Juvenile Purussaurus Attacked Ground Sloth

The giant prehistoric caiman Purussaurus is regarded as one of the largest crocodilians to have ever lived.  How big this predator of the wetlands of South America during the Miocene actually was, remains open to debate.  It is known mainly through skull material, (the largest measuring 1.45 metres in length) and isolated teeth.  It is one of several types of crocodiles associated with the Pebas Mega-Wetland System from the north-western side of the continent.  Evidence of Purussaurus interaction with prey is limited, confined to a scarred and dented turtle shell, documenting a failed attack by an adult Purussaurus.

Fossilised Tibia with Pathology

However, scientists have reported the discovery of the lower leg bone of a ground sloth that bears the tell-tale tooth marks and scratches of an attack from a Purussaurus (P. neivensis).  The sloth identified as the genus Pseudoprepotherium very probably did not survive the encounter with a four-metre-long caiman.

A Life Reconstruction of the Purussaurus Attack

Picture credit: Jorge A. González

Attacked by a Juvenile Purussaurus

Thirteen million years ago, South America was an island continent.  There was no isthmus joining the landmass to North America and therefore no opportunity for placental carnivores (Order Carnivora), to enter South America.  There were terrestrial predators, such as pouched mammals from the marsupial lineage (sparassodonts) and three-metre-high terror birds (phorusrhacids).  Competing with these creatures for the title of most feared predator was the prehistoric caiman Purussaurus neivensis and its close relatives.

Writing in “Biology Letters”, palaeontologist Rodolfo Salas-Gismondi from Cayetano Heredia University (Peru), along with co-author François Pujos conclude that the bitemarks on a tibia found during field work in north-eastern Peru represent predation by a four-metre-long, juvenile Purussaurus.

The Location of the Fossil Find and Views of the Damaged Tibia

Picture credit: Salas-Gismondi and François Pujos (Biology Letters)

The picture (above), shows the  Pebas Mega-Wetland System around 13 million years ago (a).  Na069 represents the bonebed where the tibia was found and (b) shows this bonebed location on the Napo River in north-eastern Peru.   The left tibia of the ground sloth is shown in anterior (c i) and posterior (c ii) views with accompanying line drawings.  Photographs (d-f) highlight individual toothmarks and punctures.  The red dots map the bitemarks.

Extant and Extinct Crocodilian Skulls Compared Along with Purussaurus Teeth and Damaged Turtle Shell 

Picture credit: Salas-Gismondi and François Pujos (Biology Letters)

The picture (above) shows dorsal views of an adult extant black caiman (Melanosuchus niger) skull (a) compared with the skull of a juvenile Purussaurus neivensis (b), facing the damaged ground sloth tibia.  The skull of an adult P. neivensis (c) and teeth (d).  The shell of the prehistoric turtle Podocnemis with a portion of the carapace missing attributed to a bite from a huge Purussaurus brasiliensis (tip of jaw in line drawing).

A Substantial Meal for a 4-metre Crocodilian

The interpretation of the damaged tibia as evidence of predation from Purussaurus neivensis provides a rare insight into prehistoric crocodilian/prey behaviour.  It suggests that prior to reaching adult size, young individuals fed upon terrestrial mammals about the size of a capybara.  The ground sloth is estimated to have weighed around 78 kilograms that’s the about the same as an adult human male.  If a young Purussaurus could take down an sloth weighing as much as a man, it speaks volumes for that person’s chances if they ever would have met a fully grown Purussaurus.

To read an article commenting on contenders for the largest crocodile known to science: Which was the Largest Crocodilian of All Time?

Ancient Purussaurus from Venezuela had unique adaptations to help it move: Ancient Crocodilian Evolved Unique Specialisations Due to its Size.

Prehistoric crocodilians from Peru: Peruvian Paradise for Prehistoric Crocodiles.

The scientific paper: “Predation of the giant Miocene caiman Purussaurus on a mylodontid ground sloth in the wetlands of proto-Amazonia” by François Pujos and Rodolfo Salas-Gismondi published in Biology Letters.

The Everything Dinosaur website: Everything Dinosaur.

Newly Described Multituberculate Mammal Provides Clues to Middle Ear Evolution

A team of international scientists from the Chinese Academy of Sciences, the American Museum of Natural History (New York) and the Beipiao Pterosaur Museum of China, have described a new species of multituberculate mammal that once roamed and climbed in the forests of north-eastern China during the Early Cretaceous.  The small creature has been named Sinobaatar pani and its delicate middle ear bones have been preserved providing researchers with an opportunity to study the evolutionary development of hearing.

A Life Reconstruction of the Small Multituberculate Mammal S. pani

Picture credit: Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP)

How Do Mammals Hear?

The terrestrial placental mammal ear can be divided into three sections:

1. Outer ear – collects and directs sound via the ear flap (pinna) and the outer ear canal which ends in the eardrum (tympanic membrane).
2. Middle ear – which filters and amplifies sound waves directing them to the inner ear.  The middle ear contains three, delicate and tiny bones (ossicles), the anvil, hammer and the stapes (incus, malleus and stirrup).
3. Inner ear – consisting of the cochlea (organ for hearing) and the vestibular system (associated with balance).

The Inner Bones of a Model Mammal

Picture credit: Everything Dinosaur

It is the inner ear bones that act as a bridge between the eardrum and the oval window which is the opening to inner ear.  The cochlea, which is a hollow, spiral shaped bone transduces the sound waves into electrical signals (neural impulses), that are deciphered by the brain.

Sinobaatar pani and Middle Ear Evolution

Scientists believe that bones that were once part of the reptilian jaw slowly evolved into the three bones that are now found in the middle ear.  A joint research team led by Dr Mao Fangyuan from the Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) of the Chinese Academy of Sciences and Professor Meng Jin from the American Museum of Natural History were able to model the delicate middle ear bones of S. pani by using computerised tomography to access and view the fossils whilst they were still surrounded by matrix.

The powerful, rock-penetrating X-rays allowed the scientists to construct three-dimensional computer models of the malleus, incus and the stapes and to study their shape.

The images generated, permitted the comparing of the tiny inner ear bones of the Early Cretaceous multituberculate with the embryos of different types of living mammal (placental, marsupial and monotreme).

The Ancestral Phenotype of the Mammalian Middle Ear

Dr Mao explained that her colleagues were able to examine and assess the ancestral phenotype of the mammalian middle ear.  The researchers recognised that the three main Mesozoic mammalian groups (multituberculates, eutriconodontans and symmetrodontans) share a similar middle ear structure between the incus and malleus, which they termed the “braced hinge joint”.  Although they acknowledged that the middle ear may have evolved independently in several mammalian groups, they proposed that the braced hinge joint could represent a critical feature of the ancestral phenotype of the mammalian middle ear.

The Evolution of Mammalian Hearing

Dr Mao commented:

“There are two basic patterns of the middle ear in living mammals, represented by monotremes and therians [placentals and marsupials], respectively.  In the former, the middle ear is characterised by an “abutting contact” between the incus and malleus, which is distinct from the one in therian mammals where the incus-malleus articulation is saddle-shaped.”

Sinobaatar pani Provides Clues

The abutting pattern in monotremes and the saddle-shaped joint in therians may well be derived from the braced hinge joint linking the incus and malleus as observed in Mesozoic mammals.  At the very least, these fossil forms have narrowed the morphological gap between the middle ear of protomammals, formed by the postdentary bones lodged in the lower jaw, to the middle ear of extant mammals.  The researchers proposed that the surangular bone, which is another postdentary bone in protomammals, persisted in Mesozoic mammals; its fate in living mammals remains uncertain.

Writing in the National Science Review, the researchers demonstrate that middle ear morphologies in Mesozoic mammals represent different evolutionary stages with Sinobaatar showing an advanced inner ear configuration.  Furthermore, the evolutionary changes recorded in the Mesozoic mammals are largely consistent with the way the middle ear bones develop as living mammals grow, supporting the relationship between evolution and development.

The Holotype of S. pani and Three-dimensional Skull and Teeth Images

Picture credit: Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP)

Everything Dinosaur acknowledges the assistance of a press release from the Chinese Academy of Sciences in the compilation of this article.

The scientific paper: “Exploring ancestral phenotypes and evolutionary development of the mammalian middle ear based on Early Cretaceous Jehol mammals” by Fangyuan Mao, Cunyu Liu, Morgan Hill Chase, Andrew K Smith and Jin Meng published in National Science Review.

The Everything Dinosaur website: Everything Dinosaur.