Hu et al. (2017)- Exceptionally well-preserved Middle Triassic horseshoe crabs from SW China.

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    Sadie Mills

    This week’s open access paper discussion features a recent paper by Hu et al. (2017), published in Nature’s Scientific Reports (Click here for full pdf).

    The paper, “Exceptional appendage and soft tissue preservation in a Middle Triassic horseshoe crab from SW China,” describes how well-preserved, ancient horseshoe crab specimens can help us determine how much modern horseshoe crabs have changed over time (thus adding fuel to the “living fossil” debate, which you can read about here).

    The paper’s methods include detailed descriptions and comparisons of ancient and modern horseshoe crab anatomy. If you need help with some of those anatomical terms, try this fabulous diagram from NOAA. 

    Or, if you don’t have time to read the full paper, here is a summary:

    Who: 12 extinct Yunnanolimulus luopingensis horseshoe crab specimens.

    What: The specimens show well-preserved soft tissue features, including gills and muscles.

    Where: Recovered from a Middle Triassic fossil lagerstätte in Luoping, Yunnan, China (the first record of horseshoe crabs from China!)

    When: The Middle Triassic, around 240 million years ago.

    Why: Soft-tissue preservation in fossil horseshoe crabs has been exceptionally rare, thus a lot about certain anatomical features of past species has remained unknown. The discovery of these specimens allowed researchers to learn about ancient horseshoe crab anatomy, and compare it with the anatomy of modern, extant species.

    So what?

    Learning about the anatomy of extinct horseshoe crabs can help us make informed assumptions about their past behavior and life cycle. Based on the soft tissues and exoskeleton of Y. luopingensis, the researchers inferred that they likely burrowed into soft substrates, swam upsidedown, and used their telson (rigid tail) to right themselves when flipped over.

    Comparing ancient horseshoe crab anatomical features to those of modern, extant horseshoe crab species also helps us determine how much modern species have changed over time. The authors concluded that the horseshoe crab’s basic body plan (both exoskeleton and soft tissues) has been “fundamentally conserved since at least the early Mesozoic,” meaning that horseshoe crabs have exhibited relatively little change during at least 240 million years of their evolutionary history.

    Why little change? The authors hypothesize that habitat stability (relatively little change in environment) has played a role. They argue that most horseshoe crab species live in marginal environments, which have been fairly stable over geologic time.


    Questions for Discussion:

    1. Given the relative lack of change over time in horseshoe crabs, is it appropriate to call modern horseshoe crabs “living fossils”? Why or why not?


    1. Is habitat stability the most compelling argument for why horseshoe crabs have exhibited relatively little morphological change? What are other reasons can you think of?
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