Transitional fossils: How the ankylosaurs got their club-like tails

Ankylosaurs were a group of herbivorous dinosaurs that were covered with bony armor composed of osteoderms (bones formed in their skin), much like the mammalian glyptodonts. Many of the ankylosaurs had osteoderms flanking the tips of their tails, making a club-like structure that may have been used to ward off predators or compete for females.

Ankylosaurus_raul_martin

A recent paper [1] by ankylosaur expert Victoria Arbour showed that these ornate creatures appear to have evolved these club-like tails in a two-step transition.

Scelidosaurus 199-191 million years ago*

Scelidosaurus_harrisonii_(1)

Before talking about ankylosaurs, it’s first important to mention Scelidosaurus, a precursor of sorts to the ankylosaurs. The earliest dinosaurs were bipedal, but Scelidosaurus was quadrupedal, walking on all four legs. Also unlike earlier dinosaurs, Scelidosaurus possessed osteoderms, although not the large, fused types characteristic of ankylosaurs.

dinosaur-images-042-resize

Gastonia 127-124 million years ago

m1798

After Scelidosaurus disappeared, the ankylosaurs appeared in the fossil record. They had much more elaborate osteoderms that had fused into plates and spines. The earliest fossils, such as Gastonia here, had flexible tails, often flanked with spikes, but lacked the mallet shape characteristic of later species.

Gobisaurus 93-90 million years ago

Gobisaurus_SydneyMohr711

Gobisaurus differs from earlier ankylosaurs in that it had a very stiff tail, achieved by having highly overlapping vertebral joints. These fossils have not been discovered with the tail clubs characteristic of the latest ankylosaurs, such as Euoplocephalus (78-77 million years ago) shown below:

euoplocephalus.skeleton

This suggests that a stiffened tail, which would have been strong enough for striking predators or opponents, predated the clubs of later ankylosaurs. The clubs then appeared at a later stage, presumably to amplify the power of this putative weapon.

To further illustrate this transition, here is a diagram created by Victoria Arbour**:

timeline_Arbour

*Dates are estimates from fossil distributions reported in figure 4 of Arbour et al. [2015]

**Note that she highlights different ages than I, due to her summarizing of multiple fossils

Questions for Creationists

What happened to the ankylosaurs? Why does the fossil record appear to record a transition from dinosaurs that were bipedal and lacked osteoderms, to quadrupedal species with elaborate osteoderms and club-like tails? Is it a coincidence that these species appear in this order?

References

1. Arbour, V. M., & Currie, P. J. (2015). Ankylosaurid dinosaur tail clubs evolved through stepwise acquisition of key features. Journal of anatomy227(4), 514-523.

Photo credit

Ankylosaurus by Raul Martin, Scelidosaurus 1 by FunkMonk, Scelidosaurus 2, Gobisaurus by Sydney Mohr, Euoplocephalus, Ankylosaur transition by Victoria Arbour

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3 thoughts on “Transitional fossils: How the ankylosaurs got their club-like tails

  1. Pingback: Where did the glyptodonts go? | Evolution For Skeptics

  2. gil

    hi. we can arrange cars in order–> car–> a jeep–> a truck. but its not prove any evolution. even if they was self replicating. its pissible that those are species of the same family.

  3. Hi Gil! Yes, you are absolutely correct. None of this is proof, and it should never be construed as such. However, when an apparently transitional sequence is preserved in rocks that correlate with progressive younger ages (e.g., 200 million, 150 million and 100 million years) then it’s consistent with the hypothesis that these species evolved.

    From a creationist perspective, the fossils present in this sequence are usually considered a coincidence. For example, if Noah’s flood is to be invoked, then the ankylosaurs without flexible tails without clubs are coincidentally found in lower rocks than those with stiff tails, and both are coincidentally found in rocks below species with club tails. If this was the case for just one alleged transitional series, the likelihood of it being a coincidence is quite high. If it occurs repeatedly throughout the fossil record, then it becomes more difficult to assume that chance is responsible.

    So again, none of this is proof. What is important to do instead is weigh which hypothesis (or hypotheses) these data fit best.

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