Fascinating pictures of 270 million year-old fossils tracks from the Alps were published today, sadly most of them behind a paywall. They show the tracks laid down, probably by a salamander, as it paddled across a shelving patch of mud from shallow into deeper water and eventually taking to swimming. This is the first recorded instance of the transition from walking to swimming.
Exceptionally preserved Early Permian tetrapod trackways from the Orobic Basin (Central–Western Southern Alps) offer a unique opportunity to investigate in detail locomotion in fossil vertebrates that lived on continental European landmasses. Herein are reported the results of a study on several tetrapod trackways that display a large variety of behavioral, gait and substrate related extramorphologies. They clearly document the transition from terrestrial–underwater walking to swimming and are assigned to the compound ichnotaxon Batrachichnus C Lunichnium. The use of the “C” symbol is here introduced for the first time as nomenclatural indication of a Compound trace. Producers were probably small-sized temnospondyl or lepospondyl (microsaurs) amphibians. Comparisons with living urodelan anatomy and mechanics provide evidence for conservatism of locomotor mechanics in evolutionary history among amphibians. The derived model for locomotor kinematics in Early Permian amphibians provides a reference for interpreting transitional land-to-water trackways. The shift from walking to swimming behavior in early tetrapods, as in extant urodelan amphibians, is described as a complex balance between different dynamics.
Fabio M. Pettia, Massimo Bernardia, Miriam A. Ashley-Rossc, Fabrizio Berrad, Andrea Tessarollod, Marco Avanzinia;
Transition between terrestrial-submerged walking and swimming revealed by Early Permian amphibian trackways and a new proposal for the nomenclature of compound trace fossils;
Journal of Palaeogeography, Palaeoclimatology, Palaeoecology, P. 278-289 DOI: 10.1016/j.palaeo.2014.05.032
To begin with, the salamander was dragging its tail, showing the typical wiggling gait of salamanders and most tetrapodal reptiles which causes their tail to describe a series of curves. As it transitions to swimming all we have is the marks of the claws and the swirls left in the mud by the swimming motions.
This calls to mind something I was discussing only yesterday - how humans may well be the only species which can look at tracks like this and 'read' the information and build a narrative from it. We can see what I've described above, for example, how the salamander moved from shallow to deeper water and transitioned to swimming as it entered deeper water. So we know it could both walk and swim and was very probably semi-aquatic.
We can tell how big it was, that it had a long tail and that it walked with the typical wiggling gait typical of salamanders and lizards. This means we can begin to build an image of its basic body plan and especially how its limbs were attached to the rest of its skeleton.
Unlike mammals, dinosaurs and birds and the small mammal-like reptiles that survived the dinosaur mass extinction and evolved into mammals, the limbs of amphibians and lizards stick straight out from their body, then bend downwards at the elbow or knee joint. In birds, mammals and dinosaurs, the bend occurs at the joint with the pelvic or shoulder girdle. This means the spine can be kept in a straight line when walking, running or swimming and the weight of the body is supported on (normally) four limb with little muscular effort. Reptiles and amphibians need to use energy just to lift their body off the ground.
LocomotionIn typical early tetrapod posture, the upper arm and upper leg extended nearly straight horizontal from its body, and the forearm and the lower leg extended downward from the upper segment at a near right angle. The body weight was not centered over the limbs, but was rather transferred 90 degrees outward and down through the lower limbs, which touched the ground. Most of the animal's strength was used to just lift its body off the ground for walking, which was probably slow and difficult. With this sort of posture, it could only make short broad strides. This has been confirmed by fossilized footprints found in Carboniferous rocks.
So, we also know from that wiggly tail trace that the maker of those tracks belonged to a clade which may have been ancestral to our own but was not a member of our clade. A clade being a group that share a unique common feature. If we arrange species according to the features they have in common we end up with an evolutionary tree showing how we are related. It's fascinating to think that a little newt wrote all that information in the mud 270 million years ago and we can now read it. Our highly developed pattern-recognising ability and our deep-seated desire to tell the story allows us to read and understand that message and build a picture of its author.
These basic abilities have probably made us what we are today and created conditions in which evolving a brain capable of processing and interpreting this information and even relating it to others, especially our children who so can inherit our knowledge and stories, gave us a significant advantage over other evolving hominids in African and possibly Euro-Asia and allowed us to become the dominant and ultimately the only hominid.
Unfortunately, our ability to see patterns frequently where there are none and to invent stories to explain them, probably caused us to invent religions as simple narratives to explain what we did not understand. Some people are still unable to let go of the simplistic certainties that these simplistic explanations give them and accept that the real story was far more complicated and far more magnificent than pre-wheel Bronze-Age hunter-gatherers and nomadic goat-herders could ever have guessed.
So important is it to cling to these simple certainties that some people even go out and kill people who have a slightly different version of the same invented story.
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