Tag Archives: Acanthodii

The phylogenetic comings and goings of lobe fins

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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

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SVP abstracts 25: Acanthodians misunderstood

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Schnitz et al. 2020 attempt to bring
a new understanding to the spiny sharks, the acanthodians (Fig. 1, left column). In the large reptile tree (LRT, 1751+ taxa; subset Fig. 2) acanthodians are basal bony fish transitional taxa leading to several clades of other basal bony fish.

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.

From the abstract:
“Acanthodians are a poorly understood paraphyletic group of extinct fishes from the Paleozoic.”

They are better understood now after nesting them in the LRT, which minimizes taxon exclusion. I hate to keep repeating this, but all I add to do was add taxa.

“While they show comparatively little diversity in lifestyle and range of body shape, they play a prominent part in our understanding of vertebrate evolution as part of the chondrichthyan stem-group.”

This is false. Spiny sharks are osteichthyans in the LRT, not basal to sharks. Taxon exclusion has been a continuing problem with fish workers.

Figure 2. Subset of the LRT focusing on basal vertebrates (= fish) highlighting acanthodians (=spiny sharks).

Figure 2. Subset of the LRT focusing on basal vertebrates (= fish) highlighting acanthodians (=spiny sharks).

“Their evolutionary history, however, is poorly understood, largely due to the limited preservation of their mostly cartilaginous skeleton that results in a bias towards isolated remains such as fin spines and scales.”

This is false. The LRT provides a complete evolutionary history back to headless Cambrian chordates.

“Thus, considerable uncertainties remain in how the completeness of acanthodian fossils impact on the phylogenetic narrative of both chondrichthyans and other vertebrates.”

This is false. The LRT provides the certainties that come from minimizing taxon exclusion.

“Here, we address these issues by using a variation of the previously defined Skeletal Completeness Metric (SCM), an approach that calculates how complete the skeletons of individuals are compared to their theoretical complete skeleton, to quantify the quality of the acanthodian fossil record.”

This might be the next step after first recovering a valid phylogeny from the more complete acanthodian fossils.

“Acanthodians show a significantly lower completeness distribution than many tetrapods, including theropods, plesiosaurs, sauropodomorphs, ichthyosaurs, pelycosaurs and parareptiles, but a similarly low distribution to bats. Analysis of completeness distribution between acanthodian orders reveals significant differences, with the Acanthodiformes and Diplacanthiformes showing highest overall completeness. Our assessment of completeness reveals only weak spatial biases influencing the acanthodian fossil record while temporal biases are much higher.”

In other words, no phylogenetic conclusions and sort of a waste of time because no valid phylogeny was created. Don’t follow authorities, textbooks or invalid traditions. When you minimize taxon exclusion you’ll understand acanthodian phylogeny. THEN proceed with more detailed studies.

References
Schnitz L, Butler RJ, Coates MI and Sansom IJ 2020. Skeletal and soft tissue completeness of the acanthodian fossil record through time. SVP abstracts 2020.

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wiki/Mesacanthus
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A solution to the Acanthodes problem

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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.

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wiki/Acanthodes

 

An unexpected resolution to the ‘spiny shark’ problem

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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, 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 placoderm, Entelognathus, 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. 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 5. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels.

Figure 5. Subset of the LRT focusing on basal vertebrates. Note the clade Holocephali nests apart from Chondrenchelys and kin, including moray eels. See Figure 6 for a placoderm update.

Figure 6. Updated cladogram of the placoderms.

Figure 6. Updated cladogram of the placoderms.

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.

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

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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

 

The LRT vs. The Rise of Fishes (Long 1995)

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Figure 1. The Rise of Fishes 1995 book by fish expert John Long.

Figure 1. The Rise of Fishes 1995 book by fish expert John Long.

The majority of data sources
for fish here and in ReptileEvolution.com come from the excellent ink drawings of WK Gregory 1933 and the excellent photos of John Long 1995 in his book, ‘The Rise of Fishes’ (Fig. 1). Newer editions of the book are out there, but I don’t have them. Long’s own updates may erase some of the issues raised here. 

Long 1995 writes:
“The first osteichthyans are very poorly known from fossils, represented by a few scales and a mere fragments of bone. The oldest articulated remains, showing what their bodies and heads were like, are about 400 million years old.”

The LRT demonstrates
restricting phylogenetic fish taxa to just Silurian and Devonian fossils is unnecessary and restrictive. There are plenty of extant, yet still primitive, taxa one can plug into any fish phylogenetic analysis, like the large reptile tree (LRT, 1656+ taxa), for one. Without fossils the LRT can recreate the fish ‘tree of life’ starting with lancelets, then sturgeons, then sharks, and ending with sea horses, anglerfish and tetrapods.

Long includes sturgeons in the clade of bony fish (Osteichthyes)
despite his notes that sturgeon skeletons retain large amount of cartilage and much of the external armor is bone. In the LRT jawless armored Osteostraci are basal to sturgeons prior to the evolution of terminal jaws.

Long “pulls a Larry Martin”
when he states, “Osteichyan fishes are characterized by having a well-ossified internal bony skeleton” then backtracks by noting, “although the earliest fossil forms show the least degree of ossification of the vertebrae and internal bones.”

By contrast,
in the LRT only the last common ancestor of all extant bony fish (sans sturgeons and paddle bills) determines clade membership, despite the presence or absence to certain ‘key’ traits, like a bony internal skeleton.

Figure x. Subset of the LRT focusing on fish.

Figure x. Subset of the LRT focusing on fish.

The LRT confirms Long’s 1995 fish cladogram
with regard to nesting jawless fish (Agnatha) basal to jawed fish (Gnathostomata), naturally.

In Long 1995, Osteostraci
is the proximal outgroup to the Gnathostomata.

In the LRT, a member of the Osteostraci is basal to tube-mouth sturgeons in the LRT. Members of the Thelodonti are basal to Gnathostomata in the LRT.

In Long 1995, the basal dichotomy in Gnathostomata
splits Placodermi from Acanthodii. Those two are not related in the LRT.

In the LRT basal Gnathostomata splits taxa with transverse toothless jaws (Loganellia + Rhincodon + Manta) from taxa with U-shaped toothy jaws (Falcatus) in the LRT. No suprageneric taxa are employed (exception: Reptilia, Lepidosauromorpha, Archosauromorpha).

In Long 1995, Silurian Placodermi give rise to 
“protosharks”, then Cladoselache, then Holcephalomorpha (ratfish) and Neoselachii (true sharks)

In the LRT, Placodermi include and give rise to catfish, all derived from Cheirodus/Amphicentrum. In the LRT, the second great dichotomy splits ratfish + sharks apart from xenacanthid ‘sharks‘, hybodontid ‘sharks‘ and Pachycormus.

In Long 1995, Silurian Acanthodii give rise to
Lophosteiformes (known only from ornate epidermal scales) and Actinopterygii (ray-finned fish)

In the LRT, Acanthodii are not primitive, but arise from ray-fin fish like the Cretaceous, bony-finned Bonnerichthys and the extant Osteoglossum.

Cause for concern
This major time gap between known fossils and phylogenic first appearance is cause for concern on the one hand, and a call to action on the other. At present it does not make sense that placoderms and acanthodians (spiny sharks) are present in the Silurian while intervening and more primitive taxa in the LRT are no yet known from the Silurian… and yet, we’ve seen this before with Jurassic multituberculates preceding more basal members of the placental clades, Glires, Carnivora and Primates.

Phenomic phylogenetic analyses, like the LRT, deliver
the gradual accumulation of derived traits that support evolutionary theory. Missing Silurian taxa will have to be discovered whenever someone discovers them.

On the other hand,
Long 1995 makes no attempt to provide any generic last common ancestors with jaws for Acanthodii + Placoderm, nor does he spell out any gradual accumulations of derived traits at transitional points, not does he employ more than a token number of generic or specific taxa in his cladogram. Suprageneric taxa are often a problem in taxonomy, as noted earlier. They can be too easily cherry-picked.

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

Figure 2. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

So, are placoderms and acanthodians ‘bony fish’?
In the LRT: yes, despite having less than fully ossified internal skeletons. That could be a retained primitive trait or a reversal in both cases. Both are derived from taxa in which the external scales are robust, providing support to the body, allowing the skeleton to degenerate. As for that heterocercal tail in acanthodians (Fig. 2) and placoderms… based on the evidence, that is a primitive trait, not a reversal. Intervening taxa, like extant Trachinocephalus, appear to have independently evolved a diphycercal tail and bony skeleton, which their Silurian direct ancestors probably lacked.

Cherry picking traits,
even ‘important’ traits, is ‘Pulling a Larry Martin‘, something the wise professor taught us not to do. Find your clades using specific or generic taxa tested against 200+ traits to find that last common ancestor, no matter the ‘key’ traits that tickle your fancy.

References
Gregory WK 1933. Fish skulls. A study of the evolution of natural mechanisms. American Philosophical Society 23(2) 1–481.
Long JA 1995. The Rise of Fishes. Johns Hopkins University Press. Baltimore and London.

 

Another clade no longer extinct: Acanthodii (spiny sharks)

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The LRT has invalidated several traditional clades
like Parareptilia, Ornithodira and Cetacea. The LRT has also resurrected and supported a few long forgotten clades like Enaliosauria and Volitantia. The LRT also recovered extant members for traditionally extinct clades like Placodermi (catfish), Desmostylia (mysticete whales) and Pareiasauria (turtles).

Key to phylogenetic analysis
is the idea that it does not matter if certain clade members lack one trait or another, or have traits shared by taxa that are not clade members. It only matters that a clade is determined by a unique suite of hundreds of traits compared to all other tested clades. That makes it important to test as many taxa as possible to minimize the possibility that any pertinent taxa are excluded and no inappropriate taxa are included. Monophyletic clade members include all descendants of a last common ancestor.

Some of the earliest known fish fossils
belong to spiny sharks (e.g. Mesacanthus, Fig. 1; clade: Acanthodii) in the Early Devonian. Other disarticulated specimens attributed to spiny sharks (scales and spines) are found in Early Silurian strata.

Instead of fin rays or lobe fins,
spiny sharks have sharp dentine spines trailing unreinforced membranes. Like sharks, acanthodian skeletons were made of cartilage because they do not preserve well. Unlike sharks, the scales were made of bone-like material. Due to taxon exclusion and the reliance on just a few traits (= Pulling a Larry Martin) acanthodians have been traditionally difficult to nest in prior phylogenetic analyses.

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

Figure 1. Early Devoniann Mesacanthus in situ. This 3 cm fish is a typical acanthodian here traced using DGS methods and reconstructed. Distinct from other spiny sharks, this one lacks large cheek plates, as in the extant Notopterus (Fig. 3).

With the addition of the spiny shark Mesacanthus
(Early Devonian, Fig. 1) the large reptile tree (LRT, 1654+ taxa; subset Fig. 2) nests spiny sharks between anchovies, like the extant Engraulis, and palaeoniscids like Pteronisculus (Early Triassic).

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

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

Spiny sharks are also basal
to Perleidus (Triassic) and Notopterus (extant; Fig. 3) among tested taxa in the LRT.

Figure 3. The extant knife fish or featherback, Notopterus, is an extant descendant of spiny sharks in the LRT.

Figure 3. The extant knife fish or featherback, Notopterus, is an extant descendant of spiny sharks in the LRT. Note the spiny pectoral fins and similar skull morphology.

According to Wikipedia:
“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.”

That’s only true due to taxon exclusion,
according to the LRT (Fig. 2) where Doliodus (Fig. 4) is closer to Akmonistion and Xenacanthus.

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 4. 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.

Does a spiny shark have to have spiny fins?
No. Several taxa unrelated to spiny sharks, like catfish, have more or less spiny fins and sometimes these give rise to ray fins, as they do in Notopterus (Fig. 3) for the pectoral fins, but not the pelvic fins.

Taxa in the LRT are nested at nodes based on hundreds of traits
more closely shared with sister and cousin taxa than with more distantly related taxa in toto.

Key to scoring included taxa
is the creation of a reconstruction (Fig. 1) that moves disarticulated bones back to their in vivo positions. That simply must be done to understand the data.

Mesacanthus mitchelli (renamed with Traquair 1888; Early Devonian, 410 mya; 3cm) is a basal acanthodian transitional to Notopterus and Perleidus. So spiny sharks are not extinct. Distinct from other tested spiny sharks, Mesacanthus has open cheeks exposing the quadrate and hyomandibular. Soft tissue preserves membranes posterior to all the spines.

References
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
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.
Traquair RH 1888. Notes on the nomenclature of the Fishes of the Old Red Sandstone of Great Britain. Geol. Magazine (3)5:507–517.

https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/osteoglossiformes

 

Hybodus enters the LRT as one of our direct ancestors

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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