Early Stages of Diapsid Evolution - Part II (Younginiformes)

In a past blog post, I have explained in detail the rise of the diapsids throughout the Late Carboniferous and Early Permian periods. Unfortunately, we have a rather vague knowledge about what occurred throughout the rest of the Permian because the fossil record of diapsids is very patchy between the Kungurian Stage (when Araeoscelis gracilis last lived) and the latest Permian (when a considerable amount of diapsid fossils reappear at a few localities across the world).

Figure 1: The skull of Lanthanolania ivakhnenko in left lateral view (drawn from Modesto et al., 2003).

Throughout the whole Guadalupian epoch (Mid Permian), the diapsids comprise what palaeontologists describe as a “ghost lineage” - this is because we only know that they were around back then because they are known to have lived before and after the Mid Permian, although no fossils have ever been discovered dating to that time. However, not quite: a partial skull found in the Mezen River basin in the Mezen District of Arkhangel'sk province in Russia has been described as Lanthanolania ivakhnenko, the genus name which translates as “forgotten ripper” (Modesto et al., 2003). Of all the hundreds of amniote fossils found at the Mezen River basin, the only diapsid is Lanthanolania and was initially misidentified as a specimen of Mesenosaurus, a synapsid of the varanopid type. Lanthanolania was a miniature neodiapsid, which may have been no bigger than 30 cm (1 ft) in length as estimated from the tiny 30 mm long skull.

At the time, diapsids must have been very uncommon, existing in the relative shadows of much larger anapsids, synapsids and amphibians which were a lot more successful back during the Permian period. The partial skull of Lanthanolania has been dated to the ICS Wordian stage, or the uppermost Kazanian in eastern Europe, somewhere between approximately 266 and 257 million years ago. Despite the discovery of Lanthanolania inducing a bit of excitement amongst those experts trying to fill the voids in our knowledge, it still didn’t explain much about the evolutionary history of diapsids during the Middle Permian because the remains were so incomplete. One thing we do know for certain is that between the late Early Permian and the Late Permian, the araeoscelids had already gone extinct whilst the neodiapsids managed to survive and somewhat diversify.

By the Late Permian, a mixed assortment of neodiapsids collectively known as eosuchians (meaning ‘early crocodiles’, although this may be misleading as it didn’t include crocodilians at all) had arrived on the scene. However, the Eosuchia is something of a wastebasket taxon and the phylogenetic relationships and taxonomic classification of eosuchians is rather unclear. It was initially assumed that eosuchians ought to be divided into the ancestors of archosaurs (e.g. dinosaurs, birds, pterosaurs, crocodilians) and the ancestors of lepidosaurs (e.g. lizards and snakes), but current thinking is that their is an even more complex evolutionary history than this. One recently proposed cladogram (Reisz et al., 2011) demonstrates a supposedly paraphyletic group with ‘younginiformes’ being the basalmost eosuchians (comprising both aquatic and terrestrial types), followed by the marine Claudiosaurus germaini (originally thought of as an early sauropterygian), then by some representatives of a family of terrestrial lizard-like types known as ‘paliguanids’, and lastly the gliding weigeltisaurids. They all perished by the Early Triassic, leaving no surviving descendants.

Figure 2: The skull of Youngina capensis in left lateral view (drawn from Carroll, 1981).

I shall now be covering the diversity of ‘younginiformes’ in greater detail, commencing with the terrestrial forms. Youngina capensis (Broom, 1914) was a miniscule lizard-like animal, originally described on the basis of a fragmentary skull discovered in the Daptocephalus Assemblage Zone of the Karoo Supergroup in South Africa, dated to the latest Permian at ~ 250 million years old. A particular characteristic present in Youngina is one simple row of osteoderms running midway along the back. Youngina was part of an ecosystem consisting of a diverse array of fauna such as synapsids (e.g. biarmosuchians, gorgonopsians, dicynodonts etc.) and anapsids, in an environment with a semi-arid climate. One amazing find is an assemblage of five intact, articulated juvenile skeletons, suggesting that Youngina may have been living in dens (Smith & Evans, 1996). In the subsequent years following its initial description, a few other specimens of Youngina were later discovered, predominantly skulls, each being given distinct names (such as Youngopsis and Youngoides) all of which are now believed to be synonyms of Youngina.

Figure 3: My reconstruction of Youngina capensis.

The following taxa are also classified as younginids, but with uncertainty: Heleosuchus griesbachi, first described as a species of Saurosternon by Richard Owen in 1876,  is known from just one specimen - an incomplete postcranial skeleton with posterior fragments of the skull. It was first discovered in South Africa from an obscure horizon, being dated to the Early Triassic or Late Permian. The specimen was assumed to have been lost until it was moved to the Natural History Museum of Vienna, Austria (Carrol, 1987).

Galesphyrus capensis (also described by Broom in 1914) is known from an incomplete postcranial skeleton, discovered at the bottom of the Cistecephalus Assemblage Zone of the Karoo Supergroup in South Africa, so Youngina is therefore younger.

Kenyasaurus mariakanensis (Harris & Carroll, 1977) is one species of neodiapsid discovered in the Maji-Ya-Chumvi Formation from Kenya, dated to the Early Triassic. Its phylogenetic relations are difficult to interpret because only one specimen is known, consisting of very incomplete forelimbs and pectoral girdle material as well as a missing skull, but may have been closely related to Lanthanolania. The remains of Kenyasaurus were also discovered in marine beds, but lacks adaptations for an aquatic lifestyle, just like Thadeosaurus. Thus, it was a small, terrestrial lizard-like form measuring roughly 0.5 m long. In addition, Kenyasaurus was recently found to have not been a member of the younginiformes. With the absence of well-preserved skull material, knowing the exact phylogenetic relations of all these taxa has proved to be very challenging.

Figure 4: My reconstruction of Thadeosaurus colcanapi.

Thadeosaurus colcanapi (Carroll, 1981) is a better-known neodiapsid first discovered in the Lower Sakamena Formation of southern Madagascar, known from two partial skeletons that are almost complete, but are lacking the distal (lower) segments of the limbs as well as the skull. Thadeosaurus was initially and mistakenly presumed to belong to Datheosaurus from Europe and in turn, Datheosaurus was believed to be a synonym of Haptodus (a genus of pelycosaur and therefore, a synapsid) but is now considered a basal caseid within the synapsids. Also bear in mind that two other taxa from the Permian, Apsisaurus witteri and Heleosaurus scholtzi, previously thought of as younginiformes, are now believed to have been synapsids of the varanopid family. It appears that a lot of these primitive diapsids from the Permian were misidentified as synapsids at first! One other interesting thing about the genus Thadeosaurus is that it’s merely an anagram of the genus Datheosaurus, the only anagram of a scientific name in an animal that I currently know of.

Figure 5: My reconstruction of the pelycosaurian synapsid Haptodus baylei.

Thadeosaurus was a rather small animal, estimated to be approximately 60 cm in length, and was very lizard-like in appearance, being distinguished by its particularly elongated tail. Several specimens of Thadeosaurus are known, which include juveniles (the only known skull is from a juvenile too), most of them originally being misinterpreted as Tangasaurus mennelli from northeastern Tanzania (Currie & Carrol, 1984). Thadeosaurus shows no clear specialisations for swimming despite being discovered in beds deposited in a marine environment, suggesting that it may have been a terrestrial neodiapsid living along the coast.

 

Figure 6: My reconstruction of Tangasaurus mennelli, a neodiapsid that must have lived an aquatic lifestyle as indicated by its long, powerful flattened tail. 

One intriguing aspect about the Lower Sakamena Formation is that the marine deposits consist of an extraordinarily large quantity of diapsid reptiles in comparison to all the other Late Permian formations elsewhere around the world. The age of the deposits has been determined by the correlation of vertebrate fossils with those from South Africa as well as by palynological analysis, being dated from the Capitanian stage of the late Middle Permian to the Wuchiapingian stage of the early Late Permian. Among the useful index fossils is the procolophonoid parareptile Barasaurus besairiei, similar in appearance to Owenetta rubidgei. However, Owenetta was subsequently found in Early Triassic beds too.

References: 

Broom, R. 1914. A new thecodont reptile. Proceedings of the Zoological Society of London B, 84 (4), 1072–1077.

Carroll, R. L. 1981. Plesiosaur ancestors from the Upper Permian of Madagascar. Philosophical Transactions of the Royal Society of London B, 293 (1066), 315-383.

Carroll, R. 1987. Heleosuchus: An enigmatic diapsid reptile from the Late Permian or Early Triassic of southern Africa. Canadian Journal of Earth Sciences, 24, 664-667.

Currie, P. J. 1982. The osteology and relationships of Tangasaurus mennelli Haughton (Reptilia, Eosuchia). Annals of the South African Museum, 86 (8), 247–265.

Currie, P. J., Carroll, R. L. 1984. Ontogenetic changes in the eosuchian reptile Thadeosaurus. Journal of Vertebrate Paleontology, 4 (1), 68–84.

Harris, J. M., Carroll, R. L. 1977. Kenyasaurus, a new eosuchian reptile from the Early Triassic of Kenya. Journal of Paleontology, 51 (1), 139-149.

Modesto, S., Reisz, R. R. 2003. An enigmatic new diapsid reptile from the Upper Permian of Eastern Europe. Journal of Vertebrate Paleontology, 22 (4), 851-855.

Olson, E. C. 1936. Notes on the skull of Youngina capensis Broom. The Journal of Geology, 44 (4), 523-533.

Reisz, R. R., Scott, D. 2002. Owenetta kitchingorum, sp. nov., a small parareptile (Procolophonia: Owenettidae) from the Lower Triassic of South Africa. Journal of Vertebrate Palaeontology, 22 (2), 244-255.

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, 3731-3737.

Smith, R., Evans, S. E. 1996. New material of Youngina: evidence of juvenile aggregation in Permian diapsid reptiles. Palaeontology, 39 (2), 289-303.

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.