Accidental Land Animals

John Merck

Link to cladogram and phylogram cheat-sheets

And Now For Something Completely Different: Sarcopterygii

  • Definition: All organisms more closely related to land vertebrates than to Actinopterygii. This is a total group definition, however all known members are diagnosed by synapomorphies (of course they would be.)

    Onychodus - basal sarcopterygian (left) and Cheirolepis canadensis basal actinopterygian (right).

    Skull roof of the coelacanth Rhabdoderma
    Actinistia and relatives: Actinistia (Devonian - Quaternary) and Onychodontiformes (Devonian) are united by an elongate postparietal.

    Onychodontiformes include a small number of taxa including Onychodus (above right) and Strunius. Their synapomorphy:

    But caution: A review of phylogenetic hypotheses by Ciudad Real et al., 2022 suggests that although onychodontoformes group with Actinistia, they may not be monophyletic with respect to it.

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

    Rhabdoderma from Palaeos
    As a result, the primary teeth of the upper jaw are palatal, rather than marginal teeth.

    Additionally, though not exactly a synapomorphy, most actinistians have a diphycercal tail with a fleshy central lobe, although early forms, such as Miguashaia didn't conform to this pattern.

    Axelrodichthys from BIO 370 Vertebrate Zoology
    Udo Savalli, Arizona State University
    The actinistian record:

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

    Lepidosiren paradoxa from Wikipedia
    Dipnomorpha: (Devonian - Quaternary) Total group containing Dipnoi (lungfish) and their fossil relatives.

    Attributes of living lungfish:

    Diabolepis speratus showing anterior and posterior nares from Palaeos

    Ancient dipnomorphs and their fossil record:

    The lungfish record goes back much farther than that, however. Paradoxically, they first turn up as a very speciose Devonian radiation of marine fish. Indeed, for much of the Devonian, lungfish are second only to placoderms as the most common group of marine fish.

    Skull of Dipterus in ventral and dorsal view, and schematic of dermal elements
    The problem with lungfish: From a cladist's perspective, lungfish are intractible for a two reasons:

    Holoptychus from Miguasha National Park
    Dipnomorpha: For decades, their relationships with other vertebrates were controversial. The problem seems to have been solved by the examination of Porolepiformes - (E.g. Holoptychius, from Early Devonian) less derived primitive fossil sarcopterygians that share with lungfish the synapomorphies.


    Tetrapodomorpha: All organisms more closely related to land vertebrates than to lungfish. These creatures didn't actually set out to become land animals. They were perfectly happy as fish. In fact, most of the evolutionary novelties that predisposed them toward life on land were near-term adaptations to life in water or were simply accidents. But first, what were they?

    Holoptychus from Miguasha National Park
    Our starting point is the completely aquatic Holoptychius, a porolepiform dipnomorph. Fully aquatic with external incurrent and excurrent nares. From such beginnings, we witness a series of transformations that give rise to creatures with at least the latent ability to invade the land.

    Roster of transformations

    Becoming a land vertebrate is not easy and certainly not inevitable. Major adaptations are required in the following systems:

    Kenichthys nares (a, d, and e) from Zhu and Ahlberg, 2004
    The choana: In land vertebrates, air flows through the nares into the nasal capsule. From there, it moves through am "internal naris" or choana into the oral chamber. In effect, the excurrent naris of other aquatic osteichthyans has been shifted across the upper lip. Kenichthys campbelli (Early Devonian), the basal member of Tetrapodomorpha, shows this transition in progress: With this system, a vertebrate could inhale air without having to gulp and swallow it. Instead, air can be drawn directly into the (closed) oral chamber (being sniffed by olfactory system on the way) then pumped into the lungs, using the expansion and contraction of the oral chamber and pharynx already used in prey capture.

    Choanata: The last common ancestor of Rhizodontida and land vertebrates and all descendants. Synapomorphies include:

    Rhizodus by Kahless28
    Rhizodontida: (Devonian - Carboniferous) Tetrapodomorph superlatives. Fresh water ambush predators including Rhizodus (right) which at up to 7 m may be the largest fresh water fish ever. Characterized by:

    Osteolepis sp. from Carroll, 2009
    "Osteolepids" (Devonian - Permian) A speciose paraphyletic assemblage of aquatic sarcopterygians. The synapomorphies that unite them with tetrapods to the exclusion of rhizodontids are highly technical. For us, they provide a picture of aquatic choanates that are known from long and thorough study.

    Eusthenopteron foordi
    Tristichopteridae (Devonian) A distinctive monophyletic group. for GEOL431, their significance is mostly in their being the closest sarcopterygian group to Tetrapoda not to show any particular adaptation for life in very shallow water. Thanks to the mid-20th century work of Erik Jarvik, the tristichopterid Eusthenopteron (right) is among the best known of fossil vertebrates.

    Synapomorphies of Trstichopteridae include:

    Forelimb of Eusthenopteron foordi from Palaeos
    Synapomorphy of Tristichopteridae and Tetrapoda:


    Panderichthys rhombolepis from Clack, 2012
    The first true indication of a tendency to place at least parts of the body out of the water occurs in Elpistostegalia (Latest Devonian - Quaternary - named for a poorly known member, Elpistostege.) The last common ancestor of Panderichthys and Tetrapoda and descendants. The first and most basal well-known member is....

    Panderichthys (Late Devonian) Known from the Baltic region. Up to over a meter length, it is slightly flattened with a pointed snout. Its orbits are located on the dorsal surface of the skull, as are the spiracles, as if they were meant to project above the water. The impression given is of an aquatic animal specialized for shallow water and for hunting creatures just above the water's surface. Panderichthys retains rather heavy scales.

    Synapomorphies of Elpistostegalia and Tetrapoda:

    Tiktaalik roseae from Villanova University
    Tiktaalik roseae: (Late Devonian) Described in 2006 by Daeschler et al., this creature has become the current poster-child of vertebrate evolution. It is featured in:

    In its general profile, it's similar to Panderichthys, there are two important differences.

    Tiktaalik's pelvis was described by Shubin et al., 2014. Although the hindlimb is unknown, the pelvis is surprisingly large and the hip socket is circular, indicating that the hindlimb was strong and had a wide range of motion. And yet, Tiktaalik lacked a sacrum.

    From this it seems that Tiktaalik could support at least a little of its weight on its paired fins. What would be the point? For an idea, we look at the head.

    Spiracles of a tristichopterid and a basal elpistostegalian compared from Brazeau and Ahlberg, 2006.
    Spiracular inspiration? The earliest elpistostegalians (including Panderichthys and Tiktaalik) display an interesting trend in the evolution of the spiracle, which is: Could they have been breathing through their spiracles? We see this today in the basal actinopterygian Polypterus.

    A review by Clack, 2007, maintains that this was an adaptation to air breathing, in effect, through the ears. Indeed, the ability of creatures like Tiktaalik to do "pushups" may have been to get the spiracles clear of the water's surface. Arguably, this was an adaptation to unusually low O2 concentrations during the Frasnian age of the Late Devonian, during which elpistostegalians radiated. Recent geochemical analyses indicate that this evolutionary pulse coincided with an interval of low O2 concentrations. This, Clack argues, stimulated the evolution of air-breathing adaptations. Naturally, creatures with the ability to elevate their heads above water to breathe, by supporting their body weight on their limbs would have the best shot at leaving the water altogether, if only for brief intervals.

    But note that there is a positive-feedback effect here: Modern fish who rely on air-breathing in hypoxic water environments tend to reduce their gills in order to avoid losing oxygen from the blood through outward diffusion, making them more dependent on air-breathing. Why not quickly lose gills altogether? Because it is metabolically easier to excrete CO2 through them than through the lungs because CO2 shed into water is quickly converted to HCO3-, maintaining a strong CO2 concentration gradient ( Pelster, 2021) at the gills. As we will see, tetrapods held onto their gills for quite a while for this reason. (Witzmann, 2015)

    Additional reading: