Neopterygii - Half of vertebrate diversity in one hour

John Merck

Link to Teleosteomorpha cladogram and phylogram cheat-sheets
Link to Acanthomorpha cladogram and phylogram cheat-sheets

"Say, what a lot of fish there are."
Theodor Seuss Geisel - 1960.


This remaining actinopterygians belong to Neopterygii and comprise roughly half of vertebrate diversity. Living neopterygians include:

This huge radiation seems to have resulted from signifcant morphological adaptations beginning with:

...and ending with:

Dipteronotus gibbous
The starting point: Dipteronotus, a stem neopterygian, gives an idea of the plesiomorphic condition for these animals: From this condition, remarkable transformations unfold. Some are autapomorphic or convergent on crown teleosts (E.G. pectoral fin mediated gliding by the basal neopterygian Potanichthys xingyiensis (Xu et al., 2012.)

Schematic of Cheirolepis, a non-neopterygian and Amia, a neopterygian
The mouth: In non-neopterygians, the geometry of jaw adductor muscles is similar to that in other eugnathostomes: they originate on the lateral surface of the palatoquadrate and insert on the jaw, and occupy an adductor chamber that is confined by the palatoquadrate medially and dermal skull roof bones laterally. Among neopterygians this chamber: This was accompanied by a reorientation of the lever system closing the jaw:

Whenever the mouth opens, the oral cavity expands and water flows in. Ancestrally osteichthyans did little to capitalize on this. With increasing ability to expand the oral cavity laterally, however, a cascade of adaptations to suction feeding occurred:

Parasemionotidae indet.
In the postcranium:

Lepisosteus sp.


(Triassic - Quaternary) Living Lepisosteidae, gars (Cretaceous - Quaternary) and their fossil relatives. Gars are specialized ambush predators. Their skulls are highly derived in having long, drawn-out jaws. Posteriorly, they are more primitive, with heavy ganoid scales and heterocercal tails (albeit outwardly almost symmetrical.)

Lepistosteid skulls are a fun combination of autapomorphic and plesiomorphic. On the plesiomorphic side: On the apomorphic side: In the post-cranium, we see: Morphologically, lepistosteids occupy the end of a long branch encompassing much unique evolutionary change. To break up that branch, we look at fossil ginglymodans.

Fossil Ginglymodi:

Obaichthys decoratus
Obaichthys decoratus (Middle Cretaceous - Santana Formation Brazil) In all respects a gar but for the presence of a mobile, toothed maxilla.

Isanichthys palustris from Cavin and Suteethorn, 2005
Semionotiformes (Middle Triassic - Middle Cretaceous) A very speciose Mesozoic radiation whose members share with lepistosteids: In their skulls, however, neopterygian characters are clearly visible, including:

Semionotiformes are interesting in their own right, as one of the best documented fossil examples of species flocks - radiations of closely related species occupying adjacent ecological niches in the same general environment, and distinguishable by body shape, color, and scale pattern. A modern example is that of African rift valley lake cichlids. During the Late Triassic, similar flocks of semionotiformes occupied similar environments - the rift valley lakes of the Newark Supergroup.

Amia calva from Wikipedia


(Triassic - Quaternary) Living Amia calva and fossil Amiiformes, (Jurassic - Quaternary) and their other fossil relatives. Freshwater ambush predators. In them, the rest of the body begins to catch up, evolutionarily, with the head:

Amia calva caudal skeleton. Click for comparison with ancestral halecomorph.

Watsonulus with jaw and cheek bones removed
But the mouth is not static. As emphasis increases on suction-feeding, the role of the hyomandibula in the lateral expansion of the palatoquadrate increases. In halecomorphs and Teleosts, we see the symplectic, a new ossification directly linking the hyomandibula to the quadrate, the ossification of the palatoquadrate forming the jaw joint. Together, the hyomandibula, symplectic, and quadrate form the suspensorium - the primary load-bearing attachment of the jaws to the neurocranium.

Ionoscopus analibrevis from Grande and Bemis, 1998
Fossil halecomorphs:

Amiiformes (Jurassic - Quaternary) Members of Amiiformes extend back to the Jurassic and include both fresh water ambush predators similar to the living Amia calva, marine pursuit predators such as Ionoscopus (Late Jurassic - right). Note that Amia's long dorsal fin is derived within the group.

Watsonulus eugnathoides, a paraseminotid
Parasemionotidae: (Early Triassic) Basal, small relatively unspecialized halecomorphs.

Synapomorphy of Halecomorphi:

Neopterygian phylogeny headache:

Alas, there is no consensus on the phylogenetic pattern formed by Ginglymodi, Halecomorphi, and Teleostei (the derived actinopterygians). Two hypotheses compete:

With improved information, the halecostome hypothesis has been eclipsed by the holostean hypothesis, but it is not dead.

Proscinetes sp., a pycnodontiform
A victim of the headache: Pycnodontiformes (Triassic - Paleogene) Distinguished by: By analogy with living forms, probably reef fish feeding on hard-shelled organisms. Maximum diversity in the Cretaceous, but they straggled well into the Eocene Epoch. Cute, but where do they fit on the tree? They present a confusing combination of character states: But... Pycnodont surprise: Traditionally considered close to Teleostei, the only phylogenetic analysis to address their position is that of Poyato-Ariza, 2015. This places Pycnodontiformes, instead, as a basal branch of Neopterygii.

What is Teleostei?: Wait for it.

Teleost Phylogeny

Teleostei: (Triassic - Quaternary) The vast majority of living actinopterygians belong to this highly derived clade, with significant apomorphies of:

In them, the neopterygian trends toward: reach their greatest expression.

We follow the taxonomic nomenclature of Arratia 2013 where:

Parasemionotidae indet.

Teleost Tails

Because teleost evolutionary change is focused in the caudal region, let's establish a basic vocabulary.

Ancestral neopterygian: A review. The caudal skeleton contains:

Even though in Neopterygii the tail tends to evolve toward an outwardly symmetrical shape, simply by modifying the length of fin rays, the skeleton that supports it is still clearly heterocercal. In the course of teleost evolution, the internal skeleton rushed to catch up.

Pholidophorus bechei
Basal teleost: Pholidophoridae are the basal branch of Teleostei, including small fish like Oreochima and Pholidophorus (right). In them, some changes are evident:

The net effect is:

Stem Teleosteomorph Superlatives

Aspidorhynchus acutirostris
Briefly noted - long remembered.

Aspidorhynchiformes: (Cretaceous - Paleogene) Long - bodied, medium-sized predatory fish with heavy ganoid scales, long pointed snouts, and a unique pattern of ossification of the skull. United with Teleosteomorpha by the possession of distinct uroneurals.

Hypsocormus sp. Link to reconstruction by D. Bogdanov.
Pachycormiformes: (Jurassic - Cretaceous) Large to gigantic fish with pointed snouts, long blade-shaped pectoral fins, and reduced pelvic fins. These invaded marine niches including:

As with aspidorhynchids, the connection between pachycormids and teleosts is found in the tail, where hypurals are not merely specialized but fused into a hypural fan. And yet, these animals show no tendency to ossify their notochords. In fact, these tend to be poorly ossified in comparison to other neopterygians.

Pholidophorus sp. palatal view of skull. Vomer in blue.
Synapomorphy of known teleosteomorphs:

Oreochima sp. a Jurassic pholidophorid.

Teleostei trends and synapomorphies:

Pholidophoridae, (Triassic - Cretaceous), the most basal member of apomorphy-based Teleostei is our model of their basal condition. Pholidophorids were small predatory fish.

The neurocranium: Compare the braincases of Mimipiscis, a basal "paleoniscoid" with Pholidophorus.

Mimipiscis (left) and Pholidophorus (right). Lateral views of neurocranium.

Evolutionary Trends:

The skull roof: Compare the braincases of Watsonulus, a halecomorph with Pholidophorus.

Watsonulus, a halecomorph (left) and Pholidophorus, a teleost (right). Lateral views of dermal skull roof.

Synapomorphies of Teleostei - The List:

Teleost Diversity

Gillicus arcuatus inside Xiphactinus audax from Wikipedia.
Link to D. Bogdanov reconstruction of Xiphactinus
Ichthyodectiformes: (Jurassic - Cretaceous) A major radiation of predatory fish that included top predators of the Cretaceous. Ranging from 1 m Cladocyclus to 6 m Xiphactinus (right). Ichthyodectiformes are characterized by:

Potential synapomorphies of Ichthyodectiformes and Elopocephala:

Elopocephala: (Jurassic - Quaternary) The crown-group of Teleostei. This is a very speciose group (~28,000 species). Here we must focus only on the basal pattern and largest groups:

Synapomorphies of Elopocephala: Exist but are highly technical and depend largely on reversals. E.G.:

Pholidophorus, a basal teleost (left) and Leptolepides, an elopocephalan (right). Lateral views of dermal skull roof.

Elopocephalan diversity

Anaethalion sp. (Jurassic elopomorph)
Elopomorpha: (Jurassic - Quaternary). Living forms range from unspecialized forms like tarpons and bonefish to derived eels, including deep sea gulpers. Synapomorphies:

Arapaima gigs from Smithsonian's National Zoo and Conservation Institute
Osteoglossomorpha: (Cretaceous - Quaternary). Living forms occupy the fresh waters of former Gondwana, although earlier distribution included North America and Asia. Synapomorphy:

Lycoptera (Cretaceous)

The osteoglossomorph caudal skeleton is plesiomorphic. Note that:

Diplomystus dentatus
Otocephala: (Jurassic - Quaternary). The last common ancestor of Clupeomorpha and Ostariophysi and all of its descendants. Synapomoprhy:

Otocephalan diversity: (Jurassic - Quaternary)

Diplomystus dentatus caudal skeleton
Clupeocephala: The last common ancestor of Otocephala and more derived teleosts. Potential synapomorphies include:

Gaudryella sp.
Euteleostei: (Cretaceous - Quaternary) The last common ancestor of Salmoniformes and more derived teleosts. Potential synapomorphies are beyond our technical scope, but involve reduction of the uroneurals.

Esox lucius
Neognathi: (Cretaceous - Quaternary) The last common ancestor of Esociformes (pikes and kin) and more derived teleosts. Esociformes have a "primitive look" because their dorsal fin is shifted backwards to the level of the anal fin, enabling rapid acceleration befitting fierce ambush predators. Where have we seen this before? Potential synapomorphies of include:

Neoteleostei: (Cretaceous - Quaternary) Potential synapomorphies of occur at both ends of the animal:

Striped bass Morone saxatilis from New York State Department of Environmental Conservation
Acanthomorpha: (Cretaceous - Quaternary) Here the evolutionary trajectory of Teleostei reaches its full flower. Synapomorphies:

Spines are an obvious defensive adaptation, that is developed in various ways. In many familiar acanthomorphs, the anterior spiny portion of the dorsal fin is distinct from the remainder, resulting in an anterior spiny dorsal fin and a posterior soft dorsal fin

Caveat: We can't address the fantastic diversity of Acanthomorpha, but in some of them we see the ultimate expression of the actinopterygian trends described earlier, plus some wonderful adaptations. Glancing at a small, arbitrary sample of its diversity, from the molecular phylogeny of Betancur et al., 2013 we note:

Percomorphacean phylogeny after Betancur et al., 2013
Percomorphacean diversity: Where to start? Beyond this point, we enter the realm of dragons and unicorns. Morphological systematics has failed to converge on a clear picture of Percomorph phylogeny, however molecular systematists are beginning to. Currently the last word belongs to Betancur et al., 2013 who have provided both a phylogeny and revised taxonomic nomenclature which we follow here. the major pattern:

Percomorpharian phylogeny after Betancur et al., 2013
Percomorpharia: Peeling away another layer of the endless actinopterygian onion, we reach Percomorpharia. Well known groups and interesting patterns contained within it include:

That a lot of fish.

Emerging Patterns

For all that this is lots of data to process, major patterns in teleost evolution do emerge readily from the background:

1. Acanthomorpha is stratigraphically congruent (almost):

The principal radiation of acanthomorphs occurred during the Late Cretaceous.

2. Percomorphaceae is a Cenozoic radiation:

As such it parallels other vertebrate radiations that followed the K-Pg extinction, including

3. Major radiations start in the oceans:

With few exceptions (Anabantomorphariae) major actinopterygian radiations occur in the oceans and invade fresh water subsequently. As a consequence, fresh waters serve as a refuge for groups that have been replaced in the oceans. Examples include:

It is tempting to regard: As representing stages in that transition.

Pleuronectiform sp. from Flickr

Green jack from

Amphistium paradoxum from wikipedia

Introducing "Burden"

Burden 1: Convergent approaches to flatfish

Teleost evolution shows the iterative invasion of the same ecological niches by diverse lineages. Example: Carangimorphariae - transforming something like the green jack(above), a fast predator with a strongly laterally compressed body, into a flattened bottom dweller like a flounder. One option is simply to flop onto one side, but that places one eye is in the mud.

Apparently it is easier for evolution to move one eye to the opposite side of the head than to radically change the shape of the body. This transition is documented by the Paleogene fossils Amphistium and Heteronectes (Friedman, 2008), in which the orbits have become asymmetric but orbital migration is incomplete.

A survey of groups within Carangiomorphariae reveals at least one independent derivations of left-eyed flounders (Paralichthyidae and Bothidae), one of right-eyed flounders (Pleuronectidae) and one group whose members go both ways (Psettodidae).

Dwarf gourami creates bubble nest from aquariumsetup
Burden 2: Air breathing for creatures who gave up lungs

The evolution of the physoclistous swim-bladder allowed the osteichthyan conquest of the deep oceans, but at a cost:

The "lung" could no longer be used for breathing.

Fish that find themselves in need of accessory breathing apparatuses have to improvise evolutionarily. Some strategies:

Additional reading: