Accidental Land Animals

John Merck

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 all known members are diagnosed by synapomorphies (of course they would be.)

    Synapomorphies:

    The most primitive known are:

    These, according Qiao and Zhu 2010, form a basal "Guiyu clade."


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


    Onychodus
    The remaining sarcopterygians are diagnosed by:

    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:



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


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

    Miguashaia
    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
    Dipnoi: (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

    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:

    Elpistostegalia


    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:



    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.


    A 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.

    By the time we meet elpistostegalians again in the latest Devonian, O2 concentrations had returned to familiar levels, but air-breathing adaptations had not gone away.

    We now encounter creatures with very tetrapod-like heads including Ventastega curoni (Ahlberg et al., 2007). Known only from its skull and pectoral girdle, Ventastega lacks opercular elements and the dermal pectoral elements dorsal to the anocleithrum. Thus, like Tiktaalik, it had a neck of sorts. Synapomorphy with Tetrapoda:

    What kind of appendages attached to the pectoral girdle? Unknown. Fortunately in the next case, it is not.


    Acanthostega gunnari from Clack 2012
    Acanthostega gunnari (Latest Devonian):

    Very well known from relatively complete skeletons. The first and phylogenetically most basal vertebrate definitely to possess fingers and toes! Synapomorphies with Tetrapoda include:



    Neurocranium of Acanthostega gunnari from Clack 2012
    Acanthostega's neurocranium at last begins to resemble that of a tetrapod. Note:


    Acanthostega gunnari skull from tumblr
    But was it a fish? Consider the transformations listed at the beginning of the lecture:

    The overall impression is of a fully aquatic animal with rudimentary ability to support some body weight on fore and hindlimbs and with the beginnings of the ability to use its jaw and hyoid arch to pick up sound from the ground. Maybe it came onto land occasionally to feed, disperse to new habitats, or thermoregulate, but it was primarily an aquatic animal. What did it use its fingers and toes for? Maybe not much at all.


    Ichthyostega stensioi
    Ichthyostega (Latest Devonian):

    Very well known (even in popular culture of previous generations) from relatively complete skeletons, but redescribed byAhlberg et al., 2005.

    Ichthyostega is the first animal we have seen with unambiguous adaptations to leaving the water: It's limbs, however, although more powerful than those of Acanthostega were paddle-shaped and probably did not offer much help out of the water. Maybe it lurked near the water's edge and lunged at prey on shore, or hauled itself up onto the beach in the manner of a harbor seal (Link to video). In short, it probably came onto land briefly but neither it nor Acanthostega could really walk on it.



    Tulerpeton curtum scale bar = 1 cm.
    Tulerpeton curtum: (Latest Devonian) Significant but known only from pectoral and fore and hindlimb elements, along with belly scales. Derived features include: Tulerpeton's limbs appear to be adapted to use as paddles for swimming, but it seems to have been an air-breathing adult with an increased ability to halt the production of fingers at an early stage.

    Elegant Hypothesis - Brutish Facts: We have constructed an attractive and coherent picture of the steps in the acquisition of tetrapod-like attributes:

    But there are at least two problems:


    Chastened, we consider the Carboniferous:


    Pederpes finneyae from Palaeos.
    Whatcheeridae: Clack, 2002 described Pederpes finneyae of Britain, from the middle of Romer's Gap at 350 mya. Although it retains a tiny sixth finger, has a robust lateral line system, a heavy hyomandibula/stapes, and retains a preopercular; it also has robust limbs and a skull that is compressed from side-to-side like early land vertebrates. We don't know how much time it spent on land, but it seems, at least, to have been able to walk - a new development fifteen million years after the evolution of hands and feet. Together with the later Whatcheeria deltae of Iowa, it comprises the Whatcheeriidae (Early Carboniferous).
    Early Carboniferous Landmarks: Emerging from Romer's Gap, we pick up several lineages. Their synapomorphy:


    Greererpeton burkemorani from Carroll, 2009.
    Colosteidae: (Carboniferous) Including the familiar Greererpeton. All are elongate with small limbs whose joints are formed in cartilage. They retain extensive lateral line systems, and some have gill-rakers on their ceratobranchials, indicating an open operculum and suspension feeding as an option (Schoch and Witzmann, 2011). Distinctly fresh-water aquatic. Distinctive features:


    Crassygyrinus scoticus from Theclacks.org. Total length~35 cm..
    Crassygyrinus scoticus: (Early Carboniferous) Very strange: Indicates a permanently aquatic animal. The skull combines derived and primitive features: Placed in a compromise position on our cladogram, but different analyses differ wildly about where it should go.


    Loxomma allmani from Palaeos. Total length~25 cm..
    Baphetidae:(Carboniferous) A morphologically disparate but distinct clade of aquatic predators. Known since the 19th century from skulls but hardly any postcranial material. Plesiomorphic in retention of full lateral line system and distinct spiracular notch.

    Synapomorphy:

    What was housed in this embayment is a topic of wild speculation:

    Baphetid diversity: encompasses several ecomorphs.

    Phylogenetic analyses place baphetids either below the base of crown Tetrapoda or as basal members of the Lissamphibian stem. A potential synapomorphy of baphetids and the lissamphibian line: A slender stapes (hyomandibula) suspended in the middle-ear (spiracular) chamber.

    Final word:

    Stem tetrapods are paradoxical. All show adaptations indicating increased reliance on air-breathing, the hearing of airborne sound, and some may have routinely walked on land; however Acanthostega, Greererpeton, Whatcheeria, and Megalophalus, all display osteological correlates to the presence of internal gills (Schoch and Witzmann, 2011). Thus, no matter how much time they spent out of the water, they remained fundamentally aquatic.

    Additional reading: