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The Tyrant’s Tiny Arms

Tyrannosaurus rex is likely the most well known of any species of extinct animal. It has appeared in more movies, sold more toys, and generally wowed more people than any other other dinosaur. It is known for its gargantuan size, enormous head, long sharp teeth and its…tiny arms? Why would such a massive carnivore have such feeble arms?

Tyrannosaurus, and its close relatives; the Tyrannosauridae, belong to the suborder of dinosaurs known as the Theropoda. This group contains the vast majority of all known carnivorous dinosaurs (such as T.rex) as well as the last extant dinosaurs; the birds. This vast collection of dinosaur species evolved from small carnivorous dinosaurs, similar to the Triassic Eoraptor, with smaller, more lightly built skulls, and longer, grasping arms, than were found in the Tyrannosauridae. Despite this starting point, many groups of theropods independently evolved large heads, and reduced arm and hand size, an example of what is known as convergent evolution. These adaptations occurring multiple times, in different lineages of large theropod dinosaur, indicates that they were very useful adaptations to have.

The most likely explanation for the small size of Tyrannosaurus’ arms is a simple matter of balance. The skull of T.rex is massive, with fused nasal bones and a fleet of adaptations that reinforce the skull against stresses, in turn enabling it to have one of the most powerful bight forces of any known animal. Despite intense pneumaticity in both the skull and jaw, this head weighed a great amount. To counteract this, the arms of the animal were small, as the large head made them largely redundant in hunting, and extra energy would be needed to grow larger, heavier tails to counterbalance the animals top heavy nature, alongside the added energy to move an even heavier animal. Reduced arm length and large, heavy heads are often considered adaptations necessary for large hypercarnivores and as such, these adaptations are common among large theropods outside of the tyrannosaurs.

These adaptations are perhaps taken to their most extreme conclusion in the Abelisauridae. This family of large theropods were in many ways the southern counterpart to the tyrannosaurs (the tyrannosaurs were found on the northern supercontinent of Laurasia, while the abelisaurs were found on the southern Gondwana). Both were the apex predators in their respective environments, and both displayed forelimb reduction and enlargement of the head, but the degree of limb loss in the Abelisauridae was far greater. Their elbows were non-functional, their fingers immovable, and their forearms were tiny. All of this suggests that the arms were completely vestigial; useless in any way.

However, the arms of Tyrannosaurus rex, while short compared to the animal’s body size, were clearly not vestigial. They display large muscle-insertion scars, and multiple individuals have been found to have stress fractures in the bones of the arms; indicative of repeated activity. It could be that the arms of the Tyrannosauridae were on the way out, slowly becoming smaller over time, until they would have eventually (without intervention from an titanic space rock) become as small and useless as those of the Abelisauridae. Though this sounds a likely explanation at first, there is a large, or rather, small problem. By the Campanian, and the arrival of the tyrannosaurs, the limb reduction had stopped. The tyrannosaurs around at this point already had tiny arms, and from this point, for around 15 million years, no further reduction in arm size seems to have occurred. Which leads to perhaps an more interesting question; why did the tyrannosaurs still need their small arms?

Though there are a number of competing hypothesise, from holding struggling prey to use in mating, there is no agreed upon use for the arms of a T.rex. We do, however, know exactly what that enormous head was used for, and that it alone ensured that this giant carnivore was far from ‘armless.

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Why it’s strange that the Ginkgo hasn’t Gink-gone

The ginkgo tree is a plant that any botanist or evolutionary biologist will come across at one point or another. Also known as the maidenhair tree, it is found throughout the world thanks to introduction by people over the last 300 years. This alone is not particularly special, many plant species are now found globally, being cultivated for their attractive appearance, or for use in the agriculture industry. What makes the ginkgo special is it’s genetic isolation.

The ginkgo lineage of trees used to be widespread but the maidenhair, or to call it by it’s scientific name Ginkgo biloba, is the only species left in it’s genus (Ginkgo). This genus is then the only one in it’s family, which is in turn the only remaining family in it’s order, which is the last order in it’s class. This class is also the only class with living representatives in it’s entire division. All relatives of this tree are now extinct.

The ginkgo lineage extends at least 270 million years into the past, the the closest living relative of the ginkgo must not have shared a common ancestor for at least that length of time. This closest living relative is something on which there is still debate, but the current evidence appears to suggest that the ginkgo forms a sister clade with the diverse division Cycadophyta, or as they are better known; the cycads.

The ginkgos used to be widespread, reaching maximum diversity during the Mesozoic, living alongside the dinosaurs. By the Cretaceous period, known for dinosaurs like T. rex and Triceratops alongside massive marine reptiles like the Mosasaurus, the group were in decline, but had already settled on an appearance that would change very little for the next 100 million years.

So why are they now only represented by a single species? It appears likely that the diversification of flowering plants, or Angiosperms, is the main culprit. The ginkgo used to be found in the riparian habitats alongside the banks of waterways, where the sediment was regularly disturbed, and well watered with good drainage. Ginkgos grow slowly and they have large, complicated seeds. The angiosperms are thought to have displaced them due to a collection of adaptations that saw them better suited to fill the riparian niche. These include a fast growth rate, allowing saplings to take hold before water action can remove them, and lightweight, simple seeds that can be easily transported by the water itself. Although it is not known for certain that these plants were the cause, the spread of the angiosperms does coincide with the decline of the ginkgo.

Since the Cretaceous the numbers have dwindled, until a new sequence of events nearly removed them entirely. Cooling temperatures around the planet saw the start of the last Ice Age. As glaciation increased, the remaining populations of ginkgo were covered by ice sheets. Through all this, only two, small populations are thought to have survived; in the refugia of south-western China, and on the West Tianmu Mountains of eastern China.

One of the most easily identifiable features of the G. biloba are it’s large meaty nuts, which give off a distinctive smell, often compared to that of vomit. These features indicate that the seeds have evolved to be spread by animals attracted to the smell, which would eat the nut and then excrete the seed in a new location. However, though some animals, such as dogs are attracted to the smell, ingestion of the nut often causes illness, a feature which doesn’t aid in the spread of the seeds. As no likely candidate can be found for the dispersal agent, it has been hypothesised that the glacial periods responsible for the reduction in the ginkgo habitat, may have also lead to the extinction of whatever animals used to spread the ginkgo seeds.

Despite all this, the ginkgo still clings on. This is largely due to human cultivation of the plant, which is thought to have began 1,000 years ago in China. They were likely first cultivated for their large and nutritious nuts. Though 1,000 years ago sounds early for plant cultivation, this was relatively late for a plant to first be grown by the Chinese, which suggests that by this point, very few of the trees remained, and as such, were not discovered for some time.

The plant is, perhaps surprisingly for it’s scarcity in the wild, very tenacious. They are highly resistant to fungal, viral and bacterial diseases, and are known for their insecticidal properties. Alongside this they are capable of surviving in heavily polluted and crowded areas, making them well suited to urban spaces. Perhaps there is no better demonstration of the ginkgo persistence than the case of seven G. biloba trees in the city of Hiroshima, Japan. This city is perhaps most well known for being the site of the first atomic bomb drop. While most of the city was levelled, seven maidenhairs survived at less than three kilometres from the centre of the blast, the closest of which was only 1130 metres away, surviving to prevent the Hosenbo Temple from completely collapsing.

Nowadays G. biloba is perhaps best known for the many alternative medicinal properties it is claimed to have benefits as wide ranging as aiding with blood pressure, tinnitus and dementia. Though there is no evidence of extracts of the plant being able to provide benefits to people with the first two problems, there are mixed results regarding dementia. Most studies conclude that extracts of the plant do not in fact help, while a couple of studies suggest that it shows promise.

Regardless of the supposed health benefits of the plant, I am happy to know that this link to a time long before the age of mammals is still alive, and that this unique organism, so unlike anything else alive today shows no sign of gink-going anywhere soon.

http://pubs.acs.org/doi/abs/10.1021/bk-1997-0658.ch007+http://science.sciencemag.org/content/157/3794/1270 http://www.unitar.org/hiroshima/sites/unitar.org.hiroshima/files/A-bombed%20trees%20worddoc%20as%20of%20Dec.%202011_1.pdf http://www.bioone.org/doi/abs/10.1666/0094-8373(2003)029%3C0084:ECITLF%3E2.0.CO%3B2 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3595029/ http://www.sciencedirect.com/science/article/pii/S1055790308002194 http://e360.yale.edu/features/peter_crane_history_of_ginkgo_earths_oldest_tree http://www.amjbot.org/content/88/7/1309.short

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Human remains from Jebel Irhoud, possibly the earliest known Homo sapiens. 

Figure 1:  Fossils from Jebel Irhoud on the left, demonstrating a more elongated skull than that of modern H. sapiens.

Figure 2: Facial reconstruction using known elements of a skull belonging to a Jebel Irhoud human.

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On the Origin of Our Species

It has been understood for some time that all of humanity is descended from a single common ancestor. This shared ancestor would be the first true example of Homo sapiens. For the last fifty years it was widely accepted that our species took it’s first steps in the shelter of the Great Rift Valley of Ethiopia; the Cradle of Mankind (not to be confused with the location often given the same name in South Africa). However, recent discoveries may have changed all of that.

In 1967, famed palaeoanthropologist Kamoya Kimeu discovered a fossil hominid (great ape) while on an expedition to the Omo river in Ethiopia. Louis Leakey, fellow palaeoanthropologist, identified the fossils as belonging to a Homo sapiens; a human being. When discovered they were believed to be 130,000 years old, and it wasn’t until thirty years later, using more modern dating techniques, that the fossils could be shown to be even older. They were closer to 200,000 years old, making them the oldest fossils of our species, giving further evidence to the African ancestry of humankind.

Only a couple of weeks ago, on the 8th June 2017, all that changed. An article published in the scientific journal “Nature” described what could be the oldest known fossils of our species, and they are not from Ethiopia. Instead, they were found about 60 miles west of Marrakesh, in Morocco, at a site known as Jebel Irhoud. The fossilised remains were more than 100,000 years older than the Omo remains of Ethiopia, and are thought to have belonged to people 315 ± 34 thousand years old.

In 1961 the first scientific expedition to the area was carried out, following the discovery of a human skull by a miner. Since then, the remains of at least five individuals have been found, comprising of more than twenty individual bones. Alongside the bones, sophisticated stone tools were discovered, as were the remains of animals that these ancient people are believed to have hunted.

These people looked very similar to modern humans; similar enough in fact, that you would probably struggle to notice a difference between the inhabitants of ancient Morocco, and modern people. That being said, there were some differences. The teeth in particular, were larger than those of most people alive today, and the skull was elongated, with a prominent brow. This elongated skull suggests that the brains of the Jebel Irhoud people were organised differently to the brains of modern humans.

Some do not agree with the classification of these people as Homo sapiens and instead believe them to be another species entirely. This is largely due to the shape and size of the chin and forehead, as a reduction in the size of these specific  features is sometimes used to define us as a species.

Regardless of whether these fossils should be considered a subspecies of H. sapiens, or an entirely new species, these findings are critical to our understanding of human evolution. They help to fill in the gaps in the evolution of our face shape and brain structure, and crucially, they demonstrate that we did not evolve in a single sheltered “cradle of humanity” but instead, if anywhere is to be called our cradle, it should be the entirety of Africa.