Osteichthyes

John Merck

Osteichthyes Introduced:

Definition: The last common ancestor of Actinopterygii (ray-finned fish) and Sarcopterygii (lobe-finned fish).

Osteichthyan Trends:

Osteichthyes weren't the first vertebrates with bone, but they developed and used it in novel ways that cause their skeletons to preserve much more evolutionary information: Major trends:


Synapomorphies of Osteichthyes:

A thorough survey goes beyond the technical limitations of this course. If you really want to know, consult the technical literature. For us, a general review must do:

Synapomorphies of Mandibular arch:



Hyoid and branchial arches of Mimiapiscis.
Synapomorphies of Hyoid arch:

Synapomorphies of branchial arch:


Synapomorphies of pectoral girdle: In addition the endochondral scapulocoracoid, there is a series of paired dermal elements that couple the pectoral girdle and skull.


Lepidotrichia from dkimages.com
Synapomorphies of the postcranium:



Mimiapiscis neurocranium
Noteworthy plesiomorphies of the neurocranium: Ancestrally, the occipital, otic, and sphenoid-ethmoid regions of the braincase are imperfectly co-ossified resulting in:


Latimeria menadoensis (Sarcopterygii) and Xyrauchen texanus (Actinopterygii)

Osteichthyan Diversity

Two major living groups:

Their evolution will be addressed in detail. For now, a "quick and dirty" guide to distinguishing their ancestral members (actual apomorphies in red.):


Moythomasia nitida (Actinpoterygii) and Strunius sp. (Sarcopterygii)

Body profile:


Xiphias gladius (Actinpoterygii) and Sauripteris taylori (Sarcopterygii)

Paired limb skeleton:


Mimiapiscis toombsi (Actinpoterygii) and Eusthenopteron foordi (Sarcopterygii)

Braincase:


Ganoid scale (Actinpoterygii) and cosmoid scale (Sarcopterygii)

Scales:

Osteichthyan Origins

Actinopterygii and Sarcopterygii are total groups that include all members on their stems. Thus, every member of Osteichthyes is, by definition, a member of Actinopterygii and Sarcopterygii. Nevertheless, we should expect to find "stem osteichthyans" displaying some of the osteichthyan synapomorphies. Alas, the only candidates known are:

The result is the huge concentration of synapomorphies at the Osteichthyes node. It seems probable that stem-osteichthyans have been misidentified as basal actinopterygians or sarcopterygians because:

But the most significant landmark are among basal actinopterygians and sarcopterygians. Our most informative example:


Guiyu oneiros from Pharyngula
Guiyu (Right. Silurian Zhu et al. 2009) is: Remarkably, it is actually a basal sarcopterygian. It's most interesting feature, however, may be its plesiomorphies, including:

An osteichthyan for sure, but it remembers its placoderm-grade origins.

Actinopterygii:


Cheirolepis canadensis from Wikipedia
Living actinopterygians are so highly derived that their basic synapomorphies are hard to see. These are visible, however, in fossil taxa like Cheirolepis (right) from the Devonian, the oldest well-known actinopterygian and phylogenetically the most basal. A full exploration of the synapomorphies of Actinopterygii and its constituent clades is beyond the scope of this course. The ones we do track will pertain to four major trends in actinopterygian evolution.

Actinopterygian evolutionary trends:

Actinopterygian Homology Headache:

Newcomers to actinopterygian paleontology confront a frustrating amount of disagreement about the names of actinopterygian dermal head skeletal elements. This is because in literature published prior to the 1990s, such elements were named based on the similarity of their topographic position on the skull to the cranial elements of sarcopterygians, including tetrapods. Thus, the paired bones between the orbits were called frontals, because they occupied the same general position as the frontals of tetrapods, even though they enclosed the pineal foramen. This results in two problems:

Because so much literature employs this nomenclature, you must always determine which nomenclatural system is being used.

Note, also, that in derived actinopterygians, the homologies of the elements shown at right can be obscured when:


Actinopterygian Diversity:

Groups with living representatives include:

We will address these one by one, with a selection of their fossil relatives.

Synapomorphies of Actinopterygii:


Basal Actinopterygii:

Although controversial, most phylogenies agree that the Middle - Late Devonian Cheirolepididae (monogeneric for Cheirolepis) is the basal branch followed by Cladistia. There follows a speciose and morphologically disparate paraphyletic assemblage, traditionally called "paleoniscids." They represent the major diversification of actinopterygians during the Late Devonian and Carboniferous, with many members surviving well into the Mesozoic. Alas we can only touch on them.


Cheirolepis canadensis
Cheirolepididae (Middle-Late Devonian) - The most basal well-known fossil actinopterygians, and a close approximation of what we would expect from the true last common ancestor of the group.

Plesiomorphies:

Autapomorphy:

See Giles et al., 2015 for details.


Polypterus bichir
Cladistia (Cretaceous - Holocene) The most basal living actinopterygians are generally thought to have branched very early. Their living representatives, Polypteriformes, include the living reedfish Polypterus (CT-scans at Digimorph) and Erpetoichthys, native to the fresh waters of Africa. Although derived in many respects, including: they lack the synapomorphies of most other actinopterygians.

Ecologically convergent on sarcopterygian lungfish in their ability to tolerate changing conditions in ephemeral pools and streams and their need to supplement gill-derived oxygen with air. Indeed:



Polypterus bichir after Allis, 1922.
Fossil cladistians: Most analyses (Xu et al., 2014) agree that Polypteriformes are the most basal of living actinopterygians. That, alas, places them at the end of one of the longest known vertebrate ghost lineages, stretching from the Devonian to the Neogene. Recent discoveries have shortened this a little:


Polypterus bichir neurocranium after Allis, 1922.
Potential synapomorphies of Crown-group Actinopterygii (Cladistia plus Teleostei):


Moythomasia nitida

Actinopteri:

Actinopteri (Devonian - Quaternary) The sister taxon of Polypterifromes includes all remaining actinopterygians. It's synapomorphies:

At the base of this group are "paleoniscoids" (Devonian - Cretaceous) a large paraphyletic assemblage representing the major Paleozoic actinpterygian radiation. E.G. Moythomasia (Late Devonian). Plesiomorphic features include:


Redfieldius gracilis
The comparison of Moythomasia (Late Devonian) a basal member of this radiation, and Redfieldius (Triassic - right) a more derived member shows that Redfieldius displays a condition closer to that of derived actinopterygians in most of these features. See if you can spot the differences!

Note that "paleoniscoids" also included deep-bodied forms, and forms with special feeding adaptations.

Crown-group Actinopteri:

(Late Permian - Quaternary) The last common ancestor of all living actinopterygians excluding Polypteriformes. Includes:

Synapomorphies include:

Prominent issues: Although speciose, Paleozoic actinopterygians tended to be small, and a minor component of faunas in which large placoderm-grade fish (prior to Devonian extinctions), chondrichthyans and sarcopterygians figured prominently. The crown-group Actinopteran radiation, in contrast:


Chondrostei:

Living chondrosteans:



Acipenser fulvescens from Digimorph
Among chondrosteans, sturgeons and paddlefish are sister-taxa. Their synapomorphies tend to be associated with extreme reduction of ossification:

Living chondrosteans significantly reduce their amount of endochondral ossification. Examining fossil chondrosteans, one sees that this is a derived feature.

Fossil Chondrosteans:


Neopterygii:

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:


Beishanichthys brevicaudalis from Xu and Gao, 2011.
The starting point: Members of Scanilepiformes (Triassic) like Beishanichthys (right), occupying a basal position on the neopterygian stem, give 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:


Parasemionotus
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.

Ginglymodi:

(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.)


Lepisosteus
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

Halecomorphi:

(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.

Additional reading: