In comparative anatomy,
macroevolutionary events can be diagrammed (Figs. 1, 2). Hopefully this can act as a visual shorthand that models real evolutionary events. It’s a way of checking that things look right after the software tells you whatever it recovers after ‘crunching the numbers’.
A diagram documenting the evolution of shark skulls
was posted online by ScienceDirect (Berkowitz and Shellis 2017; Fig. 1). This represents the traditional view and includes a hypothetical ancestor. That means the authors did not know the proximal outgroup for sharks.
in the large reptile tree (LRT, 1770+ taxa) shark ancestors are known back to Cambrian chordates. Sturgeons and paddlefish are shark ancestors (Fig. 2). If one proves to be not suitable, there are a dozen more. Shark descendants include bony fish.
(Fig. 1) chimaeras and sharks form a basal dichotomy from their hypothetical ancestor.
in the LRT (Figs. 2, 7) horn sharks (Heterodontus) and chimaeras (Chimaera) arise from dogfish sharks (Squalus) far from the base of sharks. The goblin shark (Mitsukurina) nests closer to the paddlefish (Polyodon).
Traditionally and in the LRT
dogfish (Squalus) gave rise to guitardfish (Rhinobatos) and skates. Traditionally skates are ray sisters, but In the LRT most rays arise from angel sharks (Squatina) and earlier sharks with giant pectoral fins (Cladoselache). The manta ray (Manta) has a separate ancestry close to whale sharks (Rhincodon, not shown in figure 2).
Pre sharks in the LRT have an operculum.
Post sharks in the LRT have an operculum. Transitional sharks do not have an operculum. Instead they have five to seven, tiny to enormous gill slits. As an added twist, chimaeras have four gill slits covered by an operculum… and goblin sharks lack an operculum, but nest with paddlefish with an giant operculum.
Pre sharks in the LRT have individual skull ‘bones’,
whether highly ossified or not. Post sharks in the LRT likewise have individual skull bones. Sharks do not. They tend to fuse several jaw bones together apart from the skull bones, which also tend to fuse together.
Lacking an operculum and individual skull bones
traditionally sets sharks apart from other fish with these traits. As readers know by now, using a few to a dozen traits to describe a clade is called ‘Pulling a Larry Martin‘. Instead, run a phlogenetic analysis with enough traits to lump and separate all included taxa. In that way convergence and reversals will sort themselves out without being hampered by traditional bias, outdated orthodoxy and cherry-picking a short list of preferred taxa with preferred traits (Fig. 1).
Gillis et al. 2011 tested genes
in a shark and a chimaera. They concluded, “the common ancestor of Elasmobranchs and Holocephalans had the gills of a shark, rather than the gills of a chimaera.”
That’s not what the LRT found
after testing traits and expanding the taxon list.
Vertebrate embryos (including humans)
have several gill pouches/clefts that either become gill slits (sharks) gill slits covered by an operculum (chimaeras and many bony fish) or throat elements (tetrapods). Since shark gill pouches are similar to those in higher vertebrates, their origin can be traced by to more primitive ancestors, like lampreys (before they evolved to become blood-suckers, Fig. 3).
The loss and later reacquisition of opercula
in sharks and bony fish respectively appears to be a novel hypothesis of traits and interrelations recovered by the LRT. If there was a prior paper on this subject, let me know so I can promote it here.
The fusion and reduced ossification of shark skull bones
has traditionally marked them as ‘primitive’, but recent studies indicate sharks arose from bony ancestors, agreeing with the LRT (Fig. 2).
(Fig. 4) nests basal to sharks along with paddlefish (Polyodon) and hatchling paddlefish (Fig. 5), which don’t have a long rostrum and look even more shark-like. Tanyrhinichys has a preoperculum (light yellow, Fig. 4), a trait traditionally lacking in sharks, but look again. In many sharks the traditional ‘palatoquadrate’ has a curved and robust lateral rim extending over the jaw joint. According to phylogenetic bracketing, that is the preoperculum now fused to the ‘palatoquadrate’ (light tan, actually the lacrimal using tetrapod homologs), which forms before the advent of the premaxilla and maxilla). Fusion of former jaw elements in sharks is a novel identification.
The long straight tail of the paddlefish hatchling
is similar to the long straight tail of the goblin shark (Mitsukurina, Figs. 2, 6), which is one more reason why the goblin shark nests at the base of sharks, along with its protrusible jaws. The similarity of paddlefish and goblin sharks has not gone unnoticed. Several authors have written about the similarity, but all have attributed the similarity to convergence.
resolves all sorts of phylogenetic issues.
Berkovitz B and Shellis P 2017. Chondrichthyes 1 Elasmobranchs in The Teeth of Non-Mammalian Vertebrates. online at ScienceDirect.com
Gillis JA, Rawlinson KA, Bell J, Lyon WS, Baker CVH and Shubin NH 2011. Holocephalan embryos provide evidence for gill arch appendage reduction and opercular evolution in cartilaginous fishes. Proceedings of the National Academy of Sciences USA, 2011, 108:1507-1512