Gorgosaurus libratus: Two new juvenile specimens shed light on the ontogeny of a Late Cretaceous Tyrannosaurid Dinosaur.

Gorgosaurus libratus is one of the best known Tyrannosaurids, with dozens of skeletons known from the Late Cretaceous Dinosaur Park Formation of Alberta and Judith River Formation of Montana. This has enabled scientists to develop a good understanding of the Ontogeny (developmental changes during growth) of this Dinosaur. The species was first described in 1914 from a mature specimen from the Dinosaur Park Formation. A second species of Gorgosaurus, Gorgosaurus sternbergi, was subsequently described on the basis of a smaller, more slender skeleton from the same formation, though this was later recognised as a juvenile, rather than a separate species, the beginning of a long process of discovery about the life history of this Tyrannosaurid.

In a paper published in the Journal of Vertebrate Paleontology on 13 April 2022, Jared Voris and Darla Zelenitski of the Department of Geoscience at the University of Calgary, François Therrien of the Royal Tyrrell Museum of Palaeontology, Ryan Ridgely of the Department of Biomedical Sciences at Ohio University, Philip Currie of Biological Sciences at the University of Alberta, and Lawrence Witmer, again of the Department of Biomedical Sciences at Ohio University, describe two new juvenile specimens of Gorgosaurus libratus from the Dinosaur Park Formation, and the implications of these for our understanding of ontogeny in the species.

The first specimen described is TMP 2009.12.14, a juvenile Gorgosaurus libratus skeleton with an articulated skull, a partial vertebral columns with ribs, a pelvic girdle, and an articulated left pectoral girdle. The skull of this specimen is almost complete on the left side, but lacks the articular, epipterygoid, jugal, lacrimal, prearticular, quadrate, quadratojugal, squamosal, and surangular bones on the right side, as well as the unpaired ethmoid and orbitosphenoid bones, while the right angular, ectopterygoid, and post-orbital are preserved, but disarticulated from the skull.

Skull of MP 2009.12.14 in lateral view. Voris et al. (2022).

The second specimen, TMP 2016.14.1, is a partial skeleton with an  articulated skull, partial vertebral column with ribs, and pelvic girdle. In this case the skull is largely intact, although theright quadratojugal is disarticulated.

Skull of TMP 2016.14.1. in lateral view. Scale bar equals 10 cm. Voreis et al. (2022).

Together, TMP 2009.12.14 and TMP 2016.14.1 represent two of the most complete juvenile Gorgosaurus libratus specimens known, and considerably to our understanding of the ontogeny of this species. Tyrannosauroids in general are known to have undergone dramatic morphological changes as they grew. 

In Gorgosaurus libratus juveniles had narrow, shallow skulls with large circular orbits (eye sockets) and ziphodont teeth (flat, sharp teeth with serrated edges), as well as uninflated sinuses and little cranial ornamentation, while adults had wide, deep skulls with incrassate (thickened) teeth, p-shaped orbits, inflated sinuses, and prominent cranial ornamentation. 

The additional data provided by the new specimens enables a better understanding of when these changes took place. The proportions of juvenile Gorgosaurus libratus individuals seems to have remained fairly constant until they reached about 50% of their maximum size, when they began to grow much more rapidly, with their skulls becoming deeper, wider, and generally more robust. Once the skulls reached about 60% of their maximum size, other adult features, including changes to the shape of the bones around the orbits, thickening of the teeth, and the development of ornamentation. When the skull had reached 80% of its maximum size the relative increases in depth and width plateaued, with the skull maintaining the same approximate proportions for the rest of its growth, and the sutures of the braincase had all closed, while the sinuses expanded and the bones around the orbit began to reach their final structures. At 90% of maximum size the transformation appeared to be complete, with the final stages including the resorbtion of the extremities of the antorbital fossa and the expansion of the flange on the posterior of the dentary.

Comparison of the growth series of Gorgosaurus libratus and Tyrannosaurus rex, demonstrating similar ontogenetic stages (and morphologies) occurring at similar relative size (percent of largest specimen skull length) but different body sizes and biological ages. Voris et al. (2022).

The development of the cranial morphology during ontogeny has also been studied Tyrannosaurus rex, enabling direct comparison of these two closely related species. Both species entered a period of accelerated growth when they reached about 50% of their maximum size. In both species this period of accelerated growth was accompanied by a significant increase in the relative width and depth of the skull, as well as a general increase in robustness. Furthermore, both species developed incrassate teeth, nasal ornamentation and changes to the shape of the eye socket when they reached about 50% of their maximum size, with the eye socket shape continuing to change until the animal reached about 80% of its maximum size and these bones began to permanently fuse. However, while the general growth trajectory of the two species was generally similar, in Tyrannosaurus this occurred when the Dinosaur was both older and larger. A Gorgosaurus with a skull length of about 720 mm would have been about 14 years old, and have had features consistent with a young adult developmental stage, whereas a Tyrannosaurus the same age would have been 11-13 years old, with completely juvenile features. This suggests that the onset of accelerated growth in Tyrannosaurids was associated with maturity rather than absolute size, and that the larger size achieved by Tyrannosaurus was linked to both the delayed occurrence and increased length of this developmental stage.

Simplified comparison of ontogenetic trajectories of Gorgosaurus and Tyrannosaurus. Relative to the more basal morphology of Gorgosaurus, the delayed onset of similar ontogenetic changes in Tyrannosaurus coupled with its more hypermorphic features may suggest sequential hypermorphosis to have played a role in the evolution of this taxon. Voris et al. (2022).

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Torosaurus in Canada.

The Ceratopsids, an iconic group of Dinosaurs which were an important part of the faunas of Late Cretaceous North America and Asia, were large, quapruped herbivores with distinctive neck frills and facial horns over their eyes and noses. One of the largest Ceratopsids was Torosaurus, known from the End Cretaceous of North America, which is thought to have weighed up to 6500 kg, and achieved skull lengths of about 3 m. However, Torosaurus, is similar to Triceratops, another large End Cretaceous Ceratopsid, which has led to an active debate in recent years, as to whether the two genera are in fact the same, and, therefore, whether or not Torosaurus is a valid name at all (in taxonimy, where a group has been named twice, the older name is considered to have priority). This argument derives from the fact that all known specimens of Torosaurus are as large as, or larger than, the largest specimens of Triceratops, while the horns and crest of Torosaurus are essentially a more exagerated variant in the pattern seen in Triceratops, which might imply that Torosaurus fossils represent more mature specimens of Triceratops, rather than a separate species.

To date, there have been two reports of Torosauraus specimens being found in Canada, both of fragments of frills that have not fully been described. The first, EM P16.1, was recovered from the Frenchman Formation of southern Saskatchewan in 1947 by Charles Morey, upon whose land it was found, and local amateur palaeontologist Harold ‘Corky’ Jones. This specimen was briefly described and illustrated by palaeontologist Tim Tokaryk in 1986, who attributed the specimen to Torosauraus, but this diagnosis has since been disputed, and it has been suggested that the specimen might in fact belong to Arrhinoceratops, which can be hard to differentiate from Torosaurus. The second specimen, UALVP 1646, was excavated from the lower Scollard Formation in the Red Deer River Valley of southern Alberta by Charles Stelck in 1964, and has been mentioned as an example of Torosaurus in a number of publications, but never formally described or illustrated.

In a paper published in the Zoological Journal of the Linnean Society on 1 March 2022, Jordan Mallon of the Beaty Centre for Species Discovery and Palaeobiology Section at the Canadian Museum of Nature, and the Ottawa-Carleton Geoscience Centre and Department of Earth Sciences at Carleton University, Robert Holmes, also of the Beaty Centre for Species Discovery and Palaeobiology Section at the Canadian Museum of Nature, and of the Department of Biological Sciences at the University of Alberta, Emily Bamforth of the T. rex Discovery Centre at the Royal Saskatchewan Museum, and Dirk Shuman of Fibics Incorporated, formally describe specimens EM P16.1 and UALVP 1646, and discuss their attribution to Torosaurus and the implications of this to the ongoing Torosaurus/Triceratops debate.

The site where specimen EM P16.1 was collected is no longer accessible, but is believed to be an outcrop of the upper-half of Frenchman Formation along the Frenchman River, making it uppermost Maastrichtian in origin (i.e. from the very end of the Cretaceous). Shortly after it's discovery, Harold Jones wrote a letter to Charles Sternberg of the Geological Survey of Canada in which he described, in addition to the crest fragment, the associated discovery of two large bones whicj he believed to be the Animal's femurs, which were in good condition and each measured 46" in length and 9" in diameter at the thinnest part (117 cm long and 23 cm in diameter). The subsequent fate of these bones has not been recorded, however, the Eastend Historical Museum, which houses EM P16.1, also hosts some partial, poorly preserved limb material bearing the field number B9, which is also the field number assigned to EM P16.1, suggesting that these are the femurs mentioned by Jones. Much of Jones' collection is known to have been damaged in 1952, when the basement of an old schoolhouse in which it was housed was flooded, so the disparity in condition between Jones' description and that of the Eastend Historical Museum material is not surprising.

 

Femoral material pertaining to Torosaurus cf. Torosaurus latus (EM P16.1). (A) Proximal portion of right femur, medial view; (B) distal portion of left femur, cranial view; (C) proximal portion of left femur, cranial view; (D) proximal diaphysis of left femur, transverse view; (E) proximal portion of left femur, caudal view. Red arrows indicate where left femur was sampled for osteohistology. Mallon et al. (2022).

The second specimen, UALVP 1646, was uncovered in a quarry at a quarry in the Red Deer River Valley in 1964, with its current assignment to the lower Scollard Formation being based upon a description that it was found approximately 8 m below the Nevis coal seam.

 

Locality information pertinent to Canadian Torosaurus specimens. (A) Map showing locations of EM P16.1 and UALVP 1646 (stars). (B) stratigraphic context of EM P16.1 and UALVP 1646, with distribution of Hell Creek Formation Torosaurus for context. Note that the precise placement of EM P16.1 within the Frenchman Formation is unknown. The precise location of MPM VP6841 within the upper Hell Creek Formation is likewise uncertain. Abbreviations: Fm, Formation; K-Pg, Cretaceous-Palaeogene boundary; Mtn, Mountain. Mallon et al. (2022).

Mallon et al. identified the three remaining fragments of limb bone associated with specimen EM P16.1 as the proximal portion of the right femur, the distal end of the left femur missing the condyles, and the proximal metaphysis and diaphysis of the left femur (excluding the fourth trochanter, which is missing). This third piece appeared to be the least damaged, although it shows signs of having been hastily repaired in places with plaster and shellac, and from this specimen Mallon et al. took a sample from the caudolateral margin of the metaphysis and caudal left femoral diaphysis for osteohistological sampling.

The frill of specimen EM P16.1 was reconstructed with plaster and clay shortly after being extracted, and previous descriptions of the specimen have been based upon that reconstruction, even though it was known to be inaccurate. The specimen was re-prepared by experts from the Royal Saskatchewan Museum in the early 2000s, and Mallon et al. present a new description of the specimen based upon that preparation.

The frill of EM P16.1 is mostly complete, although the left squamosal is heavily reconstructed with plaster, and the median parietal bar is missing entirely. The frill is of moderate dimensions, being somewhat larger than that of Arrhinoceratops brachyops, but smaller than the largest Torosaurus frills, particularly in the transverse dimensions. The frill is nearly square in outline, being just 1.1 times wider than long, as in the holotype of Arrhinoceratops brachyops. The frill is notably more triangular in the holotype of Torosaurus latus (YPM 1830, excavated from the Lance Formation of southern Wyoming by John Bell Hatcher in 1891), which is 1.79 times wider caudally than rostrally (although the parietal of this specimen is heavily reconstructed). The width of the transverse parietal bar of EM P16.1 is among the smallest known for Torosaurus, and most nearly approximates that of ANSP 15192 (which is 1225 mm). The frill is flat in the transverse plane, not saddle-shaped as in Triceratops, although this flattening may be exaggerated by taphonomic compression.

 

Parietosquamosal frill of Torosaurus cf. Torosaurus latus (EM P16.1.). (A) Dorsal surface; (B) interpretive reconstruction of dorsal surface; (C) schematic transverse cross-section of squamosal near caudal edge, based on sketch by Harold Jones; (D) schematic of frill, showing paired fossae and channels on ventral surface of squamosals, based on sketch by H. Jones. Abbreviations: p, parietal; pf, parietal fenestra; sq, squamosal; sqaf, accessory fenestra of squamosal; sqb, squamosal bar. Mallon et al. (2022).

Specimen UALVP 1646 consists of a partial parietal, including portions of the median and caudal bars. As preserved, the maximum dimensions of the specimen are 513 mm rostrocaudally by 568 mm transversely. The parietal is weakly bowed dorsally, indicating that the frill was originally saddle-shaped, albeit not to the extent seen in Triceratops. The partial midline parietal bar is thickened (20 mm dorsoventrally) and bears three sagittally aligned bumps dorsally, similar to those reported in other Torosaurus. The anterior-most of these is most prominent; the remaining two become progressively broader and lower toward the caudal margin of the parietal. Portions of the finished margin of the left parietal fenestra are preserved. The fenestra seems to have been small and probably had a maximum diameter of not much more than 15 cm. The bone surrounding the fenestra is thin (8 mm), thickening outwardly in all directions. The caudal margin of the parietal is 21–25 mm thick (compared to 10–19 mm thick in YPM 1831) and lacks a median emargination. The parietal bears three epiparietals co-ossified to the frill. The interstitial sutures are visible. These epiparietals are interpreted as the left and right P1s and the left P2. There is no epiossification straddling the midline.

 

Parietal of Torosaurus cf. Torosaurus latus (UALVP 1646). (A) Dorsal surface; (B) interpretive reconstruction of dorsal surface; (C) ventral surface; (D) interpretive reconstruction ventral surface; (E) detail of left parietal fenestra. Arrows in (B) and (D) indicate epiparietal loci. Arrows in (E) indicate unbroken margin of parietal fenestra. Abbreviations: mb, median bump; pf, parietal fenestra; pfo, parietal fossa. Mallon et al. (2022).

Triceratops is known to have ranged into Canada, but the question of whether-or-not the closely related Torosaurus also did so has until now remained open; two specimens found in Canada have previously been referred to Torosaurus, but only one of these has previously been (briefly) described, and both have had their reference to the genus questioned. Mallon et al.'s redescription of EM P16.1 rules out the exclusion of that specimen from Torosaurus, on the basis that all of the features used to reject that hypothesis were ether artefacts of the way in which the specimen had been preserved, or were also present in the holotype of Torosaurus, if not in every specimen attributed to that taxon. Specimen UALVP 1646 is less complete than specimen EM P16.1, but again has no features which preclude its inclusion within the genus Torosaurus. Mallon et al. note that specimens assigned to Torodsaurus do seem to be somewhat variable in some features, and note that Triceratops, which is closely related, is considered to have undergone some species turnover during the 200 000 years represented by the Hell Creek Formation, and it would not be unreasonable to expect a similar level of turnover in Torosaurus.

Mallon et al. also note that the frill of specimen EM P16.1 is considerably smaller than seen in many other Torosaurus specimens, although the associated femora were of comparable size to the largest known Triceratops specimens. They note that size is not always a reliable indicator of maturity in Dinosaurs, but that EM P16.1 shows a number of morphological and osteological features which lead them to conclude that it was not a mature specimen. The existence of a sub-adult Torosaurus specimen of comparable size to the largest Triceratops specimens leads them to conclude that it is unlikely that Triceratops represents an immature form of Torosaurus.

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Fireball meteor over Alberta.

Witnesses across the Alberta, British Columbia, Saskatchewan, and Montana have reported observing a bright fireball slightly after 6.20 pm local time (slightly after 1.20 pm GMT) on Monday 22 February 2021. The fireball is described as having moved from southheast to northwest, appearing over Val Soucy and vanishing to the north of Armstrong Lake. A fireball is defined as a meteor (shooting star) brighter than the planet Venus. These are typically caused by pieces of rock burning up in the atmosphere, but can be the result of man-made space-junk burning up on re-entry. 

 

Meteor seen from Oktoks in Alberta on 22 February 2021. Brenda Honish/American Meteor Society.

Objects of this size probably enter the Earth's atmosphere several times a year, though unless they do so over populated areas they are unlikely to be noticed. They are officially described as fireballs if they produce a light brighter than the planet Venus. The brightness of a meteor is caused by friction with the Earth's atmosphere, which is typically far greater than that caused by simple falling, due to the initial trajectory of the object. Such objects typically eventually explode in an airburst called by the friction, causing them to vanish as an luminous object. However, this is not the end of the story as such explosions result in the production of a number of smaller objects, which fall to the ground under the influence of gravity (which does not cause the luminescence associated with friction-induced heating).

 

 

Heat map  showing areas where sightings of the meteor were reported (warmer colours indicate more sightings), and the apparent path of the object (blue arrow). American Meteor Society.

 

These 'dark objects' do not continue along the path of the original bolide, but neither do they fall directly to the ground, but rather follow a course determined by the atmospheric currents (winds) through which the objects pass. Scientists are able to calculate potential trajectories for hypothetical dark objects derived from meteors using data from weather monitoring services.

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Canadian city flooded by ice jam.

Around 15 000 residents of the city of Fort McMurray in northeast Alberta, Canada, have been forced to evacuate after the lower part of the town began to flood on Sunday 26 April 2020. The flooding has been caused by ice jams that have formed on the Athabasca and Clearwater rivers, creating a blockage almost 25 km long, resulting in water piling up behind the obstruction to form a dam lake, which has begun to invade the town.

Flooding in Fort McMurry, Alberta, caused by ice jams on the Athabasca and Clearwater rivers. McMurray Aviation.

Ice jams can be a problem in areas where there is a strong seasonal freeze and thaw cycle. They are generally associated with a rapid spring thaw, which can cause many large chunks of ice to be deposited into a river at the same time. When these ice blocks reach an obstruction on the river, such as a narrow or shallow stretch they can become jammed together, forming a blockage. These blockages cause flooding in two ways; firstly by creating a dam lake behind them, and secondly (and more dangerously) by suddenly giving way, allowing all the water piled up behind them to escape at once, and causing a flash flood downstream. Although generally associated with the spring thaw, ice jams can also occur at the onset of the winter freeze, if an unseasonal warm spell causes a sudden thawing.

An ice jam on the Athabasca River in Alberta, Canada, on 26 April 2020. Vincent McDermott/Fort McMurray Today/Postmedia Network.

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Fireball meteor over Alberta.

Witnesses in Alberta, British Columbia and Saskatchewan reported seeing a fireball meteor on Saturday 8 February 2020. The object was seen moving weast to east over southern Alberta slightly after 4.05 pm local time (slightly after 0.05 am on Sunday 9 February, GMT). A fireball is defined as a meteor (shooting star) brighter than the planet Venus. These are typically caused by pieces of rock burning up in the atmosphere, but can be the result of man-made space-junk burning up on re-entry.

Daytime fireball meteor over southern Alberta. Mobizen/Calgary Herald.

Objects of this size probably enter the Earth's atmosphere several times a year, though unless they do so over populated areas they are unlikely to be noticed. They are officially described as fireballs if they produce a light brighter than the planet Venus. The brightness of a meteor is caused by friction with the Earth's atmosphere, which is typically far greater than that caused by simple falling, due to the initial trajectory of the object. Such objects typically eventually explode in an airburst called by the friction, causing them to vanish as an luminous object. However this is not the end of the story as such explosions result in the production of a number of smaller objects, which fall to the ground under the influence of gravity (which does not cause the luminescence associated with friction-induced heating).

 

Heat map of western Canada showing areas where sightings of the meteor were reported (warmer colours indicate more sightings), and the apparent path of the object (blue arrow). American Meteor Society.

 

These 'dark objects' do not continue along the path of the original bolide, but neither do they fall directly to the ground, but rather follow a course determined by the atmospheric currents (winds) through which the objects pass. Scientists are able to calculate potential trajectories for hypothetical dark objects derived from meteors using data from weather monitoring services.

 

On this occasion Fabio Ciceri of the Dapartment of Geoscience at the University of Calgary has estimated that the object may have weighed around 500 kg, and that there is a strong likelyhood of material from it having reached the ground intact. The University of Calgary team maintain a series of cameras accross the province which are calibrated to help determine the trajectories of meteors. However, as this object fell during the daytime the amount of recoverable footage was rather limited, and they are interested in hearing from any members of the publich who may have recorded the event.

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Analysing the skin of an exceptionally well-preserved Hadrosaur from the Late Cretaceous Wapiti Formation of Alberta, Canada.

Fossilised Dinosaur integument has been known for nearly 150 years, yet it is only recently that it has been considered more than a simple impression (trace fossil) of the original skin surface. Although feathers and filamentary protofeathers of Avian and non-Avian Theropods have received considerable attention, particularly in the past two decades, squamous (scaly) skin is more widespread and was probably plesiomorphic (the original state) in Dinosaurs. Significant advances in our understanding of the preservation and structure of squamous skin have been achieved with the use of synchrotron radiation techniques, and it is now generally accepted that labile tissues, such as skin and muscle, can preserve and remain intact millions of years after the death of the organism. Hadrosaur skin is relatively common in the fossil record, but few studies have investigated either its composition or the possible determining factors behind its preservation.

In a paper published in the journal PeerJ on 16 October 2019, Mauricio Barbi of the Department of Physics at the University of Regina, Phil Bell of the School of Environmental and Rural Science at the University of New England, Federico Fanti of the Dipartimento di Scienze Biologiche, Geologiche e Ambientali and the Museo Geologico Giovanni Capellini at the Università di Bologna, James Dynes of Canadian Light Source Inc. at the University of Saskatchewan, Anezka Kolaceke, also of the Department of Physics at the University of Regina, Josef Buttigieg of the Department of Biology at the University of Regina, Ian Coulson of the Department of Geology at the University of Regina, and Philip Currie of Biological Sciences at the University of Alberta, describe the results of a study of a study of a sample of three-dimensionally preserved squamous skin from a Hadrosaurid dinosaur from the Late Cretaceous Wapiti Formation, discovered near the city of Grande Prairie in Alberta, Canada.

The Wapati Formation of the Western Canadian Sedimentary Basin outcrops across parts of northwestern Alberta and northeastern British Colombia. It is a series of sandstones, siltstones, mudstones and related deposits laid down in a broad floodplain associated with a meandering river system between the Campanian (83.6-72.1 million years ago) and the Palaeocene (66-56 million years ago). The Cretaceous portion of this formation has yielded a variety of Dinosaur fossils, though most of the fossil-producing exposures are in inaccessible locations, limiting palaeontological efforts.

The speciemen examined by Barbi et al., UALVP 53290, is an incomplete Hadrosaur recovered from the Red Willow Falls locality, less than one kilometre to the east of the Alberta-British Columbia border. In this area, late Campanian deposits of the Wapiti Formation have been dated at 72.58 million years old and consist of repeating fining-upward sequences of crevasse-splays, muddy and organic-rich overbank deposits, and minor sandy channel fills. Sandstones are primarily formed by poorly-sorted quartz, feldspar, and carbonate clasts, commonly presenting a carbonatic cement. Thin and discontinuous altered volcanic ash beds are found at the top of fining-upward successions, where they are locally interbedded with coal lenses. The specimen comprises an incomplete articulated-to-associated Hadrosaurid skeleton with most of the thoracic region, forelimb and pelvic elements. Parts of the tail likely continue into the cliff but could not be recovered owing to the precipitous nature of the outcrop. The only cranial element found, an incomplete jugal (cheekbone), indicates Hadrosaurine affinities, though the material is not sufficient to diagnose the specimen to species level. Another Hadrosaur specimen recovered from nearby could be assigned to Edmontosaurus regalis, and Barbi et al. consider it likely that UALVP 53290 belongs to the same species.

Sheets of in situ and partially displaced fossilised integument were found close to the forelimbs of UALVP 53290, and occur in two types: as a 2 mm thick black rind preserving the three-dimensionality of the epidermal scales, and as low-relief structures covered in a thin, oxide-rich patina. The skin samples examined by Barbi et al. were slightly displaced relative to their true life position but are presumed to have come from the dorsal (anterior) surface of the forearm. The integument is composed of large (10 mm), hexagonal basement scales identical to scales on the upper surface of the forearm in other Edmontosaurus specimens.

UALVP 53290. (A) quarry map showing location of preserved integument (indicated by numerals) shown in (B) and (C). Dark grey regions are freshwater bivalves. (B) Higher magnification of (1) showing dark-coloured polygonal scales . (C) Detail of (2) showing cluster areas associated with the forearm integument. (D) Detail of dark  scales in oblique view showing sampling locations for spectromicroscopy: samples were collected with a microtome from (i) the outer surface of the epidermal scale to produce a light-coloured powder, and (ii) from a cross section of the scale that penetrated into the pale underlying sedimentary matrix to produce a dark-coloured powder. Scale bar in (A) is 10 cm. Scale bars in (B) (D) are 1 cm. Abbreviations: Dv, dorsal vertebra; H, humerus; Mc, metacarpal; Os, ossified tendon; Pu, pubis; R, rib; Ra, radius; Sc, scapula; Th, Theropod tibia; Ul, ulna. Line drawing by Phil Bell. Barbi et al. (2019).

Scanning Electron Microscope studies of a 20 μm thick cross section of the Hadrosaur skin were conducted with the intention of investigating possible markers that could discriminate the skin from the sedimentary matrix, and (b) identify potential regions for the presence of organic contents.. The thin section covers the first 2 cm of the sample starting from the outer surface of the scales To better understand the morphology of the Hadrosaur skin, histological skin samples were prepared for a Chicken, Gallus gallus domesticus, a Saltwater Crocodile, Crocodylus porosus, and a Rat, Rattus norvenicus, and compared with the Hadrosaur skin.

Optical microscopy of histological samples of the skin from three extant representatives (Gallus, Crocodylus, Rattus) reveals a thin but characteristically multi-layered epidermis and a deeper, thicker dermis. The outermost epidermal layer corresponds to the stratum corneum, a keratinous layer that aids in protection of the internal organs (including the underlying epidermal and dermal layers) from desiccation. The stratum corneum, which is composed of stratified layers of β -keratin, is the thickest component of the epidermis in Crocodylus owing to the presence of keratinised scales. As a result of this cornified layer, the epidermis is also the thickest in Crocodylus in both absolute and relative terms. The stratum corneum is comparatively thin in both Rattus and Gallus, but where epidermal scales are present on the avian podotheca, such as Gallus, they are similarly covered by a thick stratum corneum, although these were not sampled in this study. In Birds, the relative thickness of the epidermal layers differs between locations, however, the epidermis is consistently thinner in Gallus than it is in Rattus and Crocodylus, a feature that has been linked to the progressive lightening of the Avian body and the evolution of flight.

In Crocodylus, the epidermis is invaginated to form the hinge area between scales. Much deeper invaginations, invading both the epidermis and dermis, are formed by hair and feather follicles in Rattus and Gallus, respectively. Underlying the epidermis is the dermis, which contains openings for the blood vessels, fat deposits, and abundant pigment cells, the latter of which are more diverse in Crocodylus and other reptiles due to their naked skin. Glands are relatively scarce and/or small in Avian and Reptilian skin, but are a salient feature of Mammalian dermis. Thick subcutaneous hypodermis dominated by fat stores and blood vessels underlies the dermis in both Gallus and Rattus, whereas it is relatively thin in Reptiles and was not sampled in the specimen of Crocodylus.

Phase-contrast optical microscopy of the Hadrosaur skin reveals an outermost (superficial) dark-coloured layer 35 75 μm in thickness, which overlies the sedimentary matrix This outer layer is composed of clearly-defined, alternating dark and lighter-coloured layers, which typically range from ~5 mm to ~15 μm in thickness. Individual layers are typically laminar or undulatory giving the entire outer layer a stratified appearance. These finer layers may deviate around sedimentary particles that are occasionally found embedded within the outer dark layer. Aside from these occasional particles, sedimentary grains are typically restricted to the sedimentary matrix underlying the dark outer layer. In places, the dark layers appear to be composed of oval substructures measuring a few micrometers in maximum dimension. The entire dark stratified layer is, in places, capped by a pale-coloured, faintly laminated region identified as barite. No other evidence of integumentary features that could be interpreted as hair follicles, feathers or glandular structures could be identified anywhere in the sample. Other epidermal/dermal features such as osteoderms and melanosomes are also absent.

Comparative histology (transmitted light optical micrographs) of the skin of Edmontosaurus cf. regalis (UALVP 53290) (A), (B) Crocodylus porosus (C), (D), Rattus rattus (E), (F) and Gallus gallus domesticus (G), (H). In UALVP 53290, the dark outer (superficial) layer corresponds to the position of the epidermis (e) in modern analogues (C)-( F). The thickness of the region identified as epidermis in UALVP 53290 varies (B); however, distinctive layering of this region (arrowheads in B) resembles the stratified appearance and general thickness of the stratum corneum in Crocodylus (D). Boxed area in (A) encompasses the enlarged area shown in (B). (I) Phase-contrast and (J) transmitted light optical micrographs of Edmontosaurus cf. regalis (UALVP 53290) skin revealing fine laminae in the outer stratified region. The outermost epidermal layer in indicated by arrowheads. Dark laminae are, in places, composed of small, lenticular or subcircular bodies (arrows in (I)). Abbreviations: b, barite layer; e, epidermis; d, dermis; ds, dark stratified region; g, sedimentary grains; h, hinge area; hs, hair shaft; hy, hypodermis; m, sedimentary matrix; p, pigment cells; s, epidermal scale; sc, stratum corneum; sg, stratum germinativum. Barbi et al. (2019).

Histological sampling of the Hadrosaur skin reveals microscopic details of the dark outer layer associated with a single epidermal scale. Specifically, this layer is distinctly stratified, composed of alternating dark and lighter-coloured layers with a total thickness of 75 μm. The topological position, overall thickness and stratified composition of the dark outer layer in UALVP 53290 is strongly reminiscent of the stratum corneum in Crocodylus ( ~145 μm thick in Crocodylus), which forms the thickest component of the epidermis in the latter. In contrast, the entire epidermis is extremely thin in both Rattus and Gallus (less than 25 μm) and the thickness of the stratum corneum is negligible compared to Crocodylus. Given the obviously scaly epidermal covering of Hadrosaurs, including UALVP 53290, it seems reasonable to infer that the dark-coloured stratified layer represents the mineralised remains of the stratum corneum. The differing thickness in what we have identified as the stratum corneum of the Hadrosaur and that of Crocodylus could be attributable to dehydration, diagenesis, taxonomic differences or any combination of these. Similar keratinous structures to those identified in UALVP 53290, together with intact remains of α - and β-keratins have been reported in non-avian Dinosaurs and contemporaneous Birds; however, these results are largely restricted to feathers and the cornified sheaths covering the unguals rather than skin.

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