Osteichthyes and Actinopterygii

John Merck
Link to cladogram and phylogram cheat-sheets

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 Mimipiscis.
Synapomorphies of Hyoid arch:

Synapomorphies of branchial arch:


Synapomorphies of pectoral girdle: In addition to 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:



Mimipiscis 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:


Mimipiscis 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 or Sarcopterygii. Nevertheless, we should expect to find "stem osteichthyans" displaying some of the osteichthyan synapomorphies. Alas, only a handful of candidates are known:

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 landmarks are closer to the osteichthyan node. Our most informative example:


Guiyu oneiros from Wikipedia

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:


Synapomorphies of Actinopterygii:


Actinopterygian Diversity:

Groups with living representatives include:

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

Basal Actinopterygii:

Although controversial, most phylogenies agree that the Middle - Late Devonian Cheirolepididae (monogeneric for Cheirolepis) is the basal branch. There follows a speciose and morphologically disparate paraphyletic assemblage, traditionally called "paleoniscoids." 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.

From this point onward, we follow the result of Giles, et al., 2017, noting its differences with older hypothesis commonly cited in technical literature.


Moythomasia nitida

"Paleoniscoids:"

(Devonian - Cretaceous) A host of fossil forms are closer to living "crown Actinopterygii" than to Cheirolepis. Traditionally termed "Paleoniscoids" after Paleoniscum. Their derived features include:

Ancestrally, they resemble Moythomasia (Late Devonian - right). 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.


Saurichthys from Prehistoria Wikia
Of special interest - Saurichthyidae (Late Permian - Middle Jurassic) Long (0.5 - 1.5 m) slender long-jawed predators superficially resembling houndfish or gars. Vaguely like a sturgeon trying to be a barracuda. Generally considered close relatives of sturgeons, but recently placed just outside of crown-group actinopterygii by Giles, et al., 2017.

Body elongation: Within this group, Maxwell, et al., 2013 show a unique mode of elongation of the vertebral column. Whereas other vertebrates either:

Members of Saurichthyidae manage to transpose the developmental identity of neural and haemal arches onto small intercalary elements that would typically serve as spacers between them. (Link to video. )

Crown Actinopterygii:


Polypterus bichir
Cladistia (Cretaceous - Holocene) The most basal living actinopterygians are highly specialized. 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: Through extensive reversals, 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: Recent discoveries pull crown-cladistians into the Cretaceous:


Beishanichthys brevicaudalis from Xu and Gao, 2011.
New for 2017: Giles, et al., 2017 recover the Triassic group Scanilepiformes (like Beishanichthys - right) as stem Cladistians. Previous authors had considered Cladistia basal to all actinopterygians but Cheirolepis, invoking a ghost lineage extending from the Devonian to the Cretaceous! The new result eliminates this problem and places the minimum divergence age of crown Actinopterygii in the Early Triassic.

Actinopteri:

(Triassic - Quaternary) The last common ancestor of all living actinopterygians excluding Polypteriformes. The crown of Actinopteri 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 actinopteran radiation, in contrast:


Stem Actinopteri:


Birgeria stensioei from Universität Zürich
Birgeriidae (Triassic) Large (1.2 m) robust pelagic predators ecologically similar to tuna or mackerels.

The crown of Actinopteri is anchored on:

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. (Also conversantly derived in other large actinopterygian suspension-feeders.)

Fossil Chondrosteans:

Additional reading: