Sometime around 400 million years ago, the land-living, four-legged vertebrate animals properly called tetrapods evolved from their aquatic fish ancestors. Early, land-going tetrapods – animals like the famous Ichthyostega, a short-snouted aquatic animal discovered as fossils in 360-million-year-old rocks in eastern Greenland – were seemingly slow, clumsy walkers that kept their bodies close to the ground and appeared to lack agility or speed when moving on land.
Any analysis of the fossils points to them having a very inefficient gait. But if we look at the enormous variety of land-living tetrapods that evolved later during Earth’s history, including those that live alongside us today, it’s obvious that far greater agility, speed and efficiency eventually evolved.
Supposedly, it was only when more modern tetrapods evolved, such as reptiles and the ancestors of mammals – a tetrapod group known collectively as amniotes – did things change. These new animals gradually evolved energy-saving poses and an ability to hold their bodies up above the ground.
But when did the key components of ‘advanced tetrapod’ gait appear? And what were the key events that gave later tetrapods their ability to scurry about quickly and efficiently, while holding their bodies higher up off the ground?
One team is turning to sophisticated computer modelling and robotics to find out. Led by Prof John Nyakatura of the Humboldt University of Berlin, the researchers are focussing on Orobates, an early tetrapod known to have lived in the region that later became Germany around 260 million years ago and traditionally considered to be close to the ancestry of amniotes.
This probable evolutionary position is significant, since it means that Orobates might serve as proxy for the amniote ancestor. In other words, if we learn more about the biology and behaviour of Orobates, we might understand more about amniotes as a whole.
But why choose Orobates specifically as a focus of research, and not another diadectid (the tetrapod group to which Orobates belongs) or early tetrapod?
What was Orobates?
Orobates is a member of an extinct group of ancient tetrapods called diadectids and lived around 260 million years ago during what’s known as the Middle Permian period.
First, it is known from excellently preserved, complete remains. Enough is known about the skeleton of Orobates to accurately reconstruct the way its bones fitted together when it was alive, and hence make inferences about its posture and locomotion.
Second, a set of fossilised tracks found in 2007 in the same region as the Orobates skeletal remains, is of exactly the right age in geological terms, and was made by an animal that matches Orobates in size and shape. It was almost definitely made by Orobates itself, which is rare in palaeontology and important since it means we have a direct record of how this animal placed its feet while walking.
Third, Orobates is from an especially interesting part of the tetrapod family tree. “It can be regarded as a key fossil due to its placement within the tree of life, very close to the origin of the amniotes – a group that largely became independent from water,” says Nyakatura.
Orobates was not much like modern amphibians, but nor was it a reptile or a relative of mammals. “It was a medium-sized creature, about 85cm long, that was a melange of what could loosely be called amphibian-like and reptile-like features. A salamander-y, lizard-y thing,” says team member Prof John Hutchinson of the Royal Veterinary College in Hertfordshire, UK.
Nyakatura and his colleagues therefore embarked on an ambitious project. Building on previous work that looked at the locomotion of salamanders, crocodilians and other tetrapods, and combining the track evidence with data gleaned from the Orobates skeleton, they wanted to use computer modelling and real-world robotics to analyse the gait and posture of Orobates.
Digital and real-world robotic skeletons could be custom-built to fit the track and show how the animal placed its feet. But what sort of gait would be required in order to do this precisely? Their aim was to reverse-engineer the possibilities that might be available to this animal.
The robot constructed for this study is perhaps the most intriguing component of the work, and certainly the part that has received the most attention from the media. Dubbed OroBOT, it has a range of movements in its limbs and body designed to mimic that of the real Orobates, numerous mobile joints like those present in a real animal, and flexible feet that allow it to properly contact the ground with each step. It is carefully designed so that its mass distribution matches that of a live Orobates, and it can employ numerous subtle variations in gait and pose.
In action, OroBOT has a surprisingly realistic air, moving and flexing with a real-world feel and recalling the walking style of a big lizard or small crocodilian. “It was a magical moment for us when the robot first walked,” says Nyakatura. “The motor went through some tests and the robot did something that resembled push-ups. Then it started to walk for the first time. I probably will not forget this moment. We enjoyed it a lot.”
Data from living animals can be used to show how efficient given walking styles and limb postures are. The team used information from four living animals
(a salamander, a skink, an iguana and a caiman) to determine which of OroBOT’s postures and poses were biologically most likely.
They also measured the robot’s centre of mass and the way it distributed its weight across the different segments of its body. The team measured 512 gaits and mathematically scored them according to how well they performed based on our understanding of what real animals do.
“We explored a large number of potential gaits and evaluated these according to anatomical plausibility and other factors, all of which were tested in the robot,” says Nyakatura.
Some of the gaits caused OroBOT to lose its balance, bang its limbs together or put too much strain on its joints and so were considered unlikely to have been used by the living animal. Also, living animals tend to use gaits where relatively little energy is required to move their joints, so gaits that were found to be inefficient were also deemed to be unrealistic.
Surprisingly, OroBOT’s highest-scoring gaits involved it walking with a tall, erect-limbed stance where its body was held high off the ground and its limbs were held relatively close to the body, albeit sprawled out. This resembles the walking gait used by iguanas and caimans rather than that of salamanders and short-limbed lizards like skinks.
This means that Orobates – and presumably other diadectids as well – walked in an efficient, ‘modern’ fashion more reminiscent of advanced reptiles such as crocodilians, not with the less efficient, seemingly clumsier gait assumed by previous studies.
What this indicates is that tetrapods had evolved a sophisticated, complex way of moving on land relatively early in their history, prior to the evolution of amniotes.
Rather than being clumsy prototypes of amniotes, diadectids – and presumably the tetrapods related to them too – were already quite capable on land, as the features of their gait, posture and efficiency suggest. Diadectids and similar tetrapods were, it now seems, probably faster, more efficient and had better balance and coordination on land than had been assumed before.
The beauty of this study is both that it uses multiple different techniques and lines of evidence to examine the gait of a long-extinct animal and proposes numerous possibilities, some of which provide more satisfying, more realistic results than others.
The findings are compelling enough to show how reliable and useful this technique is, and ultimately mean that the locomotion of this interesting and important early tetrapod has effectively been reverse engineered with the help of biorobotics.
“We give a range of possibilities and provide some powerful tools and datasets so that others can replicate and visualise our results,” says Hutchinson.
“We hopefully show how multiple lines of evidence – footprints, skeletons, digital animation, experiments with living animals, robots and computer simulations, essentially a kitchen sink of techniques – can be combined to tackle similar questions with other organisms in the future.”
So, what’s next? The success of this technique, and of OroBOT itself, means that work of this kind will surely be applied to other ancient animals, some of which come from crucial parts of evolutionary history or are unusual and mysterious because they’re so different from living animals. “I’d love to see someone try this with a walking Triassic pterosaur,” says Hutchinson.
This article was first published in BBC Science Focus in April 2019 – subscribe here