Is Oculudentavis a Member of the Archosauria?

On the 11th March (2020), Everything Dinosaur posted up a blog article featuring the discovery of a remarkably well-preserved fossil skull that had been found in amber from northern Myanmar.

To read our blog post: Hummingbird-sized Dinosaur from Burmese Amber.

Oculudentavis khaungraae

Following publication, a number of academics have questioned the conclusions made by Xing et al with regards to this fossil representing a member of the Maniraptora.  It is certainly true that the validity of the interpretation of the fossil skull as maniraptoran has subsequently been challenged post publication (Wang Wei et al).  They comment that the shape of the skull is not unique to archosaurs, many lizards for example, show similar characteristics, the phylogenetic analysis is questioned, along with the apparent absence of an antorbital fenestra (an opening in the skull of all known archosaurs between the orbit and the naris).

The Very Bird-like Skull of Oculudentavis khaungraae But Can Appearances be Deceptive?

Picture credit: Xing et al (Nature)

It is suggested that the skull actually comes from a lizard and that the specimen is not from an archosaur at all.

The original publication noted that the spoon-shaped bones that make up the sclerotic ring were reminiscent of that seen in the eye sockets of lizards.  Scleral bones of this shape have never been found in a dinosaur or a bird, it is suggested that these bones support the idea that the fossil is that of a lizard and not a member of the Archosauria.

Trouble with the Teeth

The roots of the tiny teeth do not seem to be located in sockets in the jawbone (thecodont dentition).  This was a peculiar feature remarked upon by a number of academics once this paper had been widely circulated.  Teeth located in sockets is a characteristic of toothed-archosaurs such as crocodilians and the dinosaurs.  Other types of tetrapod also show this tooth morphology, but in Oculudentavis the teeth are not in sockets but either fused to the jaw (acrodont dentition) or located within grooves that can be found along the length of the jaw bones (pleurodont dentition).

The number of teeth in the jaw far exceeds that known for any type of ancient bird.  The tooth line extending under the eye-socket (orbit), is also highly unusual.  Such anatomical traits are associated with the Squamata (lizards and snakes), not with the Archosauria.

These arguments (along with others, such as the absence of feathers), have led some scientists to question the conclusions made in the original Nature publication.  Oculudentavis might not be a bird or a dinosaur, it might represent the preserved remains of a lizard.

An Anolis Lizard (A. equestris) Displaying its Throat Sac

Picture credit: Everything Dinosaur

The scramble to publish can sometimes lead to a lack of peer review opportunities and a foreshortening of pre-publication correspondence amongst academics.  When the “Nature” paper was published it certainly created a great deal of interest in the wider media.  Sadly, we suspect that any challenge to the original paper’s conclusions or subsequent revision will not attract anywhere near as much publicity.

We shall await developments.

Perhaps, in future we could refer to such controversies as “Oculudentavism”.

The Everything Dinosaur website: Everything Dinosaur.

Scottish Island – A Dinosaur Stomping Ground Complete with Stegosaurs

Scientists led by researchers from Edinburgh University have reported the discovery of dozens of dinosaur footprints preserved in exposed mudstones at two locations on the Isle of Skye.  The trace fossils preserve evidence of a variety of different types of dinosaurs, which helps palaeontologists to gain a better understanding of dinosaur distribution and diversity during the Middle Jurassic.

The tracks indicate a variety of dinosaur trackmakers, including bipedal theropods of various sizes, possible ornithopods and a quadrupedal ornithischian dinosaur, the tracks of which show a resemblance to the ichnotaxon Deltapodus, which is believed to represent a stegosaur.  If these prints do represent a member of the Stegosauria, then this is the first time that evidence for this type of armoured dinosaur has been discovered on the Isle of Skye.

Dinosaurs Congregating Around Mudflats on the Isle of Skye (Middle Jurassic)

Picture credit: Jon Hoad

Globally Important Fossil Discovery

During the Middle Jurassic the Dinosauria rapidly diversified and many new types evolved.  Unfortunately, the fossil record for terrestrial vertebrates from the Middle Jurassic is particularly poor.  The abundant trace fossils associated with the Isle of Skye are globally important, providing scientists with an opportunity to plot which types of dinosaurs are associated with this location.  Since the first dinosaur footprint in Scotland was found in the 1980s numerous tracksites representing several ichnotaxa have been recorded.

Writing in the academic journal PLOS One, the researchers describe two new tracksites from Rubha nam Brathairean (Brothers Point).  The sites are referred to as BP1 and BP3, site BP2, which revealed sauropod and theropod prints has already been reported upon: Isle of Skye Steps into the Jurassic Spotlight (2018).

An Aerial View and Line Drawing of BP1 Showing the Distribution of the Dinosaur Tracks

Picture credit: PLOS One

An Insight into the Fauna Around a Subtropical Coastal Area

The trace fossils at both BP1 and BP3 were formed when mudflats were exposed and dinosaurs walked over them.  Today, the mudstones comprise part of the Lealt Shale Formation of the Great Estuarine Group.  The dinosaurs inhabited a coastal environment in what was a subtropical climate.  The fossil bearing rocks might be exposed on the coast today, but the climate on the Isle of Skye today is very different to what it was like around 170 million years ago.  The notorious Scottish weather prevented the researchers from using drones on several occasions in their attempts to photograph and map the sites.

An Aerial View of BP3 and Accompanying Line Drawing Showing the Various Dinosaur Tracks

Picture credit: PLOS One

The tracks at both sites are preserved as impressions (concave epirelief) and the signs of mud cracks associated with the trace fossils suggest that the surfaces of both sites were briefly exposed before being quickly reclaimed by the return of brackish water.  Whilst the dinosaur tracks at BP3 were being mapped, an articulated pterosaur skeleton was found in the overlying limestone layer.  The state of the bones (largely unfractured) and the articulated skeleton suggest that the overlying limestone was deposited in a relatively low energy environment.  The pterosaur is currently being studied and will be covered in a future scientific paper.

Evidence of a Scottish Stegosaur

One of the tracks at location BP1 (BP1_Twy-01) shows a series of prints made by a quadrupedal dinosaur.  Although the tracks are a little distorted, distinctions between the pes (rear foot) and the manus (front foot) can be made.  The researchers conclude that these prints and other, single prints from this site are similar to the ichnotaxon Deltapodus.  Evidence of a potential armoured dinosaur from the Isle of Skye adds to the diversity of dinosaur types known from this location.

Mapping the Ornithischian Tracks (Potential Stegosaur – Ichnotaxon Deltapodus)

Picture credit: PLOS One

Together these two new tracksites demonstrate the wide variety of different types of dinosaur present in the area and will help palaeontologists gain more data on the early evolution and radiation of the Dinosauria.  As the researchers conclude, essentially BP1 and BP3 provide a snapshot of a “day in the life” of a Middle Jurassic ecosystem.

A Palaeontological Puzzle

No sauropod tracks have been described to date from BP1 or BP3, although they have been found at BP2.  The absence of sauropod evidence could be coincidental or perhaps an indication that during the time the mudflats were exposed, these large herbivores were not present in the area.  Environmental factors could help to explain the absence of sauropods.  Site BP2 represents a shallow lagoon, whilst BP1 and BP3 represent mudflats.  The sauropods may have preferred to occupy the lagoons.

The scientific paper: “Novel track morphotypes from new tracksites indicate increased Middle Jurassic dinosaur diversity on the Isle of Skye, Scotland” by Paige E. dePolo, Stephen L. Brusatte, Thomas J. Challands, Davide Foffa, Mark Wilkinson, Neil D. L. Clark, Jon Hoad, Paulo Victor Luiz Gomes da Costa Pereira, Dugald A. Ross and Thomas J. Wade published in the journal PLOS One.

Visit the Everything Dinosaur website: Everything Dinosaur.

Oculudentavis khaungraae – Tiny Fossil Skull Could Represent the Smallest Dinosaur

Team members at Everything Dinosaur had been aware that something big was brewing amongst those members of the academia with an interest in vertebrate palaeontology.  An academic paper published in the journal “Nature”, describes the beautifully preserved but very small skull of a theropod dinosaur preserved in Cretaceous amber from northern Myanmar (Burma).  The fossil might just represent the smallest dinosaur known to science.

The fossil might be tiny but this wonderful discovery could have very big implications when it comes to understanding how miniaturisation occurs within vertebrates.  It also provides yet another remarkable insight into the types of creatures that shared the Late Cretaceous with non-avian dinosaurs and pterosaurs.  Named Oculudentavis khaungraae it probably weighed about as much as the smallest living bird, the Bee Hummingbird (Mellisuga helenae), which weighs about two grammes, that is about half the weight of a single sheet of A4 paper.

The Polished Amber Nodule Reveals a Bounty of Preserved Material Including the Tiny Skull of Oculudentavis khaungraae

Picture credit: Lida Xing et al

When we Say Tiny we Mean Tiny!

The amber nodule containing the beautifully preserved skull, complete with the tongue, measures a little over three centimetres in length.  The skull, with its tiny jaws, lined with miniscule but pointed teeth, measures less than 1.5 cm long.  It is estimated that Oculudentavis had a total body length including tail of about 9 cm.  Palaeontologists have speculated that “microsaurs” – tiny dinosaurs co-existed with the giants, just as African spiny mice (genus Acomys), can be found today in the same habitats as the largest, terrestrial animals – elephants (Loxodonta).

Tantalising fossil footprints had been found that hinted at the possible existence of “microsaurs” or “tinysaurs” if you will – assuming of course that these trace fossils were not made by very young animals, with a lot of growing to do.

To read about the smallest dinosaur tracks: Smallest Dinosaur Tracks Known to Science Discovered.

A Life Reconstruction of the Tiny Oculudentavis khaungraae

Picture credit: Han Zhixin

Oculudentavis – a Possible Member of the Enantiornithes Clade

Where Oculudentavis sits on the Dinosauria family tree is uncertain.  Whilst the cranial material has provided the authors, which include researchers from the Chinese Academy of Sciences and the Natural History Museum of Los Angeles County, with a wealth of data, interpreting the taxonomy is somewhat troublesome.  If it is a member of the Maniraptora, this large clade includes dinosaurs as well as true birds (avians).  Just how closely related to the birds or whether it is a highly specialised dinosaur remains open to debate.  Oculudentavis could be an enantiornithine bird, an extinct lineage of avians that were the most common birds of the Cretaceous, or it might be more closely related to the dinosaur end of the Maniraptora spectrum.

It may be small, but the specimen does not represent a juvenile or young animal, the skull bones are sufficiently fused for scientists to confidently state that the tiny creature is an adult or at least a sub-adult.

Eye Tooth Bird

Oculudentavis demonstrates a suite of unusual anatomical characteristics.  The eyes for example, are located on the side of the head, helpful for providing all round vision but not capable of delivering stereoscopic vision to aid in the capture of small prey.  The orbits are huge, the eyes would have bulged out of the head somewhat and the bones that make up the sclerotic ring (the circle of bones in the orbit) are spoon shaped, which is a morphology previously only known in lizards.  These scleral ossicles form a cone, similar to the bones in the eyes of owls, so it can be deduced that just like owls, Oculudentavis had exceptional vision.

What is a little more surprising is that the opening at the centre of the ossicles is narrow and restricted.  This would have limited the amount of light coming into the eye, so unlike most owls this little Cretaceous creature probably operated in bright, sunny conditions – it was most likely diurnal.

A Computer Generated Image of the Skull of O. khaungraae

Picture credit: Xing et al (Nature)

Huge Eyes

Such is the size and extent of the eye socket, that they extend over the jaws and some of the upper jaw teeth are located directly under the orbit.  It is this characteristic and those large eyes, that gives Oculudentavis its name, from the Latin for eye “oculus”, “dentes” teeth and “avis” for bird.  The species name honours Khaung Ra who donated the specimen to the Hupoge Amber Museum.

The Fossil Specimen (HPG-15-3) with Computer Generated Images and Accompanying Line Drawings

Picture credit: Xing et al (Nature)

The photograph (above), shows the amber piece (a), a scan of the skull (b) with line drawing (c).  Images d, f and h represent other views of the scans, whilst images e, g and h represent the associated line drawings.  The amber specimen comes from the Angbamo site, close to the township of Tanai (Myitkyina district, Hukawng valley, Kachin province) in northern Myanmar.  The strata are believed to represent the Cenomanian stage of the Late Cretaceous, the fossil therefore is approximately 99 million years old.

Living on an Island?

The fossil discovery represents the smallest known dinosaur of the Mesozoic and it highlights the importance of amber as a means of permitting scientists to gain an insight into the ecology of a habitat thanks to the preservation of small animals and other material that would not necessarily have been preserved under other taphonomic processes.  Specimens preserved in amber are rapidly emerging as an exceptional way to study tiny vertebrates from the Late Cretaceous.

Miniaturisation in animals is commonly associated with living in isolated environments where resources are limited.  The tiny Oculudentavis lends weight to the idea that the amber deposits in northern Myanmar were produced in forests that existed on islands.  In addition, the size and morphology of this species suggest a previously unknown bauplan and a previously undetected ecology.

To read more articles about amazing fossil discoveries made in Burmese amber:

Ammonite shell discovery: Ammonite Shell Found in Amber Nodule.

Baby snake found in amber: Baby Prehistoric Snake – Xiaophis myanmarensis Preserved in Amber.

Ancient lice feeding on feathers: Lice Feeding on Feathers Entombed in Amber.

Tiny frogs preserved in Cretaceous amber: Tiny Frogs Trapped in Cretaceous Amber.

Oculudentavis and Post Publication Doubts

Following publication, a number of academics have questioned the conclusions made by Xing et al with regards to this fossil representing a member of the Maniraptora.  It is certainly true that the validity of the interpretation of the fossil skull as maniraptoran has subsequently been challenged post publication (Wang Wei et al).  They comment that the shape of the skull is not unique to archosaurs, many lizards for example, show similar characteristics, the phylogenetic analysis is questioned, along with the apparent absence of an antorbital fenestra (an opening in the skull of all known archosaurs between the orbit and the naris).

Is this the Skull of a Lizard?

It is suggested that the skull actually comes from a lizard and that the specimen is not from an archosaur at all.

The original publication noted that the spoon-shaped bones that make up the sclerotic ring were reminiscent of that seen in the eye sockets of lizards.  Scleral bones of this shape have never been found in a dinosaur or a bird, it is suggested that these bones support the idea that the fossil is that of a lizard and not a member of the Archosauria.

The roots of the tiny teeth do not seem to be located in sockets in the jawbone (thecodont dentition).  This was a peculiar feature remarked upon by a number of academics once this paper had been widely circulated.  Teeth located in sockets is a characteristic of toothed-archosaurs such as crocodilians and the dinosaurs.  Other types of tetrapod also show this tooth morphology, but in Oculudentavis the teeth are not in sockets but either fused to the jaw (acrodont dentition) or located within grooves that can be found along the length of the jaw bones (pleurodont dentition).

The number of teeth in the jaw far exceeds that known for any type of ancient bird.  The tooth line extending under the eye-socket (orbit), is also highly unusual.  Such anatomical traits are associated with the Squamata not with the Archosauria.

These arguments (along with others, such as the absence of feathers), have led some scientists to question the conclusions made in the original Nature publication.  Oculudentavis might not be a bird or a dinosaur, it might represent the preserved remains of a lizard.