Early Origins of Tyrannosaurs

The most popular dinosaur of all time may very well be Tyrannosaurus rex, but there were many other closely related dinosaurs too, collectively known as the Tyrannosauroidea, a group of coelurosaurian theropod dinosaurs consisting of the tyrannosaurids and all their early relatives, being most closely related to birds. Throughout much of their existence (commencing in the mid Jurassic some 170 million years ago), tyrannosauroids were mostly small to medium-sized animals before they attained truly gigantic sizes by the late Cretaceous. It’s only in the past couple of decades that an impressive number of early tyrannosauroid fossils have come to light.

The majority of early tyrannosauroids were only under 4 m (13 ft) long,  similar in size to other closely related coelurosaurians. Unlike the later tyrannosaurids such as T. rex, they all had longer arms and hands with three fingers as well as lightly built skulls lacking bone crunching adaptations.

The oldest known tyrannosauroid of all is Proceratosaurus bradleyi, known from a single, partial skull found in the Great Oolite Group of the White Limestone Formation at Minchinhampton, Gloucestershire, England (dated to the Bathonian Stage, Mid Jurassic). From this skull, the animal is estimated to have been around 3 metres long. What appears to be a nasal horn on the tip of the skull’s snout was likely just the base of a large, rounded, mohawk-like nasal crest analogous to Guanlong, to which we now believe is closely related.

Mistakenly described as a species of Megalosaurus at first by Arthur Smith Woodward in 1910, it was later named as a distinct genus by Friedrich von Huene in 1926, Proceratosaurus meaning “before Ceratosaurus” due to the similar-looking nasal horn-like feature on the skull, akin to the better preserved Ceratosaurus of Late Jurassic North America (von Huene, 1926). Proceratosaurus was assumed to have been ancestral to Ceratosaurus, so both dinosaurs were classified as coelurosaurians. Later in the century, Ceratosaurus was found to have not been a coelurosaur so both dinosaurs weren’t closely related after all, and it wasn’t until 2010 that Proceratosaurus was finally recognised as a definite tyrannosauroid, a member of an evolutionary lineage that would culminate into the T. rex of latest Cretaceous North America (Rauhut et al., 2010).

Figure 1: My reconstruction of Proceratosaurus bradleyi. 

One small tyrannosauroid from the mid Jurassic is Guanlong wucaii, known from two partial skeletons discovered at the bottom of a pit several feet deep in the Shishougou Formation of far western China, presumably a sub-adult trampled on by an adult after death. They were both preserved in sediment formed of volcanic ash and mud, so a natural disaster must have taken place at the time those individuals died. The name Guanlong means “crowned dragon” in reference to its thin, fragile, mohawk-like head crest, which most likely served as a display ornament for intimidating rivals and/or attracting mates. It was an agile, slender predator, with long, thin legs and a tail extending beyond the body for balance (Xu et al., 2006).

However, it was not the apex predator of its time and place, which may have been the same for other primitive tyrannosaurs like Kileskus from Russia and the dog-sized Dilong paradoxus from China.

Figure 2 My reconstruction of Kileskus aristotocus (the nasal crest is only a matter of speculation).

Kileskus aristotocus is the oldest known Asian tyrannosauroid, discovered in rocks dating to the mid Jurassic (Bathonian) from the Itat Formation of Krasnoyarsk Krai, Russia. It was named and described in 2010 and is known from a partial skeleton including the front of the snout, a fragment of the lower jaw, a tooth and some random hand and foot bones. These scrappy remains have made it difficult to estimate the length of the animal, but may have only been about 7-8 ft long. It must have lived as a small predator hunting lizards, salamanders and small mammals in the underbrush, so it would have been rather puny in comparison to its distant descendants like T.rex.

Even though the fossil lacks a nasal crest, the animal is believed to be a basal tyrannosauroid within the proceratosauridae (related to Guanlong and Proceratosaurus) because of certain skull characteristics. These include a shortened ventral margin of the premaxilla (tip of the snout in the upper jaw) and enlarged external nares (where the nostrils were situated) (Averianov et al., 2010).

Thanks to the entombing rocks being so fine-grained, the fossils of Dilong and Yutyrannus from China have both been preserved with a filamentous fur-like covering intact, which is so common in coelurosaurians. These protofeathers were likely used for insulation or for display purposes and also suggest that the common ancestors of all tyrannosaurs must have been feathered too.

Dilong paradoxus is the smallest basal tyrannosauroid (up to 2 m long), known from a partially articulated skeleton found along with the scattered bones of several other specimens, discovered in the lake deposits of the Yixian Formation in Liaoning Province, northeastern China. The protofeathers of Dilong would have certainly been used for insulation as it was an animal with a small body mass to surface area ratio, which means it could have lost internal heat quite easily. Therefore, it must have been an active predator with a high metabolic rate, relying on being agile enough to hunt prey such as smaller reptiles (Xu et al., 2004).

At 9 m (30ft) long, Yuytrannus huali is important in being the largest known theropod dinosaur yet discovered with feathers preserved, also from the same formation as Dilong. Yutyrannus was one of the largest tyrannosaurs alive during the early Cretaceous (dated to 124.6 million years BCE), but not quite as big as T.rex. It is believed to be a basal tyrannosauroid, as evident from the foot which lacks the specialised middle toe that’s useful as a shock absorber while running, as well as for supporting the weight of the animal. It also had three fingered hands, unlike the stubby two fingered hands of T. rex and its close relatives. It is therefore assumed that Yutyrannus was among those primitive tyrannosaurs evolving large body sizes independently from their later tyrannosaurid cousins (like T.rex), so they weren’t closely related. Yutyrannus has even been hypothesised to be a proceratosaurid, those tyrannosauroids with rounded nasal crests (Xu et al., 2012; Brusatte & Carr, 2016). 

Figure 3: Size chart of all the early tyrannosauroids from China I’ve already illustrated. From left to right: Dilong paradoxus, Guanlong wucaii, Xiongguanlong baimoensis, Yutyrannus huali. Height of human figure = approx 1.8 m.

One other Chinese tyrannosauroid is Xiongguanlong bohaiensis, an intermediate form between the earlier, more primitive tyrannosauroids and the larger, more advanced tyrannosaurids (such as T.rex) of the Upper Cretaceous. It was first described in 2009 on the basis of fossils consisting of a skull with a very elongated muzzle (lacking the lower jaw), a complete set of cervical and thoracic vertebrae, a right femur (thigh bone) and right ilium (largest part of the pelvis). From this incomplete fossil material, dated to the early Cretaceous some 112 million years ago, the animal is estimated to be roughly 4 to 4.5 metres long (Li et al., 2009).

All these discoveries in China help support the hypothesis that the tyrannosaurs must have had their origins in Asia.

Figure 4: My reconstruction of Juratyrant langhami.

Juratyrant langhami (“Jurassic Tyrant”) is a tyrannosauroid which, like its earlier relative Proceratosaurus, is also from the Jurassic of England. It is an important theropod dinosaur find that was discovered in the Kimmeridge Clay Formation of the Jurassic Coast in southern England, dated to the Tithonian stage of the Late Jurassic (~ 149 million years ago). It is known from an associated partial postcranial skeleton of a mature individual, consisting of incomplete leg bones, a few individual cervical and dorsal vertebrae, a complete sacrum and a complete pelvic girdle. It was first discovered by avid fossil collector Peter Langham in 1984 and later described as a species of the North American genus Stokesosaurus by Benson (2008). However, a study by Brusatte and Benson (2013), found that S. langhami is in fact a new distinct genus of tyrannosaur more closely related to Eotyrannus than Stokesosaurus. The body length of Juratyrant langhami has been estimated at 5 m (16 ft) or so.

Figure 5: My reconstruction of Eotyrannus lengi. 

Another one of those European tyrannosaurs is Eotyrannus lengi, discovered in plant debris clay beds of the Wealden Group (130-125 Ma) in the Lower Cretaceous Wessex Formation of the Isle of Wight, southern England. Those plant debris beds may represent flash floods, when lots of wood got stranded on floodplains. As the smallest, oldest tyrannosaurs are known from Asia and the largest and last are mainly from North America, Eotyrannus may help explain that some tyrannosaurs radiated out westwards from their origins in Asia to Europe, early in their history, then returned to Asia with more advanced species displacing earlier ones. Eotyrannus lengi is known from just one partial skeleton preserved as an irregular block, belonging to a juvenile or subadult. This skeleton consists of a fragmentary skull, parts of the axial skeleton, pelvic and pectoral girdle material and a few postcranial limb bones. The fused nasal bones of the skull tell us that it was indeed a tyrannosauroid, but more primitive. The individual preserved is estimated to be up to 4 metres long, but may have grown even bigger, how big exactly we just don’t know (Hutt et al., 2001).

Figure 6: My reconstruction of a running Qianzhousaurus sinensis individual.

One very interesting example of a later, more advanced tyrannosaur from China (found in a quarry near the town of Ganzhou, after which the dinosaur was named) is Qianzhousaurus sinensis, nicknamed “Pinocchio rex” because of its very elongated skull. Unlike all the other genera described above, Q. sinensis is a tyrannosaurid, a family of large, advanced theropod dinosaurs that were truly the top predators of their ecosystems and evolved from smaller tyrannosauroid ancestors. Q. sinensis is known from a partial skeleton comprising a complete skull (lacking the teeth), pectoral material, a left tibia, left femur and several vertebrae (Lü et al., 2014). These remains show us that Qianzhousaurus, along with the closely related Alioramus, formed an important, abundant clade of large, long-snouted carnivorous dinosaurs that lived in Asia during the late Cretaceous, termed the ‘Alioramini’. Q. sinensis was a tyrannosaur that lived at the same time T. rex roamed North America, existing until the K-Pg mass extinction that terminated the dinosaurs’ reign.

In conclusion, the tyrannosaurs were able to become so successful and roam so far and wide the world over because they existed while the supercontinent of Pangaea was fragmenting, so they could have dispersed freely between continents via land bridges to end up evolving in isolation over time. Many of the archaic, primitive tyrannosaurs described and illustrated above were able to exploit ecological niches as small to medium-sized carnivores in the undergrowth, which is something they were quite proficient at. 

References:

Averianov, A. O., Krasnolutskii, S. A., Ivantsov, S. V. 2010. A new basal coelurosaur (Dinosauria: Theropoda) from the Middle Jurassic of Siberia. Proceedings of the Zoological Institute. 314 (1), 42–57.

Benson, R. B. J. 2008. New information on Stokesosaurus, a tyrannosauroid (Dinosauria, Theropoda) from North America and the United Kingdom. Journal of Vertebrate Paleontology, 28, 732-750.

Brusatte, S. L., Norell, M. A., Carr, T. D., Erickson, G. M., Hutchinson, J. R., Balanoff, A. M., Bever, G. S., Choiniere, J. N., Makovicky, P. J., Xu, X. 2010. Tyrannosaur Paleobiology: New Research on Ancient Exemplar Organisms. Science, 329 (5998), 1481-1485.

Brusatte, S. L., Benson, R. B. J. 2013. The systematics of Late Jurassic tyrannosauroids (Dinosauria: Theropoda) from Europe and North America. Acta Palaeontologica Polonica, 58 (1), 47–54.

Brusatte, S. L., Carr, T. D. 2016. The phylogeny and evolutionary history of tyrannosauroid dinosaurs. Scientific Reports, 6 (20252): doi:10.1038/srep20252. Accessed 24/06/20. 

Hutt, S., Naish, D., Martill, D. M., Barker, M. J., Newbery, P. 2001. A preliminary account of a new tyrannosauroid theropod from the Wessex Formation (Early Cretaceous) of southern England. Cretaceous Research, 22, 227-242.

Li, D., Norell, M., Gao, K., Smith, N. D., Makovicky, P. J. 2009. A longirostrine tyrannosauroid from the Early Cretaceous of China. Proceedings of the Royal Society B, 277 (1679), 183–190.

Lü, J., Yi, L., Brusatte, S. L., Yang, L., Li, H., Chen, L. 2014. A new clade of Asian Late Cretaceous long-snouted tyrannosaurids. Nature Communications, 5 (3788):  doi:10.1038/ncomms4788. Accessed: 26/06/20.

Rauhut, O. W. M., Milner, A. C., Moore-Fay, S. 2010. Cranial osteology and phylogenetic position of the theropod dinosaur Proceratosaurus bradleyi (Woodward, 1910) from the Middle Jurassic of England. Zoological Journal of the Linnean Society, 158 (1), 155-195.

von Huene, H. 1926. On several known and unknown reptiles of the order Saurischia from England and France. Annals and Magazine of Natural History, 9 (17), 473-489.

Xu, X., Norell, M. A., Kuang, X., Wang, X., Zhao, Q., Jia, C. 2004. Basal tyrannosauroids from China and evidence for protofeathers in tyrannosauroids, Nature, 431 (7009), 680–684.

Xu, X., Clark, J. M., Forster, C. A., Norell, M. A., Erickson, G. M., Eberth, D. A., Jia, C., Zhao, Q. 2006. A basal tyrannosauroid dinosaur from the Late Jurassic of China. Nature, 439 (7077), 715–718.

Xu, X., Wang, K., Zhang, K., Ma, Q., Xing, L., Sullivan, C., Hu, D., Shuqing, C., Wang, S. 2012. A gigantic feathered dinosaur from the Lower Cretaceous of China. Nature, 484 (7392), 92–95.

The phytosaurs: great, big, croc-like land predators of the Triassic

Long before modern crocodiles made an appearance, other animals with a similar lifestyle occupied the same niche: the phytosaurs, which inaccurately mean "plant reptiles", because they were incorrectly assumed to be herbivores at first, from inclusions of petrified mud in the jaws of the first fossil specimens being interpreted as herbivore teeth. They are an order of long-snouted, semi-aquatic thecodont archosaurs that bore a strong resemblance to crocodiles, their armour scutes being most commonly found as fossils. Their abdomens were also toughened with densely-arranged ribs. The phytosaurs appeared abruptly in the mid Triassic, becoming especially abundant during the Upper Triassic and finally becoming extinct during the Triassic-Jurassic extinction event as a result of climate change broughtabout by an increase in carbon dioxide levels.

This group is also known more suitably as the Parasuchia (‘near the crocodiles’) because they looked very crocodile-like in their size, lifestyle and morphology, which serves as a prime example of convergent evolution. The phytosaurs were distantly related to crocodiles as both proto-crocodiles and phytosaurs shared common ancestry among the early pseudosuchia. However, true crocodiles (Crocodylia) wouldn’t appear until the late Cretaceous, long after the phytosaurs disappeared. The differences between crocodiles and phytosaurs are somewhat insignificant, but the most notable difference of all is that the nostrils of phytosaurs are situated above and between the eyes, forming blowhole-like nostrils. 

On the other hand, crocodiles have their nostrils situated far at the front of the snout. The limb position of phytosaurs were also more primitive in appearance than modern crocodiles, which were held in a semi-erect gait with their tails not dragging along the ground as determined by fossilised footprints. One other difference is that crocodiles have a secondary palate helping them to breathe whilst partly submerged in water, despite the mouth being packed chock-full of water. However, the phytosaurs lacked this feature, instead using the nostrils high on their heads to help them breathe air whilst partly submerged.

The phytosaurs can be distinguished by three different types of cranial morphology, an identification separate from phylogenetic classification: altirostral, brachyrostral and dolichorostral. These skull types are related to dental features, in particular the similarities and differentiation of teeth embedded across the jawline. I shall be briefly describing one phytosaur genus of each of these three skull morphotypes:

Altirostral (‘ high snouted’) - a skull condition intermediate between the two other types, consisting of a set of teeth that are heterodont (meaning different sized). One such taxon with this skull type is Angistorhinus grandis, known from the Popo Agie Formation of Wyoming, United States. It has an average skull length of about 90-120 cm, with an elongated, reinforced rostrum and a downturned premaxilla. The teeth were slightly larger and more compressed towards the back of the jaws (Chatterjee, 1978).

Figure 1: My reconstruction of Angistorhinus grandis basking on a river bank while ‘mouth gaping’ like modern crocodiles in order to keep cool in the very hot Triassic climate.

Brachyrostral (‘short snouted’) - broad, enormous skull, jaws and snout, with strongly heterodont teeth - the jaws consisted of distinct fang-like teeth at the front of the snout and blade-like posterior teeth at the back of the jaws which were mediolaterally-flattened, ideal for slicing up the carcasses of large tetrapods into bite-sized chunks of flesh that could be swallowed easily. The genus where these adaptations are taken to the most extreme is the huge Smilosuchus, known from the Chinle Formation of Arizona, United States. It is one of the largest known phytosaurs with an estimated length of 7-12 metres as determined by the 1.5 metre long skull (Chatterjee, 1978). 

 

Figure 2: My reconstruction of Smilosuchus gregorii, with the WIP stages included. Height of human figure = approx. 1.8 m.

Dolichorostral (‘long snouted’) - a skull condition with a relatively long, slim snout, full of medium-sized, conical, homodont teeth (meaning that they are of the same size), with distinct vertical striations. Some genera, such as the 3-8 m long Rutiodon carolinensis from the Cumnock Formation of North Carolina, were likely fish-eaters, analogous to modern day gharials of the family Gavialidae . Their long, slim snouts may have been adapted for catching fast prey such as slippery fish, whereas the weaker back teeth could secure the prey for swallowing whole, as shear could not be implemented to slice up the prey into chunks (Chatterjee, 1978).

Figure 3: My reconstruction of Rutiodon carolinensis in a late Triassic swamp.  

Certain phytosaur genera may serve as useful index fossils in determining the age of various formations because they were large water-dwelling animals, which gives them a better chance of fossilisation. For example, the fossils of genera like Angistorhinus and Paleorhinus have been used to identify the Otischalkian interval of the early Upper Carnian, whereas some like Smilosuchus served as key index fossils for identifying the Adamanian interval of the late Upper Carnian.

In a study published by Lucas (1998), a sequence of four biozones were identified, each of them distinguished by distinct phytosaur genera from the late Triassic. These intervals may have been divided by episodes of prolonged climate change, putting pressure on these large semi-aquatic archosaurs to quickly undergo speciation (when populations become isolated to evolve into separate species).

References:

Chatterjee, S. 1978. A primitive parasuchid (phytosaur) reptile from the Upper Triassic Maleri Formation of India. Palaeontology, 21, 83-127.

Hungerbühler, A. 2002. The Late Triassic phytosaur Mystriosuchus westphali, with a revision of the genus. Palaeontology, 45 (2), 377–418.

Lucas, S. G. 1998. Global Triassic tetrapod biostratigraphy and biochronology. Paleogeography, Palaeoclimatology, Palaeoecology, 143, 347-384.

Stocker, M. R., Butler, R. J. 2013. Phytosauria. Geological Society, London, Special Publications, 379, 91-117.

Earliest Stages of Diapsid Evolution - Part I (araeoscelids and kin)

Amniotes (those tetrapods that lay eggs adapted to land, equipped with a membrane encasing the embryo called an amnion) are commonly divided into several groups on the basis of the number of skull openings (or fenestrae) behind the eye sockets. The most successful of these are the diapsids, which developed two openings on either side of the skull - one upper supratemporal and one lower infratemporal fenestra. Practically all living tetrapods that we refer to as ‘reptiles’ have diapsid ancestry: snakes, lizards, crocodilians, turtles, you name it. They also include the birds (aves), which in turn are descended from dinosaurs. On the other hand, mammals along with their reptilian ancestors form the synapsid clade of amniotes; they have just one opening, which is the lower infratemporal fenestra. The synapsids and diapsids might have evolved from anapsids (a primitive clade of amniotes whose skulls lack fenestrae).

Figure 1: Simple pencil diagrams that I did to demonstrate the various skull configurations of amniotes, all in left lateral view. (a) diapsid skull of Petrolacosaurus kansensis (drawn from Reisz, 1981); (b) synapsid skull of Eothyris parkeri (drawn from Reisz et al., 2009); (c) anapsid skull of Procolophon trigonoceps (drawn from Romer, 1956). Not drawn to scale.

This differentiation may sound quite straightforward but reflects only the original primitive features and evolution can sometimes cause deception. For example, paleontologists have worked out that plesiosaurs and ichthyosaurs, the most popular Mesozoic marine reptiles, are really highly-evolved diapsids. They are characterised by having only the upper temporal fenestra behind the eye socket, which is known as the euryapsid condition. The euryapsids were initially believed to be a fourth clade of amniotes, but painstaking studies of the known fossil skulls proves that they were really descended from diapsids and that the disappearance of the lower temporal fenestra is just a secondary feature.

One other order of reptiles that have proven even more difficult to classify are the turtles, which lack fenestrae behind the eye sockets and were therefore long regarded as ‘anapsids’. In recent years, though, it now seems thoroughly proven that they too are heavily restructured diapsids, even though it took palaeontologists a very long time to work that out. If you look closely at the skulls of snakes and birds, you’ll find that they are heavily restructured with both temporal fenestrae lost, but they too are diapsids in accordance with the basic rules of phylogeny - diapsids are a monophyletic group (consisting of all descendants of a common ancestor). The diapsids are without doubt the most successful branch of amniotes with nearly 18,000 living species (when you include birds), in comparison to the approximately 6,400 species of mammals, which are the only surviving synapsids.

What exactly did diapsids evolve from and when? They may have first appeared during the Late Carboniferous from a clade of anapsid forebearers consisting of taxa like Hylonomus and Paleothyris. However, it is very difficult to tell for sure due to a patchy fossil record. The earliest true diapsids are the Araeoscelids, a family of small reptiles that superficially resembled lizards, the most well-known genera being Araeoscelis and Petrolacosaurus. The Araeoscelida are known to have lived from the Late Carboniferous to Early Permian periods.

Figure 2: My reconstruction of Hylonomus lyelli ("Lyell's Forest Mouse"), possibly the world's oldest reptile.

The oldest known diapsid of all is Petrolacosaurus kansensis, whose fossils have been dated to the ICS Gzhelian Stage of the Late Pennsylvanian subdivision of the Carboniferous period (c. 302 Ma). The binomial name translates as "rock lake reptile of Kansas", named after the locality of Rock Lake Shale where the holotype specimen was discovered, which is a partial hind limb. In 1945, it was initially described as a pelycosaur, and therefore a synapsid, but would later be recognised as the oldest known diapsid in 1977 from the discovery and description of new fossils such as a skull showing those two characteristic openings. In a swampy forest environment ruled by giant amphibians, arthropods and insects, Petrolacosaurus was considerably small, possibly measuring around 40 cm (16 in) in length, including the tail.

 

Figure 3: My reconstruction of Petrolacosaurus kansensis.

Another early diapsid from Kansas that lived around the same time is Spinoaequalis schultzei, discovered in the Calhouns Shale Formation, which also dates to the ICS Gzhelian stage, but slightly younger at ~ 300 Ma. This primitive diapsid is somewhat smaller, at ~ 30 cm (1 ft) and is considered to be an araeoscelid. One fascinating aspect of this animal is the tail, which consists of caudal vertebrae with high haemal and neural spines that are roughly equal-sized (hence the name which means ‘symmetrical spine’), forming a deep, fanned, laterally-compressed tail shape that would be ideal for swimming, which is evident of Spinoaequalis living an aquatic lifestyle. This is additionally backed up by the fact that the fossil was found in freshwater facies amongst the remains of other fully marine animals such as acanthodians (spiny sharks). Yet, the elongated, slender hind limbs are clearly those of a terrestrial animal, so it wasn’t entirely aquatic. Spinoaequalis was therefore among the earliest amniotes to have returned to the water since tetrapods first started to conquer dry land.

Figure 4: My reconstruction of Spinoaequalis schultzei.

First described as a lizard in 1910, Araeoscelis sp. dates back to the Early Permian. Like the contemporary Petrolacosaurus, it too had the same lizard-like morphology, but had a larger skull with more massive, blunter teeth specialised for processing insects with tough exoskeletons. Araeoscelis was also slightly bigger, estimated to have a total length of ~ 60 cm (2 ft), even though the tail is not known. The lower temporal fenestra is enclosed with bone which makes the skull more robust, a possible adaptation for anchoring stronger jaw muscles to help feed on a specialised diet of insects. It therefore appears to have the ‘euryapsid’ skull configuration, which would later be so commonplace in large Mesozoic marine reptiles.

 

Figure 5: The reconstructed skull of Araeoscelis gracilis in left lateral view (drawn from Reisz et al., 1984).

There are two described species of Araeoscelis, both from Texas: A. casei of the Admiral Formation (Artinskian stage) and the slightly younger A. gracilis of the Arroyo Formation (Kungurian stage). Both species are very much alike and only appear to be recognised as separate species by differences in age, A. gracilis being preserved in rocks aged ~ 275 Ma, which is ten million years younger than A. casei.

 

Figure 6: My reconstruction of Araeoscelis gracilis.

Other araeoscelid genera are not well-known, all of them also being dated to the Early Permian. Zarcasaurus tanyderus, discovered in the Cutler Formation of New Mexico, has been described from a disarticulated, incomplete skeleton consisting of a few vertebrae, fractured limb bones and a partial jaw bone (Brinkman et al. 1984). Another genus closely related to Araeoscelis (as determined by its considerably long cervical vertebrae) is Dictybolos tener whose remains have been discovered in the Wellington Formation of Oklahoma (~ 290-268 Ma) and may have been a semi-aquatic piscivore; it is estimated to be ~ 70 cm long as determined from the isarticulated bones of various individuals. In Europe, Kadaliosaurus priscus from the locality of Niederhäslich in Germany is known only from a postcranial skeleton, so its classification as an araeoscelid is uncertain. Likewise, the partial skeleton of the somewhat dubious Aphelosaurus lutevensis, known from the Les Tuilières Formation of Lodève in southern France and first described by Paul Gervais in 1858, also lacks the skull, ao again it’s difficult to tell if it really is an araeoscelid.

 

Figure 7: The skull of Orovenator mayorum in left lateral view (drawn from Reisz et al., 2011).

There is one other early diapsid distinct from the araeoscelids that dates back to the Early Permian, which is Orovenator mayorum, the most primitive and oldest known neodiapsid (a clade of amniotes consisting of all known diapsids apart from the more primitive types such as araeoscelids). It was first discovered in fissure fills at the locality of Richards Spur in Oklahoma and described on the basis of just two crushed partial skulls and mandibles. It had a rather elongated skull at about 3 cm long, so half the size of Araeoscelis and Petrolacosaurus. The locality of Richards Spur has a characteristic terrestrial vertebrate fauna of about thirty different genera and is presumed to have formed in an upland environment. This is contradictory to the araeoscelids, whose remains have been found in deposits that formed in swampy lowland environments, where fossilisation is more likely. This may mean that the separation of the diapsids into the Neodiapsida and Araeoscelida early in their evolutionary history is a consequence of adapting to life in two contrasting habitats, the neodiapsids in the uplands and the araeoscelids in the lowlands.

Overall, the early diapsids show an astonishing scope of diversity, with some types becoming semi-aquatic, such as Dictybolos and Spinoaequalis, while others like Orovenator were more adapted to drier upland habitats. Still, the fossils of these reptiles are relatively scarce and the fossil record shows a rather large void in them before they reoccur in the Late Permian. The diversity of diapsids would not significantly increase until after the Permian-Triassic mass extinction because throughout the Permian, they lived in the shadows of giant synapsids, anapsids and amphibians. This is very much similar to the small mammals living in the relative shadow of the dinosaurs, for example.

And with that, thank you very much for reading and all the best! Earliest Stages of Diapsid Evolution Part II will focus on the younginiformes...

References:

Brinkman, D. B., Berman, D. S., Eberth, D. A. 1984. A new araeoscelid reptile, Zarcasaurus tanyderus from the Cutler Formation (Lower Permian) of north-central New Mexico. New Mexico Geology, 6 (2), 34-39.

Debraga, M., Reisz, R. R. 1995. A new diapsid reptile from the uppermost carboniferous (Stephanian) of Kansas. Palaeontology, 38 (1), 199-212.

Lane, H. H. 1945. New mid-Pennsylvanian reptiles from Kansas. Transactions of the Kansas Academy of Science (1903-), 47 (3), 381-390.

Olson, E. C., Williams, P. M. 1970. New and little known genera and species of vertebrates from the Lower Permian of Oklahoma. Fieldiana: Geology, 18 (3), 359-434.

Reisz, R. R. 1977. Petrolacosaurus, the oldest known diapsid reptile. Science, 196 (4294), 1091-1093.

Reisz, R. R. 1981. A diapsid reptile from the Pennsylvanian of Kansas. Special Publication of the Museum of Natural History, University of Kansas, 7, 1-74.

Reisz, R. R., Berman, D. S., Scott, D. 1984. The anatomy and relationships of the Lower Permian reptile Araeoscelis. Journal of Vertebrate Paleontology, 4 (1), 57-67.

Reisz, R. R., Godfrey, S. J., Scott, D. 2009. Eothyris and Oedaleops: do these Early Permian synapsids from Texas and New Mexico form a clade? Journal of Vertebrate Paleontology, 29 (1), 39-47. 

Reisz, R. R., Modesto, S. P., Scott, D. M. 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society B, 278 (1725), 3731-3737.

Romer, A, S. 1956. Osteology of the reptiles. University of Chicago Press, Chicago, IL.

Williston, S. W. 1913. The skulls of Araeoscelis and Casea, Permian reptiles. The Journal of Geology, 21 (8), 743–747.

Brontosaurus excelsus - the deceptive thunder lizard

The classic sauropod as well as being among the most famous is Brontosaurus, a typical representative from the Morrison Formation of Wyoming, which is dated to the Late Jurassic (c. 156-147 million years ago). Yet, this thickly-necked, robust relative of the slimmer Diplodocus has had a long, baffling history of classification.

Figure 1: My digital reconstruction of a solitary Brontosaurus excelsus wandering around a semi-arid environment. An Allosaurus fragilis, the apex predator of the Morrison Formation, lurks in the background.

 

The eminent 19th century American palaeontologist Othniel Charles Marsh first described Apatosaurus ajax in 1877 on the basis of a partial juvenile sauropod skeleton, then found another, better intact skeleton in 1879 that he identified as Brontosaurus excelsus. In 1903, research carried out by American paleontologist Elmer Riggs confirmed that Apatosaurus ajax and Brontosaurus excelsus were so much alike that they ought to be combined as a single genus, and in accordance with the rules of zoological nomenclature, the name ‘Apatosaurus’ had priority as it was named first. As a result, Brontosaurus was regarded as an invalid genus for over a century despite its popularity. 

Then an extensive in-depth study of diplodocid phylogeny by Tschopp, Mateus and Benson (2015) using a specimen-based approach concluded that Brontosaurus was definitely a valid genus of sauropod distinct from Apatosaurus. This was determined from use of statistical analysis to more objectively assess the fossil specimens of different species of Apatosaurus; they discovered that fossils previously associated with Brontosaurus excelsus can certainly be classified as being dissimilar enough to bring it back as a valid genus. They also deduced that the genera Eobrontosaurus and Elosaurus are in fact synonyms of Brontosaurus. According to the study, Apatosaurus can be distinguished from Brontosaurus in that the former has a lower, wider neck. Both genera had greatly bifurcated cervical vertebrae (meaning that they held paired spines), which is what made the necks so wide and deep (Wedel & Taylor, 2013).

The massive, unusual and oddly-proportioned cervical vertebrae of Brontosaurus and other apatosaurine sauropods has long been subject to controversy, but one interesting recent study indicates that they may have been adapted for fighting. The cervical vertebrae show indications of possessing extended areas for muscle attachment, raised leverage for neck musculature and maybe even anchoring points for horns or bosses on the ventral surface of each vertebra, which I’ve included in my reconstruction above.  One possible hypothesis that has been proposed is that male Brontosaurus used their long, powerful necks as armoured organs to fight each other over dominance or mates (Taylor et al., 2015). Maybe someday I'll make a reconstruction of a pair of dueling Brontosaurus, as a pen and ink drawing with a touch of watercolour.

The study by Tschopp et al. (2015) resuscitating Brontosaurus has helped prove that sauropods have much greater diversity than previously thought and that the identification of Brontosaurus as a separate genus from Apatosaurus indicates that even greater mysteries remain to be fully understood...

References:

Tschopp, E., Mateus, O., Benson. R. B. J. 2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3: e857; DOI 10.7717/peerj.857.

Taylor, M. P., Wedel, M. J., Naish, D., Engh, B. 2015. Were the necks of Apatosaurus and Brontosaurus adapted for combat? PeerJ PrePrints 3: e1347v1.

Wedel, M. J., Taylor, M. P. 2013. Neural spine bifurcation in sauropod dinosaurs of the Morrison Formation: ontogenetic and phylogenetic implications. Palarch’s Journal of Vertebrate Palaeontology, 10, 1-34.

Dawn of the synapsids

The synapsids are a clade of amniotes that comprise mammals (including us humans), their ancestral reptilian groups (often incorrectly referred to as ‘mammal-like reptiles’) and other extinct relatives. Mammals are not descended from reptiles, but both classes of vertebrate shared a common ancestor. The synapsids are distinguished by having one temporal fenestra low behind each orbit in the skull and are the second most diverse clade of amniotes after the diapsids. The earliest synapsids of all constitute a family known as the Ophiacodontidae: medium-sized animals that thrived throughout the Late Carboniferous and Early Permian, which were also some of the largest terrestrial carnivores at the time.

They were an extraordinary group with unusually large skulls in proportion to their body size, being very narrow, high and elongate. Their jaws were densely packed with many small, sharp teeth that were curved slightly and started to show some differentiation in size. They also had comparatively short, sturdy limbs as well as very large pectoral girdles, presumably to provide muscle attachments for holding up the disproportionately big head.

The undisputed basalmost ophiacodontid is represented by Archaeothyris florensis (Reisz, 1972) known from fragmentary material found in Nova Scotia, Canada, dating back to the late Carboniferous (c. 306 Ma during the Westphalian stage/Mid Pennsylvanian) and is also the oldest confirmed synapsid. It resembled a modern lizard to some extent, but already had the high, elongate skull characteristic of the group. At approximately 0.5 m long, it co-existed with other primitive reptile taxa like Palaeothyris in swampy forests consisting of tree-like club mosses (lycopsids) and ruled by giant arthropods, insects and amphibians. It had strong jaws equipped with sharp, pointed teeth that were roughly the same shape, but also possessed an enlarged pair of canines at the front of the maxilla, indicating a mixed carnivorous diet (van Tuinen & Hadley, 2004).

Figure 1: My reconstruction of Archaeothyris florensis.

The best-studied and best-known genus of all is Ophiacodon (Romer, 1925; Romer & Price, 1940), numerous skeletons of which have been found in Early Permian strata from Colorado, Kansas, New Mexico, Ohio, Oklahoma, Texas and Utah, so is represented by a thorough fossil record. Six species have been described (determined by stratigraphic position and size), although it’s not 100% clear how many of these species are truly valid.

The type species O. mirus was initially described in 1878 by Othniel Charles Marsh on the basis of some vertebrae and a mandible during the “Bone Wars”, a period of intense rivalry between Edward Drinker Cope to discover and name more new fossil species. Marsh evidently wanted to outcompete Cope, who had an in-press article naming and describing the reptile, but it was deficient and written so promptly that the genus Ophiacodon was neglected by the scientific community for over 3 decades. Meanwhile, Cope published an article shortly after Marsh published his, describing three new species in a differently named genus. On the basis of individual vertebrae, these were named Theropleura retroversa, Theropleura triangulata and Theropleura uniformis. Later, the holotype specimen of Ophiacodon was re-examined and a description was made of a brand-new intact skeleton (Williston & Case, 1913). Even later, Ophiacodon and Theropleura were revealed to be synonymous, the former which had priority as it was named first, even though the species names were still kept (Romer & Price, 1940).

Figure 2: My reconstruction of Ophiacodon mirus.

These are the six species of Ophiacodon identified to this day:

 

O. hilli - known from a partial skeleton found in Kansas.

O. major - known from incomplete material found in Texas.

O. mirus - the type species, known from a few skeletons found in Oklahoma and New Mexico, including one that is almost complete.

O. navajonicus - known from incomplete postcranial remains found in New Mexico.

O. retroversus - known from numerous material found in Oklahoma and Texas, which includes an almost complete skeleton

O. uniformis - known from a few incomplete skeletons found in Oklahoma and Texas.

However, it may be that differences in size (which range from 5 - 10 ft long) only reflect various growth stages instead of different species (Brinkman, 1988). At first, Ophiacodon was thought to have been a semi-aquatic predator but is now considered to have been entirely terrestrial, as recent studies have disproved the alleged aquatic adaptations that the animal may have had (Felice & Angielczyk, 2014).

There are many other known ophiacodontids which have been described from relatively incomplete material, such as the Late Carboniferous taxa Stereorhachis dominans from France and Echinerpeton intermedium and Clepsydrops sp. from North America, as well as the Early Permian taxa Baldwinonus trux and Sterophallodon ciscoensis from North America. Protoclepsydrops haplous (Carrol, 1964), known from Joggins, Nova Scotia, may have been an ophiacodontid too, preceding even Archaeothyris florensis, but due to the paucity of fossil material it is hard to know for certain.

One other family of early carnivorous synapsids appearing in the Late Carboniferous were the even more primitive-looking varanopids. Like the ophiacodontids, they too had skulls which were narrow, deep and elongated with the orbits set far back, but had a more slender mandible with a specialised marginal dentition, as well as some extended parietal bones on the skull roof above the orbits. (Reisz & Dilkes, 2003). Other characteristics of the group also include a covering of extensive dermal osteoderms unlike other clades of synapsids, which is especially evident in Heleosaurus (Botha-Brink and Modesto, 2009). The varanopids greatly resembled monitor lizards and like their modern namesake, may have lived similar lifestyles and ecological niches; they have a stratigraphically wide-ranging, cosmopolitan distribution extending to the Mid Permian.

The type genus Varanops brevirostris was as big as a large monitor lizard (at ~ 4 ft long) with sharp, flattened, strongly-curved teeth, clearly those of a flesh-eating predator. It is known from a few individuals which include almost complete skeletons from the Garber Formation of Oklahoma and the Arroyo Formation of Texas. One of these skeletons even provides evidence of its ecology, where bite marks along with a tooth stuck between the ulna and radius shows that the body of this individual was scavenged on by a dissophoroid temnospondyl amphibian (Reisz & Tsuji, 2006).

Figure 3: My  reconstruction of Varanops brevirostris.

The largest known varanopid of all is Watongia meieri, which judging from fragmentary remains found at the Chickasha Formation (Mid Permian in age) of Oklahoma is estimated to be approximately 2 - 2.5 m long, so may have been the apex predator of its time and place. It is believed to be a varanopid and not a gorgonopsid as first thought, judging from characteristics such as large, lateral protuberances on the postorbital bone as well as marginal, backward-curving teeth unserrated on the anterior and posterior edges. It may also have had a disproportionately large head in comparison to its body (Reisz & Laurin, 2004).

 

Figure 4: My reconstruction of Watongia meieri, based on a study by Reisz & Laurin (2004). Note the bony protuberance on the postorbital (behind the eye).

One of the most primitive varanopids is Mycterosaurus longiceps, a small, agile carnivorous/insectivorous synapsid known from a partial skeleton found at the Waggoner Ranch Formation dating back to the Early Permian (Artinskian). As well as having maxillary teeth distinct from those of other varanopids, it possessed many small palatal teeth too. It also had vertebrae comparable to Varanops: the dorsal neural spine is broad in lateral view, with vertical posterior and anterior edges. The dorsal centra are considerably longer than the lumbar centra (Romer & Price, 1940). In addition, a highly mobile ankle joint present in both Varanops and Mycterosaurus suggests that they had a semi-digitigrade stance.

Figure 5: My reconstruction of Mycterosaurus longiceps.

References:

Botha-Brink, J., Modesto, S. P. 2009. Anatomy and relationships of the Middle Permian varanopid Heleosaurus scholtzi based on a social aggregation from the Karoo Basin of South Africa. Journal of Vertebrate Palaeontology, 29 (2), 389-400.

Brinkman, D. 1988. Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology, 8 (2), 172-180.

Carroll, R. L. 1964. The earliest reptiles. Zoological Journal of the Linnean Society, 45 (304), 61–83.

Felice, R. N., Angielczyk, K. D. 2014. Was Ophiacodon (Synapsida, Eupelycosauria) a swimmer? A Test Using Vertebral Dimensions. Early evolutionary history of the Synapsida, Springer Netherlands, pp. 25-51.

Reisz, R. R. 1972. Pelycosaurian Reptiles from the Middle Pennsylvanian of North America. Bulletin of the Museum of Comparative Zoology, 144 (2), 27-60.

Reisz, R. R., Dilkes, D. W. 2003. Archaeovenator hamiltonensis, a new varanopid (Synapsida: Eupelycosauria) from the Upper Carboniferous of Kansas. Canadian Journal of Earth Sciences, 40 (4), 667-678.

Reisz, R. R., Laurin, M. 2004. A reevaluation of the enigmatic Permian synapsid Watongia and of its stratigraphic significance. Canadian Journal of Earth Sciences, 41 (4), 377-386.

Reisz, R. R. Tsuji, L. A. 2006. An articulated skeleton of Varanops with bite marks: the oldest known evidence of scavenging among terrestrial vertebrates. Journal of Vertebrate Palaeontology, 26 (4), 1021-1023.

Romer. A, S. 1925. An ophiacodont reptile from the Permian of Kansas. Journal of Geology, 33 (2), 173-182.

Romer, A. S., Price, L. I. 1940. Review of the Pelycosauria. Geological Society of America Special Papers, 28, 1-538.

van Tuinen, M., Hadly., E. A. 2004. Error in estimation of rate and time inferred from the early amniote fossil record and avian molecular clocks. Journal of Molecular Evolution, 59, 267-276.

Williston, S. W., Case, E. C. 1913. Description of a nearly complete skeleton of Ophiacodon Marsh. Carnegie Institution of Washington Publication, 181, 37-59.