Eugnathostomata

John Merck

All contemporary analyses agree that the living gnathostomes: are monophyletic with respect to placoderms. This group is Eugnathostomata.

From the SUNY Orange Online Biology Library
Eugnathostome synapomorphies:


Noteworthy plesiomorphies:


Eugnathostome diversity:


Osteichthyes: Wedgetail triggerfish Rhinecanthus rectangulus (right); Chondrichthyes: Blue-spotted lagoon ray Taeniura lymna (left)

Two major living groups:

Chondrichthyes


Triakis semifasciata (Neoselachii) from Environmental Aquatic Service International

Hydrolagus colliei (Chimaeriformes) from Takako Uno

Living chondrichthyan diversity include:



Dorsal view of Xenacanthus sp. braincase
after Schultze, 1993.
  • Synapomorphies:

    Chondrichthyan diversity


    Stem chondrichthyans: The earliest unambiguous chondrichthyan remains are from the Early Silurian, but consist of isolated scales. We pick up relatively complete fossil chondrichthyans during the Devonian, however their first great radiation occurred during the Early Carboniferous. Alas, there is much disagreement about the phylogeny of basal chondrichthyans. Our review is based on Coates et al., 2017, who seem to have the last word. The cladogram at right provides a consensus of recent hypotheses. Euchondrichthyes (Pradel et al., 2011) is the last common ancestor of living chondrichthyans and all of its descendants. We consider the living groups first, then try to make sense of the stem taxa.

    Elasmobranchii:

    (Devonian - Quaternary) Chondrichthyans closer to living sharks than to living chimaeras. The cladogram (right) presents a general consensus of elasmobranch phylogeny.

    Synapomorphies:


    Elasmobranch diversity:


    Orthacanthus senckenbergianus from Jan Vanheuverzwijn-Valembois (Reconstruction)
    Xenacanthida: (Carboniferous (or Devonian?) - Triassic)

    Predators with elongate bodies. During the Carboniferous and Permian, they were among the top marine predators. After the end Permian extinction, stragglers held on in fresh water.

    Synapomorphies:

    Noteworthy plesiomorphy:



    Ctenacanthus from Oceans of Kansas
    Ctenanthidae: (Devonian - Triassic)

    Shark-like predators.

    Synapomorphies:



    Hybodus sp. model at American Museum of Natural History
    Hybodontiformes: (Carboniferous - Cretaceous) Hybodonts radiated during the late Paleozoic, along with other elasmobranchs, suffered in the end Permian extinction, but rebounded to become the primary elasmobranchs of the Triassic and Jurassic. From the late Jurassic, their diversity diminished. For most of the Cretaceous, they were restricted to fresh water. Coates 2013 argues that the Carboniferous form Tristychius (right) possessed an ecology similar to that of a living nurse-shark.

    Synapomorphies:



    White-tipped reef shark Triaenodon obesus
    Neoselachii: (Triassic - Quaternary) The last common ancestor of living elasmobranchs and all of its descendants.

    Synapomorphies:



    Shortfin Mako Shark Isurus oxyrinchus by Callaghan Fritz-Cope
    from Pelagic Shark Research Foundation
    Galeomorpha: (Jurassic - Quaternary) (Sometimes called "Galeomorphii.")Includes the sharks with five gill-slits (plesiomorphy.) The two neoselachian taxa adopted different approaches to being a shark. Galeomorpha became the scary sharks of popular culture, invading the top marine predator niches during the Middle Cretaceous. (Link to image of Cretaxyrhina attacking the mosasaur Clidastes) by Dan Varner.) Galeomorpha also includes the large suspension-feeding whale sharks and basking sharks.

    Synapomorphies:



    Spiny dogfish Squalus acanthias from Wikipedia
    Squalea: (Triassic - Quaternary) Includes the sharks with an evolutionary tendency to specialize in cold and/or bottom waters.

    Synapomorphies:

    Evolutionary tendencies:

    Diversity includes:



    Common guitarfish Rhinobatos rhinobatos from Wikipedia
    Hypnosqualea: (Jurassic - Quaternary) Includes specialized flattened bottom dwellers. We have encountered flattened bottom-feeders among Pteraspidomorphi and Rhenanida. During the Late Jurassic, a major chondrichthyan flat-fish radiation arose among the Hypnosqualea. They encompass a range of ecological strategies, but all exploit enlarged pectoral fins, highly mobile, hyostylic jaws, and modified breathing structures.


    Protospinax annectens from Wikipedia
    Sister taxon: Protospinax annectens (Jurassic), known from several specimens. Displays beginning of tendency toward enlarged pectoral fins and flattened body without specific synapomorphies of Hypnosqualea.


    Hypnosqualean schematic from Palaeos
    Synapomorphies:

    Diversity includes:

    Conflict! Alas, the "hypnosqualea hypothesis" is based on morphology. Molecular analyses find Batomorphii to be the sister taxon of all other neoselachians. Yikes.

    Euchondrocephali:


    Spotted ratfish, Hydrolagus colliei from Wikipedia
    Euchondrocephali: Total group including living Chimaeriformes and and all taxa closer to them than to Elasmobranchii (Devonian - Quaternary). Living members, the Chimaeriformes, are clearly distinct from Elasmobranchii, but as one goes back in time, the two groups become harder to distinguish.

    Synapomorphies are technically complex except for one:


    Euchondrocephali diversity:



    Denaea from Carroll, 1988.
    Symmoriida: (Devonian - Permian) Superficially shark-like. Synapomorphies:


    Cladoselache sp. from Britannica.com.
    Cladoselachida: (Devonian) Including Cladoselache, the earliest well-known chondrichthyan, and similar forms. Cladoselache known chondrichthyan from numerous well preserved skeletons. Roughly half meter adult length. Superficially shark like, anatomically suited as fast pursuit predator with tall tail, narrow trunk, and finlets to reduce drag of tail base. Stomach contents can include fish swallowed tail first, euconodonts, and invertebrates. One odd feature is the reduction of the pelvic fins and absence of anal fins. Absolutely none are known with claspers. Did these evolve later, or are all specimens female?



    Helicoprion beesonovi from Wikipedia.
    Eugeneodontida: (Carboniferous - Triassic) Shark-like predators with interesting teeth. Although many chondrichthyans retain their teeth in whorls rather than shedding them, eugeneodontids tend to have exaggerated symphyseal tooth whorls in the lower jaw that fit into corresponding whorls in the upper. Tapanila et al., 2013 have redescribed Helicoprion, long known from its tooth whorl, based on CT scans, revealing it to be a basal euchondrocephalian. Clears up a century of competing interpretations. (Link to image by Ray Troll.) Additionally:



    Debeerius ellefseni from Fossil Fishes of Bear Gulch
    Debeerius ellefseni: (Carboniferous) An exceptionally well-preserved fossil.

    In most respects similar to chimaeras, but lacking amphistylic jaw suspension. Instead, suspension is autodiastylic - The palatoquadrate articulates with neurocranium by ethmoid and basitrabecular articulations. Differs from amphistylic in that:

    Holocephali:

    Living Chimaeriformes and their closest relatives: Many develop dermal skull plates made of dentine. Generally speaking, holocephalians are slow moving and specialize on crushing hard-shelled organisms.

    Synapomorphies:

    A rogue' gallery of Holocephali:


    Assorted iniopterygiiformes by the outstanding Ray Troll from fossil.wikia.com
    Iniopterygiiformes: (Carboniferous) Proportionally similar to chimaeras except:


    Helodus simplex by Ghedoghedo from Wikipedia
    Cochliodontidae: (Devonian - Permian) Small holocephalians with holostylic jaw suspension. E.G. Helodus simplex (right). Distinguished by:


    Menaspis armata from Paleopedia
    Menaspidae: (Devonian - Permian) Small holocephalians with holostylic jaw suspension. Distinguished by:


    Belantsea montana from Wikipedia
    Petalodontiformes: (Carboniferous - Permian) Distinguished by:

    Rhinochimaera pacifica from Smithsonian Tropical Research Institute

    Noteworthy characteristics of Chimaeriformes - the living group:

    Holocephali: Chimaeriformes and their close fossil relatives. Many develop dermal skull plates made of dentine. Generally speaking, holocephalians are slow moving and specialize on crushing hard-shelled organisms.


    Early chondrichthyans:

    As we survey farther down the chondrichthyan tree, we find creatures that share chondrichthyan synapomorphies but increasingly share ancestral characters with other gnathostome groups. A sampling:


    Ozarcus mapesae from Pradel et al. 2014.
    Ozarcus mapesae (Carboniferous) A basal euchondrocephalian according to Coates et al., 2017. A small symmoriid Pradel et al. 2014 described from CT scans. Although the braincase and amphistylic jaw suspension are typically chondrichthyan, the branchial arches are not: Thus, Ozarcus is a chondrichthyan with osteichthyan-like branchial arches. (Link to image from Pradel et al. 2014 modified for PaleoStories.)


    Doliodus problematicus pectoral fin from Miller et al. 2003.
    Doliodus problematicus (Early Devonian) Roughly 1 m long, it shows classic chondrichthyan synapomorphies. Interesting because its pectoral fins have spines of dentin like those of the dorsal fins. We will meet this feature again.

    Pucapampella (Early Devonian) Known from two braincases, according to Maisey and Anderson, 2001, this animal clearly preserves a ventral cranial fissure, otherwise known only from osteichthyans.

    Gladbachus adentatus (Devonian) Little information in the literature, however in his 2013 podcast, Coates describes the results of his CT scans of Gladbachus, noting:

    A general pattern emerges in which, the farther down the chondrichthyan branch we push:



    Climatius from Wikimedia.com

    Acanthodii

    (Late Ordovician - Permian)

    A final gnathostome group demands attention: Acanthodii (aka "spiny sharks.") Small animals (10 - 30 cm) characterized by:

    Monophyly: Traditionally assumed. A potential synapomorphy: But this assumption invoked great stratigraphic incongruity. Some acanthodians seemed to exaggerate their typical features while others were similar to early Osteichthyes. E.G.:


    Climatius from Carroll, 1988
    Climatius: (Early Devonian)


    Acanthodes from Wikimedia.com
    Acanthodes: (Permian)

    Difficulty: Generally speaking, one would expect that in an evolving monophyletic group, the more derived taxa would occur later in time than the more ancestral ones. This is called stratigraphic congruence. Acanthodians display the opposite pattern. The most freakishly spiny ones are also early.


    Acanthodes neurocranium mandibular, and hyoid arches
    Adding color to the mystery was the absence of preserved internal skull elements in any acanthodian but Acanthodes, whose skull seemed similar to that of osteichthyans. At the beginning of this century, the professional consensus was that Acanthodii and Osteichthyes formed a larger clade - Teleostomi. In retrospect, a clear example of the "pull of the known."

    Acanthodian surprise: Brazeau, 2009 described a second acanthodian braincase - Ptomacanthus anglicus. This creature's neurocranium retains many plesiomorphies reminiscent of placoderms and basal chondrichthyans. Brazeau's phylogenetic analysis revealed for the first time that: Moreover, we saw the general picture that the ancestral eugnathostome had:

    Everyone was happy that the enigma had been resolved. But only until 2013.


    Entelognathus primordialis by John Long from Coolaustralia.org

    The Source of All Disquiet

    Then came Zhu et al. 2013, reporting on Entelognathus primordialis, a placoderm-like creature with extensive bony cranial and thoracic armor and:

    Now, with the addition of a heavily armored creature close to the ancestry of Eugnathostomata, the polarity was reversed. What if eugnathostomes arose from creatures with extensive armor? The lack of bony dermal plates in Chondrichthyes and Acanthodii might be a synapomorphy! Zhu et al.'s analysis recovered a phylogeny in which all acanthodians were stem chondrichthyans.

    Adding color to this pattern:


    So we now accept this pattern:

    Additional reading: