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).”
“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”.
Distinct from Burrow et al. 2016
- Sharks and ratfish are not derived from spiny sharks, but are derived from the most primitive fish with simple transverse jaws, like Rhincodon.
- 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).
- 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).
- 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.
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.
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.
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.
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.
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.
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Newman M and Davidson B 2010. Early Devonian fish from the Midland Valley of Scotland. National Palaentological Congress London 14–15.
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