An undergraduate student conduction routine fieldwork discovered mammal remains belonging to the lineage that led to humanity.

Credit: Mark Witton, University of Portsmouth

On a summer’s morning in 2015, University of Portsmouth undergraduate student Grant Smith began a typical research day. He traveled to a known fossil hotspot — the Purbeck Group Mammal Bed on the coast of Durlston Bay in east Dorset, UK — to gather samples. After the day’s work, he returned to the university with his haul.

Laborious and time-consuming sifting through rocks and other materials followed, when suddenly Smith saw the first of two teeth. Based on the site’s geology, he and his colleagues determined that the teeth were about 145 million years old.
 
“We already knew the age, but as we progressed through determining what these animals were, my excitement increased,” Smith told Seeker. “I think the significance of the find still hasn’t quite dawned on me.”
 
The teeth belonged to the earliest mammal ancestors of all humankind, according to a paper on the findings published in the journal Acta Palaeontologica Polonica.
 
Analysis of the fossils determined that the teeth are the earliest, undisputed remains of mammals belonging to the lineage that led to Homo sapiens. They are also ancestral to most mammals alive today, including species as diverse as the blue whale and pygmy shrew.
 
In fact, the mammals during their lifetimes looked a bit like shrews, lead author Steven Sweetman, also from the University of Portsmouth, told Seeker.
 
“Almost all Mesozoic mammals were shrew to rat sized, and their body plans are known from skeletons discovered in other parts of the world, including China where soft tissues are also preserved,” Sweetman explained.

Credit: University of Portsmouth

Sweetman said the animals represented by the newly found teeth “are not placental mammals, but are the earliest yet known of the group of mammals from which placentals arose, the eutherians. They are thus the ancestors of most of the extant mammals including shrews, horses, bats, and whales.”
 
Only two groups of modern animals, monotremes (platypus and echidnas) and marsupials (animals typically with pouches, such as opossums and kangaroos), do not have placentas.
 
Sweetman, senior author David Martill, and Smith named the two new eutherians Durlstotherium newmani and Durlstodon ensomi. The former was named after Charlie Newman, the landlord of the Square and Compass pub in Worth Matravers, close to where the fossils were discovered. Durlstodon ensomi’s etymology is in honor of Paul Ensom, a geologist who has studied Dorset for many years.

Credit: University of Portsmouth

In addition to looking like furry shrews and rats, the mammals are also thought to have eaten insects. The somewhat more robust tooth of Durlstodon suggests that while it was primarily an insectivore, it might have supplemented its diet with plants.
 
As for length and weight, “To make a guess, Durlstotherium was probably mouse-sized, and Durlstodon was juvenile rat-sized,” Sweetman said.
 
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The estimates are based on other known mammal fossils, some of which are even older than the Dorset finds. Animals such as 160-million-year-old Juramaia and 125-million-year-old Eomaia and Acristatherium from China had a similar shrew-like body size and resemblance. Remains for some of these early mammals have included preserved soft tissues and fur. 

Sweetman said eutherians probably evolved from animals like Juramaia. Where and when they first emerged, however, remains a mystery. A clue, however, is that the teeth of Durlstotherium and Durlstodon are highly derived, suggesting that the genus Eutheria originated sometime during the Jurassic Period (200-145 million years ago).

Credit: University of Portsmouth

The researchers also believe that man’s earliest ancestors were nocturnal, and for good reason.
 
“A variety of dinosaurs has been recorded from the Purbeck Group, including ornithopods, heterodontosaurids, sauropods, and theropods,” Sweetman said. “Among the latter was the rather small dromaeosaurid Nuthetes destructor. This would have posed a serious threat to the Purbeck mammals, including ours. However, there were a number of small crocodyliforms that may also have preyed on the Purbeck mammals.”

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Substantial other evidence supports that pressure exerted by dominant predators like these crocodile ancestors and dinosaurs contributed to the earliest mammals staying relatively small, scrappy, and nocturnal. Another recent study concluded that mammals only became diurnal, or active during the day, after non-avian dinosaurs became extinct.
                 
Smith and his colleagues suspect that at least one of the two newly discovered mammals was a burrower. This would have helped it to escape toothy predators. The tactic could have also dramatically changed the future fate of animals.

Credit: University of Portsmouth

A massive asteroid impact and heightened volcanism are thought to have wiped out non-avian dinosaurs and 80 percent of all animal species 65 million years ago.
 
In terms of the survivors, Sweetman said, “It has been suggested that burrowing forms and those able to aestivate or hibernate were most likely to survive.”
 
Techniques that first possibly evolved to allow avoidance of dominant predators like dinosaurs could then have permitted mammals to survive the extinction. In other words, the under-Earth former “underdogs” seem to have won this evolutionary battle for survival.
 
The researchers are eager to learn more about these ultimate survivors at the bottom of humanity’s family tree. They would like to unearth a complete jaw or at least additional teeth belonging to the two species.
 
 “I’m just keen to get back out and hopefully find more evidence of these animals,” Smith said.

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