Ocean tides could have driven ancient fish to walk

Feb 16, 2018

By Alexandra Witze

Tides that left fish high and dry hundreds of millions of years ago could have kick-started the evolution of land-walking vertebrates.

New calculations suggest that, around 400 million years ago, many coastlines experienced two-week tidal cycles that varied in height by four metres or more. Such a huge range could have stranded fish in tidal pools for a couple of weeks. Only the ones with fins strong enough to muscle themselves out would have been able to journey back into the ocean and survive. Fossil evidence for the earliest known land vertebrates comes from places that had such wide tidal ranges.

Hannah Byrne, who led the work while at Bangor University, UK, and is now a doctoral student at Uppsala University in Sweden, reported the findings on 15 February at the Ocean Sciences meeting in Portland, Oregon.

The idea that the first land-walking animals could have evolved from those stranded in tide pools is generally well accepted and dates back decades. “What we’re suggesting is the actual driver of why the pools formed and why they were drying out,” says team member Mattias Green, an oceanographer at Bangor University.

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8 comments on “Ocean tides could have driven ancient fish to walk

  • @OP – Ocean tides could have driven ancient fish to walk

    I think this title is misleading!

    Tides that left fish high and dry hundreds of millions of years ago could have kick-started the evolution of land-walking vertebrates.

    This text is much clearer!

    Ancestral fish were walking on fins on the sea-bed (and some modern fish still are), as a separate issue from moving around the shore abrasion platform between tides and in the inter-tidal zones!


    Some species of fish can “walk” along the sea floor but not on land.
    One such animal is the flying gurnard (it does not actually fly, and should not be confused with flying fish).
    The batfishes of the Ogcocephalidae family (not to be confused with Batfish of Ephippidae) are also capable of walking along the sea floor.
    Bathypterois grallator, also known as a “tripodfish”, stands on three fins on the bottom of the ocean and hunts for food.

    There is in fact a range of illustrated adaptations, from these sea bottom dwellers, to the amphibious fish which are also shown on the wiki link!

    Of course, because they are supported by buoyancy, the fins of sea-floor walkers carry much less load than the fins of those coming out of the water!

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  • Eamonn Shute #2
    Feb 17, 2018 at 12:03 pm

    Tiktaalik and our ancestors were fresh water fish, and amphibians all breed in fresh water, so what happens in the ocean seems irrelevant to me.

    While amphibians breed in fresh water, both some individual fish (Atlantic Salmon, eels, etc.) and species of fish over evolutionary time, have moved to fresh-water from the sea and back again.

    Several species on the wiki link @#1 move with the tides to and from rock-pools and coastal shallows.

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  • No modern amphibians have scales as Tiktaalik did. Thin skinned amphibians need to remain in freshwater in consequence. The sedimentary evidence surrounding Tiktaalik is consistent with fresh and brackish water.Tidal seas like the Baltic are less than a third as salty as the major oceans and would qualify as brackish at its periphery.

    The ideas in the article are not mutually exclusive to Tiktaalik. It is important to note that Tiktaalik isn’t necessarily the first fishapod.

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  • Oops. Tidal range in the Baltic is low. Sorry.


    Oceanic salinity was lower 400 million years ago for one thing and/or one could imagine river deltas and river tidal bores as meeting the brackish yet tidal requirements for the theory to work.

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  • Eamonn Shute #2
    Feb 17, 2018 at 12:03 pm

    Tiktaalik and our ancestors were fresh water fish


    And nestled in the rock was an especially valuable bone: a pelvis.

    It was not what Shubin and his colleagues were expecting.
    The closest lobe-fin relatives of tetrapods had tiny pelvises, which only served to attach muscles that controlled the pelvic fin during swimming.
    Tiktaalik had a massive pelvis–as big as those of the earliest true tetrapods with legs and digits. And like us, it also had a massive scoop carved out of the side, where the ball of the femur could fit.

    [The] discovery prompted Shubin and his colleagues to look back at the thousands of other fossil fragments they had found at the Tiktaalik site over the years, many of which remained puzzling to them.
    They compared the new Tiktaalik bone to those unclassified fossils and found that they had unwittingly found five other Tiktaalik pelvises.
    Until they knew what a Tiktaalik pelvis actually looked like, they didn’t know what they had.

    All those hip bones have brought Tiktaalik into sharper focus.
    For one thing, they show that the creature could get big.
    The largest pelvis bones they’ve found suggest that Tiktaalik could grow up to nine feet long. Our ancient relatives, in other words, were the size of alligators.

    Not only was its pelvis big, but its pelvic fin was big, too.
    Shubin and his colleagues envision Tiktaalik using massive muscles anchored to its pelvis to power its hind fins–not just to swim, but to walk underwater or push its way across muddy flats.

    It is clear that the fin to limb evolution, was already established in earlier ancestors, prior to the various species related to Tiktaalik arising.

    Tiktaalik had hips that were tetrapod-like in size, they were still fish-like in anatomy. Our own hips are tightly fused to our spine. It would be catastrophic for them to be floating free in our bodies, because we wouldn’t be able to hold up our torsos against the force of gravity, nor could we transmit much of the force generated by our legs to the rest of our body.
    That is true of most other tetrapods, all of which are adapted for moving on dry land rather than being supported by water.
    By 360 million years ago, early tetrapods had evolved attachments from the pelvis to the spine.

    But their forerunner Tiktaalik still had free-floating hips.
    IN other words, Tiktaalik shows that 370 million years ago the tetrapod body plan was still very much a work in progress–from head to tail.

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  • phil rimmer #4
    Feb 17, 2018 at 2:32 pm

    The sedimentary evidence surrounding Tiktaalik is consistent with fresh and brackish water.

    Rivers, lakes, and estuaries change levels with seasonal and flash flooding, in addition to tides in estuaries and mud-flats

    The ecology of shorelines is complex, but movements with tides and flood cycles has been well studied.


    Littoral migrations specifically refer to those occurring in the littoral (i.e., intertidal) zone; however, the term is often applied to any migrations occurring between inshore and offshore or between shallow and deep waters.
    By this expanded use, littoral migrations may occur in freshwater as well as marine habitats and with tidal to seasonal periodicities.
    Many fishes that inhabit rocky and muddy intertidal shores (e.g., gobies, blennies, and sculpins) migrate with the ebb and flood of tides, exploiting resources that are inaccessible to subtidal competitors while avoiding most marine predators.
    Thereby, these migrations serve alimentary and refuge functions.
    Abiotic factors (temperature, salinity, pH, and dissolved oxygen) vary dramatically in littoral habitats, and tidal or seasonal movements (climatic migrations) are often necessary for fishes to remain within physiological tolerances.

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  • It is worth remembering, that unlike modern shore dwelling semi-amphibious fish and arthropods, early fish and tetrapods, were not subject to predation from the birds which had not yet evolved!
    They could therefore concentrate on feeding and avoiding aquatic predators.

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