Acanthodes in situ.

Reports that acanthodians are the last common ancestors
of sharks and bony fish (Friedman and Brazeau 2010, Davis, Finarelli and Coates 2012) are not supported by the LRT. However, acanthodians are basal to a wide variety of stem lobefin bony fish and placoderms in the LRT.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

Figure 2. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

According to Davis et al. 2012:
“Acanthodes bronni remains the only example preserved in substantial detail, central to which is an ostensibly osteichthyan braincase.”

That’s because it nests with bony fish in the LRT (subset Fig. x).

Figure x. Newly revised fish subset of the LRT

Figure x. Newly revised fish subset of the LRT

Davis et al. continue:
“These data contribute to a new reconstruction that, unexpectedly, resembles early chondrichthyan crania. Principal coordinates analysis of a character–taxon matrix including these new data confirms this impression.”

Every time principal component analysis is used instead of phylogenetic analysis, things go awry. For example, Bennett 19xx is infamous for giving gender identities to large and small Pteranodon specimens, not realizing that phylogenetic analysis nests small taxa with Germanodactylus outgroups and large taxa as highly derived.

Davis et al continue:
“However, phylogenetic analysis places Acanthodes on the osteichthyan stem, as part of a well-resolved tree that also recovers acanthodians as stem chondrichthyans and stem gnathostomes.”

Unfortunately, Davis et al. do not employ a longer list of taxa to understand the tree topology the LRT recovers. Here acanthodians are recovered as stem tetrapods. The basal split between Amia and spiny sharks is missing from the Davis et al. 2012 cladogram.


References
Anonymous 1880. Royal Physical Society of Edinburgh (1880). “Proceedings of the Royal Physical Society of Edinburgh”. V: 115.
Davis SP, Finarelli JA and Coates MI 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486:247–250.
Friedman M and Brazeau 2010. A reappraisal of the origin and basal radiation of the Osteichthyes. Journal of Vertebrate Paleontology 30(1):36–56.

wiki/Acanthodii
wiki/Acanthodes

 

An unexpected resolution to the ‘spiny shark’ problem

Short summary for those who are skimming:
Rather than being odd sideshow characters, acanthodians (spiny-sharks; Fig. 1) were key players in the evolution of stem tetrapods, placoderms and several clades of ray-fin fish within the stem tetrapod lineage as today recovered by the The Large Reptile Tree (LRT, 1683+ taxa, subset Fig. 2). Acanthodians have nothing to do with sharks, ratfish or most other bony fish (e.g. sea horses, tuna, flounders, etc. in the bowfin clade).

The problem:
Earlier here, here, here and here the LRT nested a few acanthodians in the stem tetrapod branch. Back then the few tested acanthodians nested far from sharks with similar spines on their dorsal fins only. These include Cladoselache, Hybodus, and Gregorius (Fig. 1). Back then the few tested acanthodians also nested far from other fish with spines on their pectoral fins, but not on the dorsal fins, like Doliodus and the catfish, Clarias.

Were these examples of spine convergence?
Or homology? Or are we looking for a third answer? As usual, in order to get to a phylogenetic solution that minimizes taxon exclusion, we add taxa to the LRT (Figs. 1, 2).

Asterisk*
Some readers may remember that the extant Amazonian bronze featherback, Notopterus, (Fig. 1) earlier nested with the few tested acanthodians, despite having ray fins everywhere, but spines for pelvic fins. So that pattern (Fig. 1) has been hinted at. Today an added taxon, Ptomacanthus (Fig. 1), is a closer transitional spiny shark taxon to Notopterus. Today additional spiny sharks likewise nest basal to ray-fin fish and others.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale.

Figure 1. Click to enlarge. Acanthodians and their spiny and non-spiny relatives in the LRT (subset Fig. 2), not to scale. Earlier I provisionally named ‘Proacanthodes’ ‘Proamia’ when it nested between Gregorius and Amia. Today that mistake is corrected.

Backstory:
According to Wikipedia“Acanthodii or acanthodians (sometimes called spiny sharks) is an extinct paraphyletic class of teleostome fish, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They represent several independent phylogenetic branches of fishes leading to the still extant Chondrichthyes.”

Based on the earlier nesting of a few acanthodians deep in the bony fish clade, phylogenetic bracketing indicates that the apparent lack of a bony skeleton in acanthodians is likely based on a skeletal reversal or decay based on a slow-moving lifestyle and perhaps a deep-sea niche.

Wikipedia continues:
“In a study of early jawed vertebrate relationships, Davis et al. (2012) found acanthodians to be split among the two major clades Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish). The well-known acanthodian Acanthodes was placed within Osteichthyes, despite the presence of many chondrichthyan characteristics in its braincase.

In the LRT Acanthodes is also a basal member of the bony fish (Osteichthys).

“However, a newly described Silurian placodermEntelognathus, which has jaw anatomy shared with bony fish and tetrapods, has led to revisions of this phylogeny: acanthodians were then considered to be a paraphyletic assemblage leading to cartilaginous fish, while bony fish evolved from placoderm ancestors.”

The LRT does not support those hypotheses of interrelations.

“Burrow et al. 2016 provides vindication by finding chondrichthyans to be nested among Acanthodii, most closely related to Doliodus and Tamiobatis. A 2017 study of Doliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a transitional fossil and further reinforcing this idea.”

The LRT nests Doliodus with spiny sharks and close to the base of stem bony fish.

“Many palaeontologists originally considered the acanthodians close to the ancestors of the bony fishes. Although their interior skeletons were made of cartilage, a bonelike material had developed in the skins of these fishes, in the form of closely fitting scales (see above). Some scales were greatly enlarged and formed a bony covering on top of the head and over the lower shoulder girdle. Others developed a bony flap over the gill openings analogous to the operculum in later bony fishes. However, most of these characteristics are considered homologous characteristics derived from common placoderm ancestors, and present also in basal cartilaginous fish.”

The LRT nests acanthodians as basal bony fish. Highly derived placoderms are likewise in the stem tetrapod clade of bony fish, close to catfish. Both are derived from spiny sharks (Figs. 1, 2). When workers ‘consider characteristics’ they are relying on a few traits, which may converge or reverse. So that puts us on shaky ground. It is much better to let software decide placement in a tree topology, based on hundreds of traits and taxa to minimize the ever-present problem of pertinent taxon exclusion.

Figure 3. Distribution of spiny sharks and other fish with dorsal spines in the LRT.

Figure 2. Distribution of spiny sharks and other fish with dorsal spines in the LRT. Will adding taxa link catfish to traditional acanthodians? Add taxa to find out. Lunaspis and Dicksonosteus have a structurally different sort of pectoral fin, but a connection to acanthodians may someday be revealed.

Traditionally
fish workers thought of spiny sharks as enigmatic, odd and somewhat apart from the rest of fish evolution, leaving no living descendants with similar spine fins (see above).

By contrast,
In the LRT (subset Fig. 2), which tests a wider gamut of taxa, spiny sharks appear at the genesis of stem tetrapods (Fig. 1) and give rise to a wide variety of ray fin fish, placoderms and lobe fin fish. A wide-gamut cladogram is a powerful tool for resolving enigma taxa and clades. This problem was resolved today without guesswork—despite this clade leaving no living descendants with spine fins.

This is probably the best cautionary tale yet
for not “Pulling a Larry Martin” (i.e. depending on a few traits to define a clade). Always define a clade based on a wide-gamut cladogram that includes a last common ancestor and all of its descendants. Based on that methodology, spiny sharks are not extinct. Several extant fish are derived from spiny shark ancestors, and so are all tetrapods—all without spine fins.

This presentation appears to be
a novel hypothesis of interrelations. If not, please provide an earlier citation so I can promote it here.


References
Brazeau MD 2012. A revision of the anatomy of the Early Devonian jawed vertebrate Ptomacanthus anglicus Miles. Palaeontology. 55 (7227): 355–367.
Davis SP, Finarelli JA and Coates MI 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature. 486 (7402): 247–50.
Maisey JG et al. 2017. Pectoral morphology in Doliodus : bridging the ‘acanthodian’-chondrichthyan divide. American Museum Novitates 3875:1–15.
Miles R 1973. Articulated acanthodian fishes from the Old Red Sandstone of England, with a review of the structure and evolution of the acanthodian shoulder-girdle. Bulletin of the British Museum (Natural History), 24, 111–213.

wiki/Acanthodii

Revisiting the origin and living relatives of spiny sharks (Acanthodii)

In today’s somewhat lengthy post
there’s going to be a set-up (so you can see traditional thinking)
and a take-down (so you can see what happens when you add taxa).

According to Wikipedia:
Acanthodii or acanthodians (sometimes called spiny sharks) is an extinct paraphyletic class of teleostome fish, sharing features with both bony fish and cartilaginous fish. In form they resembled sharks, but their epidermis was covered with tiny rhomboid platelets like the scales of holosteans (gars, bowfins). They represent several independent phylogenetic branches of fishes leading to the still extant Chondrichthyes.”

“Although not sharks or cartilaginous fish, acanthodians did, in fact, have a cartilaginous skeleton, but their fins had a wide, bony base and were reinforced on their anterior margin with a dentine spine.”

“The earliest unequivocal acanthodian fossils date from the beginning of the Silurian Period, some 50 million years before the first sharks appeared. Spiny sharks died out in Permian times (250 Million years ago).”

Figure 1. Cladogram from Burrow et al. 2016 (colors added here) showing the origin of Acanthodii from Placodermi using only Silurian and Devonian taxa. Compare to figure 3.

Figure 1. Cladogram from Burrow et al. 2016 (colors and labels added here) showing the origin of Acanthodii from Placodermi using only Silurian and Devonian taxa. Compare to figure 3, which includes extant taxa.

More from Wikipedia:
“Davis et al. (2012) found acanthodians to be split among the two major clades Osteichthyes (bony fish) and Chondrichthyes (cartilaginous fish).”

“Burrow et al. 2016 (Fig. 1 above) provides vindication by finding chondrichthyans (sharks + ratfish) to be nested among Acanthodii, most closely related to Doliodus (Fig. 5) and Tamiobatis (Paleozoid shark based on multi cusp teeth). A 2017 study of Doliodus morphology points out that it appears to display a mosaic of shark and acanthodian features, making it a transitional fossil and further reinforcing this idea”. 

By contrast,
the LRT found Doliodus (Fig. 5) nested with xenacanthid ‘sharks’ basal to bony fish, far from spiny sharks.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

Figure x. Updated subset of the LRT, focusing on basal vertebrates = fish.

After adding more taxa, like the spiny shark,
Climatius (Fig. 3, ), and a long list of extant taxa in the large reptile tree (LRT, subset Fig. 2) the tree topology in figure 1 changes greatly.

Distinct from Burrow et al. 2016

  1. Sharks and ratfish are not derived from spiny sharks, but are derived from the most primitive fish with simple transverse jaws, like Rhincodon.
  2. Placoderms and pre-lobefin fish (like Cheirolepis) are not basal to spiny sharks, but are related through a last common ancestor in Bonnerichthys (Figs. 3, 4).
  3. Spiny sharks arise from Silurian sisters to extant taxa, like lizard fish (Trachinocephalus, and arowana (Osteoglossum Fig. 3) in the newly recovered clade of short-face fish (clade: Breviops) distinct from fish with the orbit set further back on the skull (at least initially, the long-face fish (clade: Longiops) that starts with the bowfin (Amia).
  4. Spiny sharks give rise to Triassic Perleidus and extant featherbacks (Notopterus, Fig. 3), both of which have traditional ray-fin fins, though Notopterus pelvic fins remain tiny spines.
Figure 5. Acanthodians, their ancestors and sisters.

Figure 3. Acanthodians, their ancestors and sometimes extant sisters. Presently tested spiny sharks are all quite tiny as adults. Larger ones are known.

Placoderms are not extinct
They exist today as catfish. Spiny sharks are not extinct. They exist today as anchovies (Engraulis) and featherbacks (Notopterus, Figs. 3, 4) in the LRT, where taxon exclusion recovers novel hypotheses of interrelationships. Spiny shark sisters don’t have spines for fins. Using a single trait, even one like ‘spines for fins’, would be “Pulling a Larry Martin.” IN order to be a spiny shark sister, a taxon just has to nest closer to spiny sharks than any other included taxon. In your own analyses, include more taxa and the transition from one to another will become more and more gradual and apparent.

Figure 4. Acanthodian skulls, plus those of ancestors and related taxa.

Figure 4. Acanthodian skulls, plus those of ancestors and related taxa. Notopterus is a living featherback. Engraulis is a living anchovy.

Acanthodes bronni (Anonymous 1880; Early Permian 290 mya; 20cm; Fig. 4) is the latest occurring acanthodian, the largest and has the most ossified braincase. Davis et al. mislabeled the hyomandibular as a giant quadrate and the preopercular as the mislabeled hyomandibular (Fig. 4). Acanthodes is toothless and presumed to have been a filter feeder. No extra spines or fins are present. Other species can reach 41cm.

Reports that acanthodians are the last common ancestors
of sharks and bony fish (e.g. Friedman and Brazeau 2010, Davis, Finarelli and Coates 2012) are not supported by the LRT.

Figure 1. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Figure 5. Doliodus skull and pectoral region with lateral reconstruction at right. Note the narrow pectoral region relative to the wide spread occiput. Apparently this fish had a narrower body than head.

Doliodus (Fig. 5) has similar spiny fins,
but nests elsewhere in the LRT, with Xenacanthus. Catfish (e.g. Clarias) often have spines anterior to their pectoral fins, but are not related to spiny sharks. the giant Cretaceous predator, Xiphactinus, bundles fin rays into a spine, but is not related to spiny sharks. Yet another Cretaceous giant, Bonnericthys, (Figs. 3, 4) likewise bundles fin rays into a spine, and is basal to spiny sharks.,

Remember this as you finish reading:
Presently some (not all) spiny sharks appear earlier  in the fossil record (early Silurian) than do many precursor taxa in the LRT, some of which wait to appear until the Late Carboniferous, Jurassic and Cretaeous. Others are only known as extant taxa. Loganellia, the tiny primitive whale shark sister, is also from the Early Silurian, 444 mya.  Guiyu, a basal lobefin (Fig. 6), and Psarolepis are from the Late Silurian. So every taxon in the LRT preceding Guiyu and Psarolepis will someday be found somewhere in Silurian strata.

Figure 2. Guiyu in situ, DGS colors added here and used to create the flatter, wider reconstruction with paddles preserved.

Figure 6. Guiyu in situ, DGS colors added here and used to create the flatter, wider reconstruction with paddles preserved.

Fossilization is rare.
Finding a fossil-bearing locality of the right age is also rare. So it is wise not to put too much exclusionary weight on chronology (as in Fig. 1 above). Keep adding taxa and the puzzle of evolution will ultimately become a coherent picture. The gaps keep getting smaller as enigma taxa, like the spiny sharks, are better understood in a phylogenetic context, using extinct AND extant taxa.


References
Anonymous 1880. Royal Physical Society of Edinburgh. Proceedings of the Royal Physical Society of Edinburgh. V: 115.
Baron MG 2015. An investigation of the genus Mesacanthus (Chordata: Acanthodii) from the Orcadian Basin and Midland Valley areas of Northern and Central Scotland using traditional morphometrics. PeerJ. 3: e1331. doi:10.7717/peerj.1331
Brazeau M 2009. The braincase and jaws of a Devonian ‘acanthodian’ and modern
gnathostome origins. Nature 457, 305–308.
Burrow C, den Blaauwen J, Newman M and Davidson R 2016. The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotland. Palaeontologia Electronica 19 (1): Article number 19.1.10A.
Davis SP, Finarelli JA and Coates MI 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486:247–250.
Egerton P de MG 1860. Report of the British Association for Science for 1859.
Transactions of the Sections. 116.
Friedman M and Brazeau 2010. A reappraisal of the origin and basal radiation of the Osteichthyes. Journal of Vertebrate Paleontology 30(1):36–56.
Miller RF, Cloutier R and Turner S 2003. The oldest articulated chondrichthyan from the Early Devonian period. Nature 435:501–504.
Newman M and Davidson B 2010. Early Devonian fish from the Midland Valley of Scotland. National Palaentological Congress London 14–15.
Traquair RH 1888. Notes on the nomenclature of the Fishes of the Old Red Sandstone of Great Britain. Geol. Magazine (3)5:507–517.
Woodward AS 1892. On the Lower Devonian fish-fauna of Campbellton, New Brunswick.. Geol. Mag. 9, 1–6.

wiki/Acanthodii
wiki/Ischnacanthus
wiki/Mesacanthus
wiki/Acanthodes
wiki/Climatius

 

Hybodus enters the LRT as one of our direct ancestors

Updated January 27, 2020
with new interpretations of Hybodus and many dozen addition taxa helping to settle Hybodus in a node basal to the basal dichotomy that splits most bony fish (see cladogram below, Fig. 3).

It should come as no surprise
that Hybodus (Figs. 1, 2) was basal to the spiny sharks (Acanthodii), but the surprise is there are several intervening taxa between these nodes. Hybodus is also transitional from chimaeras to lobefins + humans in the LRT. So this is a ‘key players’.

Figure 1 (added 01/27/2020 with a current interpretation of skull bones on Hybodus, plus a reconstruction. Note the retention of external gill bars.

Figure 1 (added 01/27/2020 with a current interpretation of skull bones on Hybodus, plus a reconstruction. Note the retention of external gill bars.

Figure 1. Diagram of Hybodus in vivo and skeleton plus teeth.

Figure 2. Diagram of Hybodus in vivo and skeleton plus teeth.

Traditionally considered an odd sort of shark with dorsal spines,
Hybodus (Fig. 1) nests in the large reptile tree (LRT, 1583 (now 1643) taxa; Fig. 2) between sharks + chimaeroids and placoderms leading + two large clades of bony fish. Apparently this hypothesis of interrelationships has been overlooked until now, but it answers so many long-standing questions. Hybodus also greatly resembled the basal placoderm, Coccosteus (Fig. 1) another overlooked hypothesis of interrelationships. And catfish, too.

FIgure 3. Taxa highlighted in today's blog are highlighted here in this subset of the LRT.

FIgure 3. Taxa highlighted in today’s blog are highlighted here in this subset of the LRT.

Hybodus basanus (Agassiz 1837; H. reticulatus (Early Jurassic skull); 2m in length, Permian –Late Cretaceous) nests between sharks + chimaeroids and spiny sharks + bony fish. This relationship was overlooked until now. Note the spines on the dorsal fins. These are homologous with spines on spiny sharks like Diplacanthus (below). Spines are transitional betwen fleshy shark fins and transparent ray fins. The skull is also transitional between sharks and bony fish, despite the presence of large gill bars (yellow) lateral to the jaws.

Figure 3. Diplacanthus, a Mid-Devonian acanthodian with proportions similar to those of a young Hybodus, shorter with longer spines.

Figure 4. Diplacanthus, a Mid-Devonian acanthodian with proportions similar to those of a young Hybodus, shorter with longer spines.

Diplacanthus crassisimus (Miller 1841; Duff 1842; 13cm ; holotype NMS G.1891.92.333, widespread in the Middle Devoinian; Fig. 4). Skull details are vague, so it was not added to the LRT.

According to Davis et al. 2012:
“Acanthodians, an exclusively Palaeozoic group of fish, are central to a renewed debate on the origin of modern gnathostomes: jawed vertebrates comprising Chondrichthyes (sharks, rays and ratfish) and Osteichthyes (bony fishes and tetrapods)… These new data contribute to a new reconstruction that, unexpectedly, resembles early chondrichthyan crania. Principal coordinates analysis of a character–taxon matrix including these new data confirms this impression: Acanthodes is quantifiably closer to chondrichthyans than to osteichthyans. However, phylogenetic analysis places Acanthodes on the osteichthyan stem, as part of a well-resolved tree that also recovers acanthodians as stem chondrichthyans and stem gnathostomes.”

The LRT nests two acanthodians in the stem lobefin clade (Fig. 2).
Earlier we looked at the central nesting of acanthodians between basal taxa and bony fish. Hybodus further confirms this hypothesis of interrelationships now seeking confirmation or refutation from an independent study using a similar taxon list and a new character list.

With more taxa,
and more knowledge of the 137 taxa at hand, note that catfish no longer nest with placoderms, but transitional between placoderms and ray fin fish (Fig. 2).


References
Agassiz L 1837 in Agassiz L. 1833-1843. Recherches sur les Poissons fossiles-I, I, III, Neuchatel, pp 1420.
Burrow C, Blaauwen J, Newman M and Davidson R 2016. The diplacanthid fishes (Acanthodii, Diplacanthiformes, Diplacanthidae) from the Middle Devonian of Scotland. Palaeontologia Electronica 19.1.10A: 1-83.
Davis SP, Finarelli JA and Coates MI 2012. Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes. Nature 486:247–250.
Duff P 1842. Sketch of the Geology of Moray. Forsyth and Young, Elgin
Maisey JG 1983. Cranial anatomy of Hybodus basanus Egerton from the Lower Cretaceous of England. American Museum Novitates 2758:1–64.
Miller H 1841. The Old Red Sandstone. (first edition). Thomas Constable and Sons, Edinburgh.

wiki/Hybodus
wiki/Diplacanthus