Introduction:

Oviraptorosauria is a group of theropod dinosaurs that first appeared around 125 million years ago, during the early Cretaceous period. They evolved into diverse forms before being wiped out 66 million years ago in the extinction at the end of the Cretaceous, an event that killed all the non-avian dinosaurs. The first known oviraptorosaurian — called Oviraptor — was unearthed in 1923 by the American Museum of Natural History expedition team in the Gobi Desert of Mongolia. It was found near some eggs assumed to be from the dinosaur Protoceratops, which led researchers to infer that Oviraptor stole the eggs of other species — the name means ‘egg thief’. This interpretation was refuted after researchers discovered oviraptorosaurian fossils brooding nests of their own eggs, but this iconic name remains. This is perhaps one of the most well-known incidents in the history of dinosaur research. However, oviraptorosaurians are more than just the main character of this story — they are anatomically and ecologically diverse, and key to the understanding of dinosaur–bird evolution.

Definite oviraptorosaurian fossils are known from Asia and North America, comprising more than 40 named genera. Although some fragments found in Argentina and Australia were once classed as oviraptorosaurians, this was later refuted. The earliest known oviraptorosaurian was Incisivosaurus, from Jehol Group in northeastern China (Fig. 1). It is a small dinosaur with a skull around 10 centimetres long. However, some oviraptorosaurians became enormous. Anzu from Hell Creek Formation in Montana represents the largest known oviraptorosaurian from North American, estimated to be about 3.5 metres long. Gigantoraptor, the largest of all oviraptorosaurians (and probably of all bird-like dinosaurs), reached a body length of about 8 metres (Fig. 2A). Both Anzu and Gigantoraptor belong to a subgroup of oviraptorosaurs called Caenagnathidae, which is one of  the two major lineages of later-diverging oviraptorosaurians (the other is Oviraptoridae). Caenagnathids have been discovered in North America and Asia, whereas oviraptorids are only known from Asia. Nemegt Basin of Mongolia is one of the most prolific regions for oviraptorosaurians, yielding Avimimus, one caenagnathid and several oviraptorids.

Dinosaurs without teeth

Caenagnathids and oviraptorids are some of the most peculiar dinosaurs. Unlike most of their theropod relatives, they are completely toothless (Fig. 2). Theropods that have lost their teeth are thought to have had a rhamphotheca — a beak made of keratin, as seen in living birds and turtles. Researchers think this because they have found some fossils with exceptionally preserved beaks (including a specimen of the ornithomimosaur Gallimimus). Other fossils have numerous holes called foramina at the front of their upper and lower jaws. In modern beaked animals, foramina provide nutrients to the beak, so in extinct dinosaurs they probably had a similar function.

The earliest oviraptorosaurian, Incisivosaurus, had teeth in its upper and lower jaws (Fig. 1), although it had fewer teeth than more ancestral theropods. Caudipteryx, another oviraptorosaurian that diverged from other species early on, had teeth only at the very front of its upper jaw. Later-diverging groups, such as Avimimus, caenagnathids and oviraptorids, are all toothless. However, their skulls were different in other ways. For example, the general shape of caenagnathid and oviraptorid lower jaws, or mandibles, are very different. Caenagnathid mandibles are usually slender with an upturned tip at the front, whereas those of oviraptorids are turned down at the front and more robust in general. Caenagnathids also have a special protrusion in their mandibles called a lingual triturating shelf, which probably helped them to pick up food. This structure is absent in Avimimus and oviraptorids, except for a small Mongolian oviraptorid called Gobiraptor, which has a weakly developed shelf.

Some later-diverging oviraptorosaurians have a tall crest on their skulls, similar to that of a cassowary (Fig. 2B), made up of several individual bones. Among the caenagnathids, only one species has been found with a substantial portion of the cranium preserved: Anzu, which has a prominent crest. More oviraptorids have been found with a complete cranium, and a number of these are crested. Rinchenia and Corythoraptor are the best examples of crested oviraptorids (Fig. 2B): their crania are as high as their skulls are long, and sometimes even higher. Given that we can’t study the behaviour of extinct animals directly, it remains unclear why some oviraptorosaurians evolved a cranial crest. But by comparing the crests of Corythoraptor and cassowaries, researchers have suggested that oviraptorosaurians might have used their crests to attract mates.

Diets of oviraptorosaurians

It is unclear what oviraptorosaurians ate, because their skeletons lack obvious adaptations to a particular diet. The diets of extinct dinosaurs can be inferred using a variety of approaches, one of the most common being looking at their teeth. For example, hadrosaurs had numerous teeth specialized for grinding vegetation, whereas the curved, serrated teeth of dromaeosaurids suggest a carnivorous diet. Some early-diverging oviraptorosaurians that had teeth were preserved with stones in their stomachs, something often seen in herbivorous dinosaurs, implying that they ate vegetation to some extent.

However, there is no direct evidence for the diets of later-diverging oviraptorosaurians, so they have to be inferred by other means. So far, there are two major hypotheses. The first suggests that both caenagnathids and oviraptorids were herbivorous, but that oviraptorids, with their more robust jaws, ate tougher plants. The second hypothesis proposes that oviraptorids were herbivorous, whereas some caenagnathids were omnivorous or even carnivorous. Caenagnathid hindlimbs were adapted for running, and these, along with the upturned tips of their lower jaws, have been considered adaptations for predation. Functional analysis of how their jaws would have moved has also reinforced this hypothesis. Although the diets of caenagnathids are still uncertain, it seems generally accepted that most oviraptorids were adapted to herbivory.

Implications for dinosaur evolution

Oviraptorosaurians are always described as bird-like dinosaurs, but they are not birds. Analyses of their evolutionary relationships suggest that Oviraptorosauria are part of Pennaraptora (Fig. 3), a group including the birds and non-avian theropods that possess bird-like feathers with a central quill. Given the close relationship with birds, studying oviraptorosaurians is essential to understanding the ancestral condition of various aspects in early bird evolution, one of which is reproductive biology. Studies of oviraptorosaurian eggs demonstrate that several reproductive features found in birds are also shared by non-avian dinosaurs; these include coloured eggs, ovulation of a single egg per oviduct per time, and egg-brooding behaviour. Oviraptorosaurians have also been used as models for studying the loss of teeth in theropods, a significant macroevolutionary change accompanying the rise of modern birds. Oviraptorosaurians are not the most iconic dinosaurs, but the knowledge stemming from their fossils should not be underestimated.

Suggestions for further reading

Funston, G. F., Mendonca, S. E., Currie, P. J. & Barsbold, R. Oviraptorosaur anatomy, diversity and ecology in the Nemegt Basin. Palaeogeography, Palaeoclimatology, Palaeoecology 494, 101–120 (2018). DOI: https://doi.org/10.1016/j.palaeo.2017.10.023

Lü, J., Li, G., Kundrát, M., Lee, Y. N., Sun, Z., Kobayashi, Y., Shen, C., Teng, F. & Liu, H. High diversity of the Ganzhou Oviraptorid Fauna increased by a new “cassowary-like” crested species. Scientific Reports 7, 6393 (2017). DOI: https://doi.org/10.1038/s41598-017-05016-6

Ma, W., Wang, J., Pittman, M., Tan, Q., Tan, L., Guo, B. & Xu, X. Functional anatomy of a giant toothless mandible from a bird-like dinosaur: Gigantoraptor and the evolution of the oviraptorosaurian jaw. Scientific Reports 7, 16247 (2017). DOI: https://doi.org/10.1038/s41598-017-15709-7

Xu, X., Cheng, Y. N., Wang, X. L. & Chang, C. H. An unusual oviraptorosaurian dinosaur from China. Nature 419, 291–293 (2002). DOI: https://doi.org/10.1038/nature00966

Zelenitsky, D. K. Reproductive traits of non-avian theropods. Journal of the Paleontological Society of Korea 22, 209–216 (2006).

Introduction:

Ichnology is the study of trace fossils, the physical evidence for the activities of organisms that lived millions of years ago. Trace fossils depict activities such as walking, resting, feeding and burrowing, which can be represented by tracks ranging from recognizable large footprints to long, grooved trails (Fig. 1). One organism can be responsible for multiple trackways: for example, the extinct invertebrate arthropods called trilobites are known to have produced the burrowing trace Cruziana as well as the resting trace Rusophycus.

This field of study supplements the fossil record from preserved skeletons and soft tissue, allowing us greater insights into the lifestyles of ancient organisms and the environments they lived in. In particular, ichnology is known to be important when considering the footprints from tetrapods (extinct and living reptiles, amphibians and mammals with four limbs), which can enhance sparse records of skeletal material in the geological past. This article will focus on footprints from amphibians and reptiles, including some that are particularly abundant from the Carboniferous and Permian periods (356 million to 252 million years ago) of what is now Great Britain.

Globally, tetrapod footprints can be found in Nova Scotia in Canada, New Mexico in the United States, and numerous sites in Germany, Italy, France, Spain, Poland, Morocco and Argentina. Many types of footprints can be found across Europe and North America; these include the tracks that have been named Amphisauropus, Dimetropus, Dromopus, Hyloidichnus, Ichniotherium and Limnopus (Limnopus), along with its smaller morph Limnopus (Batrachichnus).

Key terminology

• Ichnite: fossilized footprint, including actual footprints, trackways and trails.
• Ichnotaxon (plural ichnotaxa): the name given to a particular fossil footprint, usually defined as ichnogenus followed by ichnospecies (see below).
• Ichnogenus and ichnospecies: ichnotaxa can be classified in a similar way to organisms, with Latin names for a genus and species. For example, in the trilobite burrowing trace Cruziana problematica, Cruziana is the ichnogenus and problematica is the ichnospecies.
• Manus: forefoot of a tetrapod (the ‘hand’).
• Pes: hindfoot of a tetrapod (the ‘foot’).
• Ichnofacies: a collection of trace fossils that can be used to get an idea of past environmental conditions, such as water depth, current energy, salinity and oxygenation.

Formation: making a fossil

Footprints are best preserved in soft mud or sand near to a body of water, with moist sediment enabling the particles to stick together and maintain the shape of the footprint. The imprint must be quickly covered by sediment to fill it in and preserve the impression before it can be eroded by water or wind. If the infilling sediment has different properties from the sediment the footprint is in — for example, if mud fills a sand mould — this can help to differentiate the impression. With time, burial and compaction from overlying sediment allow prints to be preserved along the bedding planes of rocks.

Methodology: if those feet could talk

Sometimes, when out in the field exploring geology, we can stumble on trace fossils. At other times, we search through literature to find particular rock types that yield trace fossils, then study geological maps to work out where there are outcrops of similarly aged rocks.

When we have found the trace fossils, we can make 3D models of them using techniques such as photogrammetry (taking multiple photographs of a specimen from different angles to input into software and generate a model) and digital scanning. This enables us to see more details of the trace fossils, which helps us to identify them and observe the interactions between the organisms that made them.

We can also take measurements of the footprints and trackways (Figs 2, 3) to help estimate:

• Trackmaker size: measuring between the hands and feet estimates the distance between the shoulders and hips (called the glenoacetabular distance*), which can be doubled as an overall guide to the size of the animal including its head and tail.
• Trackmaker speed: the spacing between footprints can indicate the movement of the organism at the time of formation: the larger the spacing, the greater the speed.
• Gait: this refers to whether the limbs sprawl outwards to the sides of the body (as seen in tetrapods that walk by moving opposite limbs) or the animal has an erect stance, as seen in certain dinosaurs that walked on two feet (bipedal).

Significance: the perks of being a trace fossil

The greatest benefit of ichnology is that it supplements the relatively limited record of tetrapod body fossils. Bones and footprints are preserved under different conditions, so the two are rarely found together. As a result, ichnology allows us to gain insights from a wider geographical distribution than do body fossils alone. It generates the possibility of reconstructing past environments and communities through snapshots of the organisms present in a given environment, and their abundance at a particular time.

Because the footprints are in a slab of rock, it is possible to analyse the composition of the sediment, which can provide more information about the past environmental conditions. It is also possible to study trace fossils left by other organisms in the vicinity, allowing more accurate recreation of palaeocommunities and the interactions between organisms living at a precise moment in time (Fig. 4). This could include evidence about movement in herds, predation and how organisms escaped environmental hazards. It might also help us to understand the environment, because certain organisms might be known to have lived in particular conditions of water depth, salinity and sediment oxygenation.

Furthermore, comparisons can be made with sites around the world where similar footprints are found, to help determine distributions of ichnotaxa (and therefore the trackmakers) and their migrations across continents in the geological past.

Issues: disappearing without a trace

A key issue with ichnology is that footprints can be affected by numerous factors that lead to different (or new) identifications, so a single type of track might be classified as more than one ichnotaxon. This makes identification — and attempts to match a footprint to the bones of a trackmaker — difficult. Confounding factors include differences in:

• Sediment wetness: the sediment must be sufficiently moist to hold the shape of the footprint.
• Clay content: sediment containing soft, wet clay can adhere to the organism’s foot and create a sucker effect when the limb is lifted, causing deformation of the footprint.
• Environmental stability: wind and water erosion might occur before the mould is infilled, so the footprint isn’t preserved, or is preserved incompletely.
• Subjectivity of researcher: those identifying the specimen might see differences in the footprint’s features that affect their conclusions.

Multiple traces can be produced by a particular organism and a specific footprint morphology might have been produced by numerous biological taxa. Therefore, attributing footprints to a particular species can prove challenging and might not reflect genuine evolutionary patterns. Moreover, organisms that were more active than others or had larger footprints can be over-represented in the fossil record, hence biasing estimates of population sizes and the taxa present in a location.

We also see bias in distribution over time and location: only areas that experienced the right preservational conditions reveal the steps of ancient organisms. Additionally, there is human bias in terms of where people have explored the relevant-aged and exposed rocks through mining and engineering.

People who collected footprint fossils in the early days of geological exploration did not always make sufficient notes to give detailed information about which rock unit it came from, so we might not be able to ascertain the age and environmental conditions of the specimen. Similarly, some specimens are noted in early papers but have since been lost to science.

Case study: tetrapods of Great Britain

The Carboniferous period in Great Britain saw a shift in environmental conditions from humid rainforests to semi-arid climates, often linked to the formation of a mountain belt (the Variscan Orogeny) to the south of the country. The aridity created ‘Red Beds’ in Britain, notably red-tinged sandstones.

This environmental transition affected the biological development of organisms. Amphibians must lay their eggs in water, and so found the aridity unsuitable, whereas reptiles were better adapted to the emerging terrestrial environment because their eggs had hard shells and so could survive on land. Thus, we expect to see a marked change from amphibian to reptile dominated systems from the late Carboniferous Westphalian age (around 315 million years) and into the early Permian Cisuralian age (299 million to 273 million years ago).

Records of tetrapod footprints in Britain are limited, mainly occurring as a few specimens in various museums around the country. Notable exceptions include the extensive collection from the Warwickshire Group of the West Midlands, particularly the well-studied specimens from Alveley, Shropshire, and Hamstead, Birmingham.

Studies of British tetrapods, in fact, reveal a time and space bias towards specimens from between part of the Westphalian called the Westphalian D and the lower Stephanian age (311 million to 304 million years ago) of the West Midlands, highlighting that this area experienced optimal conditions for preservation at this time. However, the aforementioned issues with ichnology can explain some of this apparent bias.

Nevertheless, the period 311 million to 304 million years ago represents the greatest diversity of tetrapods in terms of ichnogenera and ichnospecies, including Dimetropus leisnerianus, Dromopus lacertoides, Hyloidichnus bifurcates and Limnopus (Batrachichnus salamandroides). Other species found in Britain include Dimetropus salopensis, Anthicnium salamandroides, Chelichnus duncani, Limnopus vagus, Limnopus (Batrachichnus plainvillensis) and ?Laoporus ambiguous.

Certain ichnotaxa in these assemblages, such as Dromopus (Fig. 5), are indicative of terrestrial ecosystems. They are often associated with dune environments, hence signalling the aridifying trend in the late Carboniferous.

However, as a standalone data set, British tetrapod trace fossils provide insufficient evidence to reconstruct evolutionary transitions from amphibians to reptiles in response to the changing environment. This does not necessarily oppose the hypothesis that this transition occurred, but rather is a lack of data to support it. By comparing British ichnotaxa with those found at sites around the world, it is possible to infer that Britain experienced the same aridification seen elsewhere, and that this would have resulted in the abundance of amphibians decreasing while reptiles diversified.

Summary

As a supplement to tetrapod body-fossil records, ichnology can offer an illuminating insight to the environment in which organisms lived and how the organisms interacted with each other. Through detailed study, footprints can be used to reconstruct palaeocommunities in ways that singular body fossils cannot, and comparison across global sites can highlight distribution and migratory patterns as well as evolutionary developments through key geological intervals. Although trace fossils are sometimes problematic to identify, ichnology has undeniable potential to enhance palaeontological interpretations.

One small step millions of years ago can now be one giant leap for science.

Suggestions for further reading

Bromley, R. G. Trace Fossils: Biology, Taphonomy and Applications 2nd edn (Chapman & Hall, 1996).

Clack, J. A. Gaining Ground: the Origin and Evolution of Tetrapods 2nd edn  (University of Indiana Press, 2012).

Delair, J. B & Sarjeant, W. A. S. History and bibliography of the study of fossil vertebrate footprints in the British Isles: Supplement 1973–1983. Palaeogeography, Palaeoclimatology, Palaeoecology 49, 123–160 (1985). DOI: 10.1016/0031-0182(85)90007-0

Falcon-Lang, H. J., Gibling, M. R., Benton, M. J., Miller, R. F. & Bashforth, A. R. Diverse tetrapod trackways in the Lower Pennsylvanian Tynemouth Creek Formation, near St. Martins, southern New Brunswick, Canada. Palaeogeography, Palaeoclimatology, Palaeoecology 296, 1–13 (2010). DOI: 10.1016/j.palaeo.2010.06.020

Leonardi, G. Glossary and Manual of Tetrapod Footprint Ichnology. (Ministerio Minas Energia, Departmento Nacional da Producao Mineral, 1987).

Meade, L. E., Jones, A. S & Butler, R. J. A revision of tetrapod footprints from the late Carboniferous of the West Midlands, UK. PeerJ, 4, e2718 (2016). DOI: 10.7717/peerj.2718

Milner, A. R. The Westphalian tetrapod fauna; some aspects of its geography and ecology. Journal of the Geological Society of London 144, 495–506 (1987). DOI: 10.1144/gsjgs.144.3.0495

Niedźwiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M. & Ahlberg, P. E. Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463, 43-48 (2010). DOI: 10.1038/nature08623

Sahney, S., Benton, M. J. & Falcon-Lang, H. J. Rainforest collapse triggered Carboniferous tetrapod diversification in Euramerica. Geology 38, 1079–1082 (2010). DOI: 10.1130/G31182.1

Tucker, L. & Smith, M. P. A multivariate taxonomic analysis of the late Carboniferous vertebrate ichnofauna of Alveley, southern Shropshire, England. Palaeontology 47, 679–710 (2004). DOI: 10.1111/j.0031-0239.2004.00377.x

Introduction:

Imagine you are an avid fossil hunter and have just dug up a skull of an extinct vertebrate. You are the first human ever to see it. Not only is that amazing, but you are also at the start of a journey into discovering how this organism lived: whether it was diurnal (active during the day) or nocturnal, whether it hunted above ground or burrowed, had poor vision or an exceptional sense of smell. Despite the millions of years that may have passed, the growing field of virtual palaeontology provides a new world of analysis techniques that can help palaeontologists to peer inside the skull and uncover some truly fascinating insights.

What are digital endocasts?

Virtual Palaeontology is the non-destructive study of fossils using digital methods, enabling, for example,  both analyses of external skull features, alongside identification of internal structures in the form of digital cranial endocasts (3D models depicting the skull interior).

For the creation of digital endocasts, computed tomography (CT) scans of fossils (comprised of potentially thousands of images of thin slices through it) are used. These pictures are loaded into imaging software such as Avizo or SPIERS. In the programme all parts of the images that are lighter than a certain level are marked as part of the skeleton, and all parts that are darker as sediment fill. This process, known as thresholding, is then applied to the image stack to separate skeletal elements from sediment. Coloured masks are then added by filling in the cavities containing the feature of interest (think paint-by-numbers, but a little more scientific, Fig. 1). Through editing the stack image-by-image, the changing shape of anatomical features can be defined, producing 3D models of the brain, inner ear and neurovascular anatomy, for example. Once you have 3D models of desired anatomical structures … let the research commence! From simple anatomical descriptions and comparisons with extant (living) relatives to the frequency of sounds it could hear or its relative level of intelligence (and how this changed over the evolution of the fossil group), the analytical world is your oyster.

What can cranial endocasts tell us?

To put endocasts into context, a recent study considered the changing endocranial anatomy of the skulls of cynodonts, a group of therapsids (fossil reptiles) that appeared during the late Permian period (around 259 million to 252 million years ago) and expanded during the Triassic period (252 million to 201 million years ago), with one branch leading to modern mammals. Understanding how the brain of mammals evolved from those of primitive cynodonts, developing the ability to carry out the complex functions that make modern mammals distinct, is of great importance to palaeoneurology — a field of research that has focused on the evolution of the human brain from those of our primate ancestors.

3D models of cynodont brains (Fig. 2) enable palaeontologists to describe new aspects of the anatomy of little-known species, such as Thrinaxodon liorhinus, allowing them to identify anatomical (and potential functional) changes in cranial features in the mammalian lineage. However, the most exciting revelations come from quantitative analyses, in which linear and volumetric data of the shapes and sizes of anatomical features can shed light upon a species’ senses, and possibly help researchers to observe differences  between members of the same species (intraspecific changes) and during the life cycle of an individual (ontogenic changes). In turn, this may indicate variations in mode of life. Other methods of analysis include:

• 3D point cloud analysis in CloudCompare, which uses a colour scale to map variations in shape between 3D models, allowing for variations in brain shape to be identified and mapped.
• Calculation of encephalisation quotients, in which brain volumes determined from the 3D models are used to estimate a relative level of cognitive ability (intelligence) for a species relative to other species through time. Higher encephalisation quotient values are associated with social interactions and foraging behaviour, so the quotient offers information about ways of life.
• Assessing hearing capability, or auditory acuity, which can be assessed through measurements of the length of a cavity in the inner ear called the cochlea.
• Measuring the shape and size of semi–circular canals of the inner ear, which provide information on an individual’s agility, once more indicating behavioural patterns.

From an in-depth study of cranial endocasts, three stages of development have been identified in the mammal brain over time (Fig. 3). First, development of body hair led to increased tactile sensitivity and olfactory acuity (sense of smell) from whiskers (processed by the brain). A secondary improvement in olfactory capabilities produced an observable increase in brain size. A third pulse in development occurred due to changes in the nasal cavity, ultimately leading to enhancement of hunting abilities.

Endocasts are pretty great …

You can access your specimens anywhere in the world, at any time, as long as you have your data sets. Digital reconstruction saves your data for the future, in case the original specimen is lost or damaged. CT scanning the fossil also means that you do not have to destroy the original by grinding away thin layers to take photographs of cross-sections to make a 3D model.
More importantly, however, cranial endocasts help to overcome some of the bias in the fossil record. Soft tissues are rarely preserved, yet can provide some of the most pivotal information about form, function and mode of life for extinct organisms. Based on the assumption that the brain followed the shape of the brain case, the soft tissues of the endocranial cavity can be reconstructed, with only the meninges (three layers of protective tissue) known to also fill the cranial cavity in modern mammals. Furthermore, image thresholding on the CT scans enables digital density filtering – separating fossil structures from obstructive sediment infill, giving a clearer picture of the internal features than visual inspection offers. Therefore, digital reconstruction techniques are paramount to understanding more about functional morphology than the fossils themselves can offer from a skeletal perspective.

… But they have their limitations

Despite endocasts, and virtual palaeontology itself, providing fantastic leaps forward in fossil research, digital reconstructions do have some limitations. First, the assumption made during endocast creation that the intracranial space correlates directly with the original soft-tissue structures of the brain is problematic. Modern mammals have additional fluid and tissues in the intracranial space (for example, meninges, blood vessels and nerves). The external surface of the mammal brain is also known to be convoluted with ridges (gyri) and grooves (sulci), but no direct impression of this can be discerned on the braincase. Therefore, palaeoneurological studies are limited to external features of the endocasts and 3D models also do not provide any direct information about the brain’s internal structures (such as neuron morphology, density and connectivity) — the perils of working with fossils.

Furthermore, the fossilisation process itself causes a bias in the types of endocast that can be produced. Skulls are often deformed or fragmented, and broken pieces can become offset. As a result, only a few specimens are preserved well enough to produce useful endocasts, and palaeoneurological studies have to rely on small data sets. The individual animals can affect 3D models, too: juveniles, for example, often don’t have fully ossified skulls (particularly, they have insufficient bone growth associated with the inner ear) and thus parts of the braincase are absent, hindering reconstruction efforts to fully constrain cranial features. Similarly, some parts of the skull never become ossified, with no bones confining the base of the brain and olfactory bulbs (Fig. 4), causing difficulties in defining neurovascular anatomy and how far the olfactory bulbs extend. This also results in underestimates of brain volumes, as the full depth of the brain within the skull cannot be determined. This, in turn, affects quantitative analyses, including calculation of encephalisation quotients. However, the bias in this volumetric analysis can be reduced by morphological comparison with the brain of an extant relative (such as Monodelphis domestica, the grey, short-tailed opossum) to define the brain shape. The model can also be compared with models from other cynodonts to determine the shape of the olfactory bulbs, inferring the symmetrical morphology of later cynodonts onto the poorly reconstructed olfactory bulbs of their predecessors.

Technology itself can be a problem. Standard CT scanning takes images using X-rays; these have fairly low resolution, so it can be difficult to see fine detail. Fossils scanned with micro-CT will produce much higher resolution images than X-ray scanning, making the former preferable for acquiring fine detail data sets to study.

And finally, the biggest limitation to endocasts can be the researcher. Everyone has their own opinions on what they are looking at, depending on prior knowledge, which ultimately affects what is included within the mask that defines the model. Small differences during endocast construction could lead to potentially significant morphological and volumetric discrepancies, affecting all subsequent analyses. The specimens chosen for study could limit your findings if they are all of a similar age range, hampering any chance of identifying ontogenetic variation. Small sample sizes also prevent discovery of possible intraspecific variation. Additionally, although digital studies hold many advantages, there is nothing quite like getting up close to a fossil and really getting to see all of the curiosities it has to offer.

Conclusions

Endocasts are invaluable to reconstructing soft tissues lost to time, and to inferring how these anatomical features may have impacted the manner in which the individual lived, and how behavioural patterns may have changed during the evolution of a species or a larger part of the mammalian lineage. 3D models provide the closest possible approximation palaeontologists can obtain of what lay beneath the surface of fossil skulls, and provide valuable information on the development of the mammalian brain through time. Moreover, cranial endocasts offer possibilities for determining how sensory capabilities impacted upon the size and shape of various brain regions.

Whilst reconstruction and analysis methods have their limitations, further research will continue to fine-tune these methods, providing an exciting future for the emerging field of virtual palaeontology — aiding not only fossil studies, but also public engagement with scientific research.

Suggestions for further reading:

Cox, P. G. & Jeffery, N. Semicircular canals and agility: the influence of size and shape measures. Journal of Anatomy 216, 37–47 (2010). (DOI: 10.1111/j.1469-7580.2009.01172.x)

Gleich, O., Dooling, R. J. & Manley, G. A. Audiogram, body mass and basilar papilla length: correlations in birds and predictions for extinct dinosaurs. Naturwissenschaften, 92, 595 – 598 (2005). (DOI: 10.1007/s00114-005-0050-5)

Jasinoski, S. C., Abdala, F. & Fernandez, V. Ontogeny of the Early Triassic cynodont Thrinaxodon liorhinus (Therapsida): cranial morphology. The Anatomical Record 298, 1440–1464 (2015). (DOI: 10.1002/ar.23116)

Kemp, T. S. The Origin and Evolution of Mammals. (Oxford University Press, 2005).

Macrini, T. E., Rowe, T. & VandeBerg, J. L. Cranial endocasts from a growth series of Monodelphis domestica (Didelphidae, Marsupialia): a study of individual and ontogenetic variation. Journal of Morphology 268, 844–865 (2007). (DOI: 10.1002/jmor.10556)

Rodrigues, P. G., Ruf, I. & Schultz, C. L. Digital reconstruction of the otic region and inner ear of the non-mammalian cynodont Brasilitherium riograndensis (Late Triassic, Brazil) and its relevance to the evolution of the mammalian ear. Journal of Mammalian Evolution 20, 291–307 (2013). (DOI: 10.1007/s10914-012-9221-2)

Rodrigues, P. G., Ruf, I. & Schultz, C. L. Study of a digital cranial endocast of the non-mammaliaform cynodont Brasilitherium riograndensis (Later Triassic, Brazil) and its relevance to the evolution of the mammalian brain. Paläontologische Zeitschrift 88, 329–352 (2014). (DOI: 10.1007/s12542-013-0200-6)

Rodrigues, P. G., Martinelli, A. G., Schultz, C. L., Corfe, I. J., Gill, P. G., Soares, M. B. & Rayfield, E. J. Digital cranial endocast of Riograndia guaibensis (Late Triassic, Brazil) sheds light on the evolution of the brain in non-mammalian cynodonts. Historical Biology 30, 1–18 (2018). (DOI: 10.1080/08912963.2018.1427742)

Rowe, T. B., Macrini, T. E. & Luo, Z.-X. Fossil Evidence on the Origin of the Mammalian Brain. Science 332, 955–957 (2011). (DOI: 10.1126/science.1203117)

¹School of Geography, Earth and Environmental ciences, University of Birmingham, UK.

Introduction:

The Morrison Formation is renowned worldwide as one of the world’s most significant locations for dinosaur fossils. It covers more than 150 million square kilometres, running from Alberta in Canada to New Mexico in the United States, and from Idaho across to Nebraska (Fig. 1). The Morrison dates to the Oxfordian stage of the late Jurassic period, some 155 million to 148 million years ago. It is what is known as a Konzentrat-Lagerstätten, meaning that it has a very high concentration of fossil remains, with extensive bone beds created by flash floods depositing lots of bones in one place. The Morrison provides palaeontologists with remarkable insight into a late Jurassic terrestrial ecosystem. Not only does the formation contain some of the largest dinosaurs ever found, but it also hosts the most diverse group of mammal remains yet known from the Mesozoic era (252 million to 66 million years ago).

Geological setting:

The Morrison Formation comprises mainly mudstones, sandstones, siltstones and limestones. The rocks are light grey, greenish grey and red owing to fossilized soils, called palaeosols (Fig. 2). The formation once included a range of environments, from swamps complete with coal deposits in the north to desert conditions in the south. Overall, the environment was dry and savannah-like, prone to flash floods.

In the states of Colorado, Utah and Wyoming, the rocks in the Morrison Formation are mostly made from river and lake deposits. Many of the fossils are partly disarticulated, or broken up, because the animal’s body was carried along by a river before being buried in a sand bar. The formation contains some species that would have lived in fresh water, such as amphibians, which suggests that the area contained permanent freshwater lakes.

The Morrison was dated to the Oxfordian stage using radiometric dating — which measures the abundance of radioactive chemical elements and their decay products — and biostratigraphy, in which layers of rock are dated by comparing the fossils found in them. The Morrison Formation is also a major source of uranium ore; in fact, the radiometric dating used uranium and lead (U–Pb).

Dinosaur National Monument:

The Morrison Formation is best known for its dinosaur bone beds, such as Dinosaur National Monument and the Cleveland–Lloyd Dinosaur Quarry. A bone bed is a sedimentary deposit that contains remains of multiple individuals in unusually high concentrations.

Established in 1915, the Dinosaur National Monument is an 850-square-kilometre national park on the border of Colorado and Utah. It contains stream, lake and swamp deposits. The dinosaurs and other ancient animals were washed into the area and buried, presumably during flooding. The park includes a quarry with a tilted rock layer that contains hundreds of dinosaur fossils. This is now enclosed by the Dinosaur Quarry Building to protect it.

The Cleveland–Lloyd Dinosaur Quarry:

The Cleveland–Lloyd Dinosaur Quarry in Utah contains some of the best evidence of theropod dinosaurs — the group that includes Tyrannosaurus rex — anywhere in the world. It is also the largest collection of theropods in the world, having yielded more than 50 Allosaurus skeletons, representing a number of stages of growth. Allosaurus is the most common species at the site; other theropods are known, but in much lower numbers. Herbivorous dinosaurs are also present, but are very rare. The ratio of predators to prey is 3:1, so the site was probably a ‘predator trap’. The rock entombing the specimens was once mud, and it is theorized that herbivorous dinosaurs became trapped in the mud around a small pond. As the herbivore struggled, its calls alerted predators and scavengers that themselves became trapped. Overall, at least 70 different dinosaur specimens are known from the quarry.

Plants:

Vegetation from the Morrison is similar to that in other Jurassic sites, such as the Lourinhã Formation of Portugal and the Tendaguru of Tanzania. It is dominated by ferns, conifers, cycads — plant that look similar to palm trees but aren’t closely related to them — and ginkgos. Insects became more diverse in the Jurassic, with some groups becoming involved in plant reproduction by transferring pollen from one plant to another. It is generally accepted that flowering plants (Angiosperms) evolved in the early Cretaceous period, between 146 million and 100 million years ago.

Pterosaurs:

In the skies above the dinosaurs were flying reptiles called pterosaurs. Like the dinosaurs, they evolved in the Triassic period (starting 252 million years ago) and died out at the end of the Cretaceous, 66 million years ago. Although some species became very large, they were able to fly because they had hollow bones. Their wings were made of a thin skin membrane supported by an extra-long fourth finger. Pterosaurs are preserved only rarely in the Morrison Formation, mainly because their bones were very fragile. Pterosaur finds from the Morrison include both the long-tailed rhamphorhynchoids and the more advanced short-tailed pterodactyloids. Morrison pterosaurs fed on fish and scavenged dinosaur carcasses.

Mammals:

The Morrison is the most important Jurassic mammal assemblage in the world, providing invaluable insights into early mammalian evolution. Mammals at this time occupied a wide range of ecological niches. For example, Fruiafossor was a termite-eater that closely resembled living anteaters. This is a remarkable example of convergent evolution, because numerous skeletal features suggest that the two groups are unrelated, so they developed a similar body plan in response to a similar lifestyle.

The mammalian groups found in the Morrison include: symmetrodonts, the carnivorous eutriconodonts, dryolestoids and the rodent-like multituberculates. Of these groups, only the multituberculates survived the extinction at the end of the Cretaceous that also killed most of the dinosaurs. They did go extinct during the Eocene epoch, between 56 million and 34 million years ago, but they still have the longest fossil history of any mammal group, lasting for more than 140 million years.

Mesozoic mammals were small and mainly nocturnal. A study in 2009 concluded that the genus Docodon was the largest at 141 grams, and Fruitafossor was the smallest at just 6 grams.

Ornithopods:

Ornithopods were herbivorous dinosaurs that walked on two legs and came in several types. Small hypsilophodonts included Drinker, Nanosaurus and Othnielia. Larger but similar-looking dryosaurids were represented by Dryosaurus, and the camptosaurids by Camptosaurus. Both dryosaurids and camptosaurids were early iguanodonts, a group that would later spawn the duck-billed dinosaurs. Duck-billed dinosaurs (or hadrosaurs) were the most successful group of dinosaurs in the Cretaceous. As well as bones, ornithopod egg shells have been found in the Morrison.

Thyreophorans:

Thyreophorans are often called armoured dinosaurs because of their armour plating. The most famous of these from the Morrison is Stegosaurus. It had a row of plates running down its back that might have been used to regulate its temperature. Its tail sported a set of four spikes, which would have been used for defence (Fig. 3). At least three species are known from the Morrison Formation, from the remains of about 80 individuals.

No armoured dinosaurs apart from stegosaurs were known in the formation until the 1990s. Since then, two have been named: Gargoyleosaurus and Mymoorapelta. Both are ankylosaurs, with Gargoyleosaurus being one of the earliest of this group represented by complete fossils. Ankylosaurs are best described as dinosaurian tanks, with their heavy armour projecting them from predators. Later ankylosaurs from the Cretaceous even had tail-clubs and armoured eyelids.

Heterodontosaurs:

Heterodontosaurs first evolved in the late Triassic period and continued until the early Cretaceous. The name comes from the Greek for ‘different tooth’, and indicates that each animal had several kinds of teeth. Many species had a large third tooth that resembles the canines of carnivorous mammals.

The only heterodontosaur known from the Morrison is Fruitadens. It was the smallest dinosaur from the formation, at 75 centimetres long. It, and other heterodontosaurids, might have been omnivorous. A study in 2012 suggested that Fruitadens was an ecological generalist, eating both plants and insects or other invertebrates.

Sauropods:

Sauropods were the largest land animals that ever existed. Complete fossil finds are rare: many species, especially the largest, are known only from isolated and disarticulated bones. Sauropods were herbivorous, generally with long tails, long necks and tiny heads. They had pillar-like legs and massive bodies. Such huge animals would have had an enormous impact on the environment, being capable of clearing sections of forests. Sauropods, like most dinosaur groups, could not chew, so had to swallow food whole. To help grind food down and make digestion more efficient, they swallowed stones called gastroliths.

Diplodocus was a gigantic dinosaur at 27 metres in length. The neck had 15 vertebrae and was held horizontal to the ground (Fig. 4). Diplodocus could use its tail as a whip. Some palaeontologists have suggested that the tails were used as defensive weapons. More recently, several have speculated that, like bullwhips, such tails were noisemakers used for communication.

Apatosaurus was like diplodocids but had a bulkier skeleton. Diplodocus weighed about 12 tonnes, but Apatosaurus would have been 20 tonnes even though it was 7 metres shorter.

The middle stages of the Morrison Formation were dominated by the giraffe-like Brachiosaurus, which was able to hold its head vertically so it could browse the tops of trees up to 16 metres tall. This titan reached a length of 20 metres and a weight of 50–80 tonnes, and required 240 kilograms of food per day. Brachiosaurus is almost identical to Giraffatitan from East Africa, so much so that some palaeontologists consider them to be the same genus.

Theropods:

The group Theropoda was mainly carnivorous, although several members evolved herbivorous, omnivorous or insectivorous diets. Theropods were the sole large terrestrial carnivores from the early Jurassic until at least the close of the Cretaceous. They are the only group of dinosaurs that have descendants alive today: birds evolved from small, specialized coelurosaurian theropods in the Jurassic. Today, there are between 9,000 and 10,000 living species.

Allosaurus was a massive carnivorous theropod that lived in what is now North America, East Africa and Portugal, making it one of the only dinosaurs known from multiple continents. Most specimens, however, come from the Morrison Formation. In 1988, it became the state fossil of Utah: more than 60 Allosaurus skeletons, ranging from juveniles to adults, are known from this state. This speciesis the most common large theropod in the Morrison Formation, accounting for 70–75% of theropod specimens (Fig. 5). The best known Allosaurus was Big Al, made famous by the television series Walking with Dinosaurs in 2000. Al’s skeleton has 19 injuries, including a badly infected toe (Fig. 6).

Ceratosaurus could reach 6 metres long. It had a horn on its nose that might have been for display and would have been brightly coloured. This species had some of the largest tooth-to-body ratios of any theropod and, interestingly, it had a long, flexible body, with a deep tail shaped like that of a crocodilian. Palaeontologist Robert Bakker has suggested that Ceratosaurus could have hunted aquatic prey such as fish and crocodiles (Fig. 7).

The megalosaur predator Torvosaurus is known from a series of incomplete specimens, so was probably much rarer than Allosaurus, although it was around the same size, at 9 metres long. The species Torvosaurus gurneyi, discovered in Portugal, is the largest theropod known from Europe, at 11 metres long.

Comparison with the Tendaguru Formation:

The Tendaguru Formation in Tanzania, containing the richest deposits of late Jurassic Africa, is comparable to the Morrison because the sites date to the same period and have a similar range of dinosaurs. For example, the Morrison’s Brachiosaurus and Stegosaurus would have occupied the same ecological niches as Giraffatitan and Kentrosaurus in the Tendaguru. Allosaurus is known from both formations but is much rarer in Africa.

A noticeable difference between the two sites is the depositional environment. Whereas the rocks of the Morrison Formation were deposited in a terrestrial basin, the Tendaguru Formation has some marine strata. As a result, the Tendaguru contains marine organisms such as ammonites, corals and other inhabitants of a shallow sea.

Conclusions:

The Morrison gives a remarkable insight into late Jurassic ecosystems, including not only some of the largest dinosaurs of all time, but also animals that existed alongside them. Specimens from the Morrison Formation can be seen in museums all over the world, such as the Natural History Museum in London, the American Museum of Natural History in New York City and the Royal Ontario Museum in Toronto, Canada.

Suggestions for further reading:

Butler, R. J., Porro, L. B., Galton, P. M. & Chiappe, L. M. Anatomy and cranial functional morphology of the small-bodied dinosaur Fruitadens haagarorum from the Upper Jurassic of the USA. PLoS One 7, e31556 (2012). (DOI: 10.1371/journal.pone.0031556)

Carpenter, K., Sanders, F., McWhinney, L. & Wood, L. Evidence for predator–prey relationships: Examples for Allosaurus and Stegosaurus. In The Carnivorous Dinosaurs (ed. Carpenter, K.) pp. 325–350 (Indiana University Press, 2005).

Foster, J. Jurassic West: The Dinosaurs of the Morrison Formation and Their World  (Indiana University Press, 2007).

Foster, J. Preliminary body mass estimates for mammalian genera of the Morrison Formation (Upper Jurassic, North America). PaleoBios 28, 114–122 (2009).

Gates, T. The Late Jurassic Cleveland-Lloyd Dinosaur Quarry as a drought-induced assemblage. Palaios 20, 363–375 (2005). (DOI: 10.2110/palo.2003.p03-22)

Maier, G. African Dinosaurs Unearthed: The Tendaguru Expeditions (Life of the Past) (Indiana University Press, 2003).

Stevens, K., Parrish, J. Neck posture and feeding habits of two Jurassic sauropod dinosaurs. Science 284, 798–800 (1999). (DOI: 10.1126/science.284.5415.798)

Trujillo, K., Chamberlain, K. & Strickland, A. Oxfordian U/Pb ages from SHRIMP analysis for the Upper Jurassic Morrison Formation of southeastern Wyoming with implications for biostratigraphic correlations. Geological Society of America Abstracts with Programs 38, 7 (2006).

¹Camborne School of Mines, University of Exeter, UK.

Introduction:

At the turn of most years, the some of the editorial board at Palaeontology [online] takes the opportunity to reflect on the past year in palaeontology. Given that we published a wonderful overview of Diploporitans in January, this year we’ve moved our look over our favourite studies from last year to February. Palaeontology and associated disciplines are fast-moving and exciting areas of science — looking back at 2018 lets us highlight just a few of the key developments that really show this. Picking just one article each is difficult, and we have been forced to miss out many of the hundreds of exciting papers published in the past 12 months. Nevertheless, we hope that our choices reflect the breadth and depth of palaeobiological research in the twenty-first century. The papers include insights into the evolution of animals, the origin of colour in dinosaur eggs, the discovery of new, unusual fossil echinoderms, and insights into the origins of plant roots. So, in alphabetical order, here are the members of our editorial board with their highlights.

Russell Garwood — Deline et al., ‘Evolution of metazoan morphological disparity’

One of my favourite papers of the past year has been the work of Deline et al., which was published in August. This work looks at the animal kingdom as a whole, and analyses the variation that we see across it. The authors have compiled a cladistic character matrix that includes members of every major animal group. In brief, they have put together a list of the characters (for example, presence of a head, eyes, legs) possessed by 212 animal species within 34 phyla. In total, they managed to code 1,767 characters. This is, to me, a really significant achievement in its own right — and a valuable resource that scientists can use in the future. The authors then ran a wide range of tests on this new data set to try to improve understanding of how animals have evolved.

A key question they addressed is whether groups reach their maximum disparity — that is, variety of forms or morphologies (Fig. 1) — shortly after they evolve, or whether they continue to become more diverse as they evolve. The answer? Well, it seems both could be the case. Many groups reach the disparity we see today quite early in their evolutionary history, but some really important ones — for example, vertebrates and their kin, and arthropods — become increasingly diverse as they evolve. In short, major episodes of innovation don’t necessarily occur at the beginning of a group’s history.

The data also suggest that phyla look so different from each other today because of the extinction of intermediate forms — that is, lineages that share characteristics of major modern groups have gone extinct over time. The authors also have a go at investigating the driving force behind the patterns they report. They find that disparity is correlated with the size of the genome, and the range in a group of molecules called microRNAs, which regulate the expression of genes. Deline et al. suggest that the evolution of this system could have influenced the disparity we see in animals, but also that factors outside the groups’ genetics — such as the environment that they live in — also have an impact.

I like this paper for a number of reasons. It is asking a wide range of interesting and important questions about evolution, using a major and really diverse group. The team has created a really useful data set, and then used it to explore a number of areas in one go: I’ve only highlighted one here! It’s a really neat way to do science, and also quite a brave one (what would have happened if the data set hadn’t shown anything?). And, like lots of exciting science, it leaves us with a whole bunch of new, but potentially more specific, questions we can ask in the future.

Stephan Lautenschlager — Wiemann et al., ‘Dinosaur egg colour had a single evolutionary origin’

My favourite paper of 2018 was published in October, although Easter would have been much more fitting. Wiemann and colleagues’ study focuses on eggs — more specifically, dinosaur eggs. This by itself is not that spectacular. We have known for more than a century that dinosaurs laid eggs like modern birds; for several decades, we have also had clear fossil evidence that birds are the descendants of dinosaurs. So what makes this study so interesting? The eggs of modern birds are unique among vertebrate eggs in showing a huge variety of colours, hues and patterns. It was generally assumed that this was a fairly modern innovation of birds, but Wiemann and colleagues’ study demonstrates that coloured eggs date back more than 150 million years and probably had a single origin, within dinosaurs. Using Raman microspectroscopy (a technique that exploits the scattering pattern of light to identify molecular structures and material composition), the authors searched for traces of pigments in fossilized dinosaur egg shells (Fig. 2). With this method, they were able to identify two pigments — one responsible for giving the egg a blue-green hue, and another resulting in a red-brown hue. However, not all dinosaur eggs had the same colour and pattern. Some theropod dinosaurs, such as Deinonychus, laid blueish-green eggs with dark speckles, whereas others laid white to beige eggs. Interestingly, many herbivorous dinosaurs, including sauropods and hadrosaurs, laid eggs without any pigments. Those were probably more similar to the eggs of modern crocodiles.

But why did different dinosaur groups lay differently coloured eggs? Probably for the same reason that modern bird eggs show such a colourful variety. Coloured and speckled eggshells probably helped to camouflage eggs in open nests, and might also have helped the breeding parents to identify their own eggs (perhaps to avoid nest parasitism from cuckoo-like dinosaurs planting their eggs in other nests). The plain eggs of sauropods and ornithischians, by contrast, were probably left in nests covered by soil and dirt, making camouflage unnecessary. Eggs of different species have in fact been found in different styles of nest, which does support these results.

This study is one of several showing how many features associated with modern birds (such as feathers) have been around for millions of years in their ancestors. At the same time, this goes to show that although we have known many of these fossils for years, if not decades, new results can be obtained with the help of new methods and technologies.

Imran Rahman — Lefebvre et al., ‘Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes’

My favourite paper was actually only officially published this month, but given that it first came online in November 2018, I think it still counts! The study in question is by Lefebvre and colleagues, who described new fossil echinoderms from 480-million-year-old rocks in Morocco. The fossils belong to an extinct class called the stylophorans, which are arguably the most controversial group of fossil echinoderms ever known. The evolutionary relationships of stylophorans are hotly contested by palaeontologists, in part owing to disagreements about what soft parts were located inside the single appendage extending from the main body. Some have suggested that this appendage was a muscular tail for moving the animal over the sea floor, whereas others thought it represented a feeding arm that housed tube feet, similar to those of modern echinoderms. This disagreement stems from the fact that soft parts had never previously been described in a fossil stylophoran, leaving the interpretation of the appendage uncertain.

Lefebvre and colleagues were able to address this debate through the discovery of new, exceptionally preserved fossils. The fossils come from the Fezouata biota, a deposit, or Konservat-Lagerstätten, in southeastern Morocco. Similar to other famous Lagerstätte, Fezouata preserves the soft parts of animals, as well as their hard parts, and thus offers unique insights into past life. Incredibly, some of the stylophorans described by Lefebvre et al. have soft parts preserved inside the single appendage. In at least one specimen, these soft parts consist of a canal along the length of the appendage, with tube-like extensions coming off it to the sides (Fig. 3). Chemical analyses demonstrate that the soft parts are rich in iron, clearly distinguishing them from the rest of the fossil. The organization of the soft parts is almost identical to what we see in many modern echinoderms, which possess a series of fluid-filled canals (the water vascular system). This has a number of functions in living echinoderms, but one of the most important is feeding; for example, tube feet are used to capture food particles suspended in the water. The new Moroccan fossils make it highly likely that stylophorans used their single appendage primarily as a feeding arm, although they might also have been capable of moving slowly over the sea floor.

The finding that stylophorans possessed a feeding arm does not in itself unambiguously resolve the group’s relationships with other echinoderms. Nevertheless, this is a key discovery that greatly enhances our understanding of the palaeobiology of an important fossil group. It is discoveries such as this that drive palaeontology forward, by shedding new light on old questions. Who knows what we will find next!

Alan Spencer — Hetherington and Dolan, ‘Stepwise and independent origins of roots among land plants’

One of my favorite papers of 2018 was published by Hetherington and Dolan in Nature and shed new light on one of the fundamental questions associated with early plant terrestrialization: when did plants evolve roots?

All vascular plants we know today produce roots — which have important roles in nutrient and water uptake, ground anchorage and the symbiosis, or close mutual relationship, of plants and fungi. Roots are characterized by a self-renewing structure called a root meristem. This appears at the root tip and is covered by a ‘root cap’ allowing the roots to be guided by gravity through the substance in which the plant is growing. To date, the fossil record has hinted at root-cap development with known examples from the Carboniferous period (356 million years ago to 299 million years ago) and the Permian period (299 million years ago to 252 million years ago), but poor preservation of earlier land plants has hampered our understanding of where, when and how they first developed. In their paper, Hetherington & Dolan re-investigated historical plant specimens from the 407-million-year-old Rhynie chert Lagerstätte (Scotland, UK) and discovered in the species Asteroxylon mackiei the earliest meristems without root caps associated with a terrestrial ecosystem (Fig. 4).

Through a series of observations using the exceptional preservation of other plant tissues as a basis, they ruled out the loss of root-caps through taphonomic processes. They also looked for evidence at the cellular level in the preserved promeristem, where cell-division patterns can indicate root-cap formation. They found that the pattern was inconsistent with the development of root caps. However, the cell patterns indicated that A. mackiei had a continuous surface called the epidermis. To test this hypothesis, Hetherington & Dolan constructed 3D models of the meristem surface using confocal microscopy.

This is a modern technique that uses several laser beams (at varying wavelengths) to capture 2D images at different focal depths in a sample. Their results showed that there was a continuous and smooth layer of epidermis covering the meristem, and that there was no evidence of tapering or cells breaking off. This led to them to conclude that A. mackiei  developed rooting axes from a previously unknown type of meristem, which lacked both root caps and root hairs. Furthermore, this indicates that the evolution of rooting axes in the plant group lycopsids, to which A. mackiei belongs, occurred in a stepwise fashion. Their findings point towards independent root evolution in different plant lineages at differing points throughout geological time, with them all deriving from a common rootless ancestor. This study once again proves that using historical material with new techniques can throw up unexpected but welcome discoveries!

Suggestions for further reading:

Deline, B., Greenwood, J. M., Clark, J. W., Puttick, M. N., Peterson, K. J. & Donoghue, P. C. Evolution of metazoan morphological disparity. Proceedings of the National Academy of Sciences of the United States of America 115, E8909–E8918 (2018). (DOI: 10.1073/pnas.1810575115)

Hetherington, A. J. & Dolan, L. Stepwise and independent origins of roots among land plants. Nature 561, 7722 235–238 (2018). (https://doi.org/10.1038/s41586-018-0445-z)

Lefebvre, B., Guensburg, T .E., Martin, E. L. O., Mooi, R., Nardin, E., Nohejlová, M., Saleh, F., Kouraïss, K., El Hariri, K. & David, B. Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes. Geobios 52, 27–36 (2019). (DOI: 10.1016/j.geobios.2018.11.001)

Wiemann, J., Yang, T.-R. & Norell, M. A. Dinosaur egg colour had a single evolutionary origin. Nature 563, 555–558 (2018). (DOI: 10.1038/s41586-018-0646-5)

¹ The Palaeontology [online] editorial board can be found here.