By the inspiring Ray Troll, the king of fish artists.

Eugnathostome Diversity

Intro: Todays we begin with Osteichthyes - the bony fish, and their close fossil relatives the "Acanthodii".
Cladists use the following monophyletic group names in referring to these creatures:


This definition points out a basic difference in the ways in which we can define monophyletic groups. For this discussion, we will use the following fantasy cladogram:

.

Synapomorphies of Eugnathostomata:

Osteichthyes

Fossil Material: The earliest osteichthyan fossils are of Silurian age, but they only become abundant in the Devonian.


Guiyu oneiros from Nature
Guiyu (Right. Silurian! Described in 2009) is our closest well-known approximation of the last common ancestor of Osteichthyes. Remarkably, it is actually a basal sarcopterygian but also the oldest well-known gnathostome!

This creature has features seen in both primitive sarcopterygians and actinopterygians, but retains others typical of primitive teleostomes and placoderms, such as the stout pectoral spine.


Actinopterygii:


Cheirolepis canadensis from Wikipedia
Living actinopterygians are so highly derived that their basic synapomorphies are hard to see. These are visible, however, in Cheirolepis (right) from the Devonian is the oldest well-known actinopterygian and phylogenetically the most basal.

Synapomorphies:


Actinopterygian evolutionary trends briefly noted:


  • Swim bladder: Lungs were a plesiomorphy for actinopterygians. Some members continued to use them for their original purpose: as a supplimentary oxygen source. Others modified them into a unique organ, the swim baldder. This is used to maintain neutral bouyancy. Gasses are either pumped from the blood into the swim bladder or dissolved from the swim bladder into the blood as circumstances require. Note: Unlike the ancestral lung, the swim bladder does not communicate with the esophagus. We don't usually expect to see this in fossils, but fossil fish whose body shapes are ill-suited for resting on the bottom (like the Jurassic Dapedium) almost certainly had this.


    Actinopterygian diversity

    On could teach an entire course about them.

    Sarcopterygii

  • Definition: All organisms more closely related to land vertebrates than to Actinopterygii.


    Eusthenopteron foordi from Palaeos
  • "Synapomorphies":

  • The limbs articulate with the girdles by a single element, the humerus (as in Sauripteris on right) or femur.


  • Living Diversity:


    Latimeria chalumnae
    Actinistia: (Dev - Rec) Commonly called coelacanths. Represented by one living genus, Latimeria.


    Rhabdoderma from Palaeos
  • Synapomorphy:
    Additionally, though not exactly a synapomorphy, most actinistians have a diphycercal tail with a fleshy central lobe.


    Axelrodichthys from BIO 370 Vertebrate Zoology
    Udo Savalli, Arizona State University
    Ancient actinistians and their fossil record:

    Actinistian biology: Latimeria tells us much about the biology and ecology of ancestral sarcopterygians.



    Lepidosiren paradoxa from Wikipedia
    Dipnomorpha: Lungfish and their fossil relatives.


    Climatius from Wikimedia.com

    Eugnathostome Headache:

    When we consider basal teleostomes (i.e. teleostomes that are not proper osteichthyans) we enter the nether-world of Acanthodii, the Paleozoic "spiny sharks." Two examples:
    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 one is also the earliest.

    How do we reconcile these positions, and what, exactly, are acanthodians? Wait for it.