Archaeopteryx

Type Species: Archaeopteryx lithographica

Classification: Dinosauria – Saurischia – Theropoda – Coelurosauria – Maniraptora – Paraves – Avialae – Archaeopterygidae

Time Period: Late Jurassic 

Location: Europe 

Diet: Carnivore

Archaeopteryx’s name means ‘Ancient Wing/Feather’, and for a long time it was thought to be the oldest known member of the Avialae (‘birds’), at least until earlier specimens were unearthed in China as far back as the Middle Jurassic. This Late Jurassic paravian lived among the cycad- and conifer-dotted wooded islands of prehistoric Germany, mingling with small lizards, pterosaurs, other paravians, and small theropods such as Compsognathus. Multiple specimens have been uncovered in what’s known as Solnhofen Lagoon, which during the Late Jurassic was interspersed with dozens of small islands. Archaeopteryx was similar in size to a Eurasian magpie, though larger individuals could reach the size of a raven. The largest species grew to about one and a half feet in length and would’ve weighed just over two pounds. Though originally considered to be the first bird, many paleontologists decry this statement, for Archaeopteryx had more in common with non-avian theropods – particularly dromaeosaurs and troodontids – than with birds. Just for beginners, it had toothy jaws, three clawed fingers on each hand, a lony bony tail, and hyper-extendable ‘killing claws’ on its feet. Though it had feathers, these are a shared morphology between birds and many different theropod classes. Because it’s morphologically closer to non-avian dinosaurs than modern birds, many have considered it a ‘transitional fossil’ between non-avian dinosaurs and avian dinosaurs and their modern-day descendants. Ironically, this ‘transitional fossil’ appears millions of years after more advanced paravians such as Serikornis and Anchiornis.

Archaeopteryx’s feathers were similar in structure to modern-day bird feathers. Its feathers were asymmetrical and showed the same structure as modern birds’ flight feathers: vanes given stability by a barb-barbule-barbicel arrangement. The tail feathers, too, were assymetrical with firm vanes. Its thumb didn’t yet bear a separately movable tuft of stiff feathers. Serious studies of Archaeopteryx feathers have been done one specimen dubbed ‘the Berlin specimen’. In this species, its legs had ‘trousers’ of well-developed feathers that are firm and capable of supporting flight. Pennaceous feathers ran along its back; these were asymmetrical and firm, though not as stiff as the flight-related feathers; thus these were similar to the contour feathers of the body plumage of modern birds. Aside from these feathers, the rest of the Berlin specimen was covered in a type of ‘proto-down’ similar to that found in the Early Cretaceous Sinosauropteryx. This ‘proto-down’ was decomposed and fluffy, and it may even have resembled fur in real life (though the fact that it wasn’t fur would be clear up-close under a microscope). This ‘proto-down’ went as far as the lower neck. In all species, the upper neck and head are clear of feathers. This is explained in one of two ways: either Archaeopteryx simply lacked feathers or proto-down on the upper neck and head, or this is an artifact of preservation. In the latter scenario, Archaeopteryx indeed had feathers (or, more likely, proto-down) on its upper neck and head, but these features were lost in death. Scientists believe most Archaeopteryx specimens became embedded in anoxic sediment after drifting for quite some time on their backs in the sea; in these anoxic waters, their corpses wouldn’t be scavenged, for marine life would be limited to smaller organisms that can survive such oxygen-depleted waters. While drifting on the waves, the head, neck, and tail bent downwards, with the body floating atop. They began to rot before they sank under the waves, resulting in loosening tendons and muscles resulting in the infamous ‘death pose.’ The skin, already softened by decay and by underwater friction, wouldn’t be able to keep hold of feathers or proto-down. Before the corpse settled to the seabed, those parts underwater – the neck, head, and tail – would begin to lose their feathers. The more firmly attached body feathers would’ve remained intact by the time the corpse settled on the seabed to be covered with sediment and fossilized for our enjoyment. 

Scientists have used electron microscopy and energy-dispersive X-ray analysis to detect the structure of the melanosomes of one of Archaeopteryx’s wing feathers and compared them with those of over eighty modern bird species; the conclusion was that Archaeopteryx’s original feather color was black with heavier pigmentation in the distal tip. In 2013 another study was done that indicated that Archaeopteryx’s flight feathers had complex light- and dark-colored plumage with heavier pigmentation in the distal tips and outer vanes. This type of coloration is consistent with many modern birds in which black melanosomes have structural properties that strengthen feathers for flight. This leads, of course, to the ultimate question: ‘Was Archaeopteryx capable of powered flight?’ Some scientists doubt that it could fly on its own, insisting that it was a ‘glider’ who would climb high into a tree, launch off, and glide to another tree or to the ground. Others argue that it was indeed capable of flight, though its flight mechanisms would’ve differed from those seen in modern birds. That it was capable of some sort of flight is undisputed. The fact that its feathers are asymmetrical hints at flight, for flightless birds tend to have symmetrical feathers; however, some flightless birds have asymmetrical feathers similar to those of Archaeopteryx. However, the degree of asymmetry in this dinosaur’s feathers more closely resemble those of slow-flying birds than flightless ones. If Archaeopteryx was capable of flight, it would’ve been more ungainly than what we’re used to; for instance, recent studies of flight feather barb geometry reveal that modern birds possess a larger barb angle in the feather’s trailing vane, and Archaeopteryx, lacking this large barb angle, would’ve been a weaker flier. Furthermore, the lack of a bony breastbone upon which flight muscles could attach also testifies to weak flying capabilities; detractors from this conclusion speculate that its strong flight muscles may have attached to the thick, boomerang-shaped wishbone, the plate-like coracoids, or even to a cartilaginous sternum. However, even if it had strongly-anchored flight muscles, its shoulder anatomy made it unable to lift its wings above its back, a requirement for the upstroke used by modern flying birds. However, it may have utilized a downstroke for powered flight. If it were capable of powered flight, its power would’ve been hindered by its large wings, which would’ve resulted in a low stall speed and reduced turning radius. The short, rounded shape of the wings would’ve increased drag, but scientists point out that it would’ve also improved its ability to fly through cluttered environments dense with trees and brush. Such heightened aerial mobility would’ve been improved by its hind wings on its legs. 

While much of the debate over Archaeopteryx’s flying ability (or lack thereof) revolves around its feather and anatomical structure, in 2004 some scientists took a different approach, asking themselves, ‘Did Archaeopteryx have a brain suited for flying?’ By analyzing a detailed CT scan of the braincase of an Archaeopteryx, the scientists found that its brain was proportionately much larger than those of most dinosaurs. The regions associated with vision took up nearly a third of the braincase, and other well-developed areas included hearing and muscle coordination. These are all factors seen in modern birds and necessary for aerial locomotion. The scientists also examined the inner ear and discovered that it more closely resembled the ears of modern birds than those of non-avian dinosaurs. Archaeopteryx had a cerebrum-to-brain-volume ratio 78% on the way to modern birds from the condition of non-coelurosaurian dinosaurs such as Allosaurus, which had a crocodile-like brain and inner ears. All this together suggests that Archaeopteryx had the keen sense of hearing, balance, spatial perception, and coordination needed to fly. The studies of Archaeopteryx’s braincase were revolutionary in the debate over its place in the sky, and in 2014 a team of dedicated scientists reported that their consensus was that Archaeopteryx was indeed capable of powered flight, but in a manner distinct and different from that of modern birds. Its modern equivalents would be pheasants and other burst flyers. 

If we were to step back in time to the Late Jurassic Period in Germany, we would find ourselves in the Solnhofen Lagoon among scattered low-lying, semi-arid, sub-tropical islands. The vegetation of these islands consisted of low-lying shrubs and cycads; interestingly, the islands of this lagoon seem to have lacked conifers. Few trunks have been found in the sediments, and tree pollen is absent. It was in this environment that Archaeopteryx lived, and it was a prime environment for a pheasant-like burst-flying hunter. It most likely hunted small prey, seizing it with its jaws if it was small enough, or with its claws if it was larger.  

Sciurumimus

Type Species: Sciurumimus albersdoerferi

Classification: Dinosauria - Saurischia - Theropoda - Carnosauria - Megalosauroidea

Time Period: Late Jurassic 

Location: Europe 

Diet: Carnivore

The small bipedal theropod Scuirumimus lived among the large islands and archipelagos of prehistoric France and Germany. During the Late Jurassic, much of modern western Europe was submerged by the Oxford Sea and shallow sea armlets of the Tethys Sea that were ringed with coral reefs. Emerging from this vast spider-web network of waterways were dry, forested islands cut by streams and lagoons. These islands catered to smaller terrestrial organisms: lizards, mammals, and a number of small-fry dinosaurs, Scuirumimus among them. This theropod lived alongside the compsognathids Compsognathus and Juravenator, as well as a number of paravians, such as Ostromia, Wellnhoferia, Alcmonavis, and of course the infamous Archaeopteryx.

Sciurumimus’ name means ‘Squirrel-mimic’ for its tail’s resemblance to that of the modern tree squirrel. It’s known from a single juvenile specimen, so its adult length is unknown. While classified as a megalosaurid, this classification is hotly debated. The single specimen was preserved with traces of feather-like filaments. The specimen’s body proportions – the short forelimbs, the lack of fusion in the skeleton, as well as the fact that its skull is 156% the length of the femur – indicate that it was a juvenile and perhaps as young as a hatchling. 

Juravenator

Type Species: Juravenator starki 

Classification: Dinosauria – Saurischia – Theropoda – Tetanurae – Coelurosauria – Tyrannoraptora – Compsognathidae   

Time Period: Late Jurassic 

Location: Europe 

Diet: Carnivore

The small bipedal theropod Juravenator lived among the large islands and archipelagos of prehistoric France and Germany. During the Late Jurassic, much of modern western Europe was submerged by the Oxford Sea and shallow sea armlets of the Tethys Sea that were ringed with coral reefs. Emerging from this vast spider-web network of waterways were dry, forested islands cut by streams and lagoons. These islands catered to smaller terrestrial organisms: lizards, mammals, and a number of small-fry dinosaurs, Juravenator among them. This theropod lived alongside its larger cousin Compsognathus, the squirrel-like theropod Sciurumimus, and a number of paravians, such as Ostromia, Wellnhoferia, Alcmonavis, and of course the infamous Archaeopteryx. Juravenator is known from a single juvenile specimen that was twenty-nine inches head-to-tail. This fleet-footed hunter likely preyed on lizards, mammals, and insects. The scleral rings of Juravenator resemble those of nocturnal birds, suggesting that it was a night-time hunter (but some believe these scleral rings are due to the fact that it was a juvenile and that adults would’ve been daytime hunters). A patch of Juravenator’s skin shows dinosaur scales as well as traces of proto-feathers. 

Compsognathus

Type Species: Compsognathus longipes

Classification: Dinosauria – Saurischia – Theropoda – Tetanurae – Coelurosauria – Tyrannoraptora – Compsognathidae   

Time Period: Late Jurassic 

Location: Europe 

Diet: Carnivore

The small bipedal theropod Compsognathus lived among the large islands and archipelagos of prehistoric France and Germany. During the Late Jurassic, much of modern western Europe was submerged by the Oxford Sea and shallow sea armlets of the Tethys Sea that were ringed with coral reefs. Emerging from this vast spider-web network of waterways were dry, forested islands cut by streams and lagoons. These islands catered to smaller terrestrial organisms: lizards, mammals, and a number of small-fry dinosaurs, Compsognathus among them. This theropod lived alongside its smaller cousin Juravenator, the squirrel-like theropod Sciurumimus, and a number of paravians, such as Ostromia, Wellnhoferia, Alcmonavis, and of course the infamous Archaeopteryx. Compsognathus itself may have island-hopped all the way down to Portugal, for teeth that may belong to this dinosaur have been found in the Lourinha Formation. 

Compsognathus is known from two nearly complete specimens, one from France and one from Germany. The French specimen was forty-nine inches long, and the German specimen was 35 inches long. This lithe theropod likely weighed as little as seven pounds full grown. Compsognathus had long hind legs and a long tail, which it used for balance while running; some scientists have postulated that it could reach speeds up to forty miles per hour. Its forelimbs were smaller than its hindlimbs, and each hand bore two large clawed digits and a third, smaller digit that may have been non-functional. Their delicate skulls were long and narrow with tapered snouts. Its eyes were large in proportion to the skull, indicating that it was a visually-oriented predator that relied more on sight than smell for hunting. Its long neck enabled it to move its head side-to-side without moving its body, helping it spy out prey; and the long neck would’ve come in handy when plunging its head into the undergrowth to root out hiding prey. This dinosaur was designed for catching small and fast-moving prey like lizards that would’ve scurried into the undergrowth as soon as they were seen. Compsognathus’ speed is attested by a proportionately small humerus when compared to the lower legs, a trademark of a fast runner. It balanced itself on its toes rather than the flat of its foot, another design appropriate for speed; this digitigrade stance meant that the main foot bones extended the length of the leg and thus increased its stride. Its tail, held erect off the ground, would’ve served as a balancing aid as it chased prey through the wooded islands of prehistoric western Europe. Its small, sharp teeth were suited for a diet of small vertebrates and perhaps even insects. Its frontmost teeth were unserrated, but those farther back in the jaw were flattened and recurved. The specimens we have preserve part of its diet: in the French specimen, the remains of unidentified lizards and sphenodonts (lizard-like reptiles) are found in its thoracic cavity; in the German specimen, the articulated remains of a full lizard are in plain view. These lizard remains were originally thought to belong to an embryonic Compsognathus, which would’ve rewritten the book on dinosaur reproduction, but further research showed that it belonged to a Bavarisaurus lizard. This lizard was fast and agile, which meant that Compsognathus would’ve also needed to be fast and agile in order to hunt it. Compsognathus may also have preyed on small mammals, as its close Early Cretaceous relative Sinosauropteryx has been discovered with the remains of small mammals in its stomach. 

Some artistic renditions depict Compsognathus with feathers while others do not; this is because the presence of feathers or feather-like features on this theropod is hotly debated. While some of Compsognathus’ relatives have been preserved with the remains of simple feathers covering the body like fur, this isn’t the case with Compsognathus. Archaeopteryx, found in the same sediments as Compsognathus, is preserved with plenty of feathers, which means that the environment was suitable for feather preservation. A patch of fossilized skin from the tail and hind-limb of its close relative Juravenator show mainly scales, though there is some indication that simple feathers were present in some areas. There are thus three possibilities when it comes to Compsognathus: it may have been featherless, it may have had partial feathers such as may be the case with Juravenator, or it may have been fully feathered like its more distant cousins. The jury is out until more evidence comes in. 

a Compsognathus perches atop a dead juvenile dwarf sauropod

Miragaia

Type Species: Miragaia longicollum

Classification: Dinosauria – Ornithischia – Thyreophora – Stegosauria – Stegosauridae – Dacentrurinae

Time Period: Late Jurassic 

Location: Europe 

Diet: Herbivore

The stegosaur Miragaia was closely related to its contemporary cousin Dacentrurus. Miragaia could grow eighteen to twenty feet in length and would clock in at around two tons. Paired triangular plates ran down the midline of its neck; these plates were asymmetrical with a convex outer side and a concave inner side. They were obtuse but lightly hooked at the front. A long, narrow, and straight spike has been preserved; some paleontologists believe this was a shoulder-spike seen in some stegosaurs, but others believe it was part of the tail. Miragaia’s tail anatomy isn’t known, but it’s usually reconstructed with a four-spiked thagomizer like that seen in its near relatives. Because Miragaia’s front limbs were almost as high as the rear limbs, its overall posture was more horizontally level to the ground than what we find in most other stegosaurs, whose bodies slope down to the ground so that their heads were better situated for low browsing.

Miragaia is noteworthy for its elongated neck, which was built by seventeen vertebrae. It had the longest neck of any known stegosaur, and most scientists believe this represents the pinnacle of a trend toward longer necks in stegosaurs. Thyreophorans – the dinosaur clade to which stegosaurs belong – originally had nine neck vertebrae, and one of the most basal stegosaurs, the Chinese Huayangosaurus of the Middle Jurassic, had nine, as well. While more advanced stegosaurs such as Stegosaurus had twelve or thirteen vertebrae, Miragaia outdoes them – and it even outdoes most sauropods of its time. Only a few Chinese sauropods – such as Euhelopus, Mamenchisaurus, and Omeisaurus – had as many neck vertebrae as Miragaia; most sauropods of the Late Jurassic had only twelve to fifteen widely-spaced vertebrae. Scientists have come up with two explanations for Miragaia’s elongated neck. The first holds that it developed due to sexual selection: if longer-necked stegosaurs of this species were seen as more attractive, then it makes sense that necks would get longer over time. Another theory, and one which is more likely, is that the long neck developed as an aide to niche partitioning. Miragaia lived alongside the obviously low-browsing Dacentrurus (and Stegosaurus may have even ocean-hopped to reside in Portugal); because of this, these two species would be in direct confrontation. Niche partitioners coexist by eating different foods, and it may be that Miragaia evolved as a medium-browser, able to reach foods inaccessible to other stegosaurs. Its limb structure, which indicates a shift away from a low-browsing stance, supports this theory for the origin of Miragaia’s long neck. 

Lusotitan

Type Species: Lusotitan atalaiensis

Classification: Dinosauria – Saurischia – Sauropoda - Gravisauria - Eusauropoda - Neosauropoda – Macronaria – Titanosauriformes – Brachiosauridae

Time Period: Late Jurassic 

Location: Europe 

Diet: Herbivore

The Late Jurassic sauropod Lusotitan lived in prehistoric Portugal as part of Portugal’s Lourinha Formation, which resembled North America’s Morrison Formation and which emerged as a result of the genesis of the Atlantic Ocean. This brachiosaurid was related to the North American Brachiosaurus, and it grew up to seventy to eighty feet in length. Lusotitan walked on four pillar-like legs; its front legs were longer than its back legs, so that its body sloped downwards towards its short tail. Some scientists believe it could rear back on its hind legs to reach super high foliage, though some believe it kept its four legs on the ground at all times. Lusotitan’s neck reached vertical rather than horizontal like diplodocids. This gave it a feeding advantage: whereas most of its sauropod contemporaries were low-browsing diplodocids, Lusotitan could browse foliage up to fifty feet off the ground to feed on high conifers and ginkgoes. It undoubtedly practiced niche partitioning alongside the low-browsing diplodocid Dinheirosaurus. As part of the Lourinha Formation, it also lived alongside large predators such as Allosaurus and Torvosaurus, stegosaurs such as Dacentrurus and Miragaia, the early ankylosaur Dracopelta, and numerous early ornithopods. 

Dinheirosaurus

Type Species: Dinheirosaurus lourinhanensis

Classification: Dinosauria – Saurischia – Sauropoda – Gravisauria - Eusauropoda - Neosauropoda - Diplodocoidea – Flagellicaudata – Diplodocidae – Diplodocinae

Time Period: Late Jurassic 

Location: Europe 

Diet: Herbivore

The Late Jurassic sauropod Dinheirosaurus lived in prehistoric Portugal as part of Portugal’s Lourinha Formation, which resembled North America’s Morrison Formation and which emerged as a result of the genesis of the Atlantic Ocean. Dinheirosaurus had the general diplodocid form: a slender body, long neck carried low over the ground, and a long, whip-like tail. Its squared snout, similar in appearance to its North American relatives Diplodocus and Supersaurus, indicates that it was a non-specific low-browsing herbivore. It grew up to eighty feet in length, the same length as Diplodocus but smaller than Supersaurus, to whom it was more closely related. Dinheirosaurus is one of the few sauropods for whom gastroliths have actually been found alongside the type specimen. This sauropod lived alongside large predators such as Allosaurus and smaller ones such as Aviatyrannus (the latter of which could be squashed under Dinheriosaurus’ feet if it lacked spatial awareness). Other herbivores of the environment included the stegosaur Dacentrurus, the early ankylosaur Dracopelta, and numerous early ornithopods. 

Supersaurus

Type Species: Supersaurus vivianae

Classification: Dinosauria – Saurischia – Sauropoda – Gravisauria - Eusauropoda - Neosauropoda - Diplodocoidea – Flagellicaudata – Diplodocidae – Diplodocinae

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore

Supersaurus was a Late Jurassic diplodocid of the Morrison Formation. Though remains have been discovered from several individuals, these remains have been scanty, and so little is known about Supersaurus apart from its length and weight. From this scientists have deduced that it was the largest sauropod of the Morrison Formation, reaching up to 112 feet in length and weighing up to forty tons. We can assume that it behaved like other diplodocids, traveling in herds across the floodplains and savannahs of Late Jurassic western North America and feeding nonstop in order to provide enough energy for its massive size. It would’ve had few predators, as it would’ve dwarfed even Allosaurus and Torvosaurus, but Supersaurus would’ve been vulnerable in its early growth stages. Given its sheer size, it may be that Supersaurus herds were greedily eyed and followed by roving theropods looking to pick off the aged or weak. A Supersaurus carcass could’ve fed dozens of large theropods. 

Marshosaurus

Type Species: Marshosaurus bicentesimus

Classification: Dinosauria - Saurischia - Theropoda - Carnosauria - Megalosauroidea

Time Period: Late Jurassic 

Location: United States 

Diet: Carnivore

While little is known about Marshosaurus, it’s noteworthy as the only medium-sized theropod of the Kimmeridgian stage of Late Jurassic North America. At fifteen feet in length and around five hundred pounds in weight, it was significantly larger than the smaller Coelurus and Ornitholestes of its habitat whilst being overshadowed by the much larger theropods Allosaurus and Torvosaurus. Unlike its smaller counterparts, Marshosaurus was built for strength: it had a robust skull and its powerful arms were built for holding onto and subduing prey. It’s likely that Marshosaurus preyed on medium-sized dinosaurs such as Camptosaurus but may have also hunted young, sick, or weak sauropods. 

Ornitholestes

an artistic depiction of Ornitholestes with a nasal horn

Type Species: Ornitholestes hermanni

Classification: Dinosauria – Saurischia – Theropoda – Tetanurae – Coelurosauria – Tyrannoraptora – Maniraptoromorpha  

Time Period: Late Jurassic 

Location: United States 

Diet: Carnivore

The ‘bird robber’ Ornitholestes is known from a single partial skeleton with a badly crushed skull found in Wyoming at the turn of the 20th century (making it the first theropod discovering of the 1900s). Ornitholestes was a short-bodied theropod that grew to about eight feet in length and stood three feet tall at the hips. It had a relatively short neck with a slight sigmoidal (or S-shaped) curve. Its tail was long and whip-like and accounted for half its length. Its skull was proportionally smaller than that of most theropods, but it remained heavily built with a short snout and robust lower jaw. Its front teeth were conical-shaped with few serrations whereas the back teeth were recurved and sharply serrated like those of most theropods; the teeth didn’t extend as far back as its eyes, and neither tooth row spanned more than a third of the skull length. It’s likely that Ornitholestes used its front conical-shaped teeth for tearing flesh from its prey while the recurved back teeth were used to slice through the meat before swallowing. Ornitholestes’ forelimbs were relatively long, slightly under two-thirds the length of its hind legs. The first and second fingers of its three-clawed hands were the same size, but the claw bone was likely stunted. Ornitholestes was able to bend its forearm at an angle more acute than ninety degrees, which is characteristic of maniraptors but absent in more primitive theropods such as Coelophysis and Allosaurus. When Ornitholestes bent its elbow, its arms moved inwards towards its body; this would’ve increased in grasping ability. Though Ornitholestes is often portrayed as a fast, agile runner, its lower limb bones were relatively short. Some scientists believe that its innermost toe was larger than its other toes and suggest that it may have had a sickle claw similar to those of the dromaeosaurs. Many modern reconstructions depict Ornitholestes with a covering of feathers; this isn’t because feathers for this dinosaurs have been discovered but because they are present in so many of its kin that it’s thought by many that all coelurosaurs (to which Ornitholestes belongs) had some sort of rudimentary feathery covering. These feathers would’ve have been used for flight but for display, insulation, or for brooding eggs. Early depictions of Ornitholestes show it with a nasal horn akin to a chicken’s comb. When the specimen was discovered, it had an area of broken bone near the nostril that appeared to bulge upwards. Paleontologist Gregory S. Paul interpreted this as a nasal horn, but this interpretation has been rejected; the upward flare of crushed bone was likely due to post-mortem crushing of the skull. Below is a modern depiction of Ornitholestes - covered in fur, eating an early mammal, and lacking a nasal horn:

Debate swirls around Ornitholestes’ diet. When it was first discovered, it was argued that it utilized its grasping ability and the balancing power of its tail to catch birds. Though this theory had its detractors – a simultaneous theory was that its strange tooth design indicated a ‘transitional’ stage between a carnivorous and herbivorous diet – the original theory gained steam and became the dominant theory (hence the meaning of Ornitholestes’ name, ‘Bird Robber’). Over the decades, both the ‘bird robber’ and transitional diet theories have faded out of vogue. Most modern paleontologists believe that Ornitholestes preyed upon small terrestrial vertebrates such as mammals, lizards, frogs, and even dinosaur hatchlings. Another theory is that Ornitholestes used its front conical teeth to catch fish in the many streams, rivers, and lakes of the Morrison Formation, and some scientists  believe that Ornitholestes engaged in pack hunting behavior to bring down medium-sized prey such as the ornithopod Camptosaurus. Some argue that Ornitholestes and its slightly larger contemporary Coelurus lived side-by-side by utilizing niche partitioning in which Coelurus hunted at night and Ornitholestes hunted during the day. 

Hesperosuchus

a mother Hesperosaurus defends its child against an Allosaurus

Type Species: Hesperosaurus mjosi  

Classification: Dinosauria – Ornithischia – Thyreophora – Stegosauria – Stegosauridae

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore

The infamous Stegosaurus wasn’t the only stegosaur roaming the Morrison during the Kimmeridgian; its slightly smaller cousin Hesperosaurus was present, too (though it was likely more closely related to Dacentrurus of Europe). Hesperosaurus grew up to twenty to twenty-two feet in length – making it slightly smaller than Stegosaurus, which could grow up to thirty feet long – and weighed two to three tons. Like Stegosaurus it had two rows of plates running along its back; asymmetrical bases imply that the rows were staggered rather than symmetrical. Hesperosaurus’ plates were wider than those of Stegosaurus but not as tall, and its skull was deeper than that of its Morrison kin. Like Stegosaurus, Hesperosaurus had a four-spiked thagomizer on its tail; the front pair was thicker than the latter pair, and the latter were more horizontally directed towards the rear. Hesperosaurus’ tail spikes angled slightly backwards, pointing away from the body, implying that they were used in defense against predators such as Allosaurus and Torvosaurus. At least one Allosaurus specimen shows damage to its vertebrae that seems delivered by a stegosaur’s tail spike. 

CAT-scans on Hesperosaurus’ plates show that they had thin but dense outer walls and were filled with thick but spongy bone. This bone shows signs of being remodeled during the growth process. The plate bones were supplied with blood via long and wide arterial vessels. CAT-scans of the tail spikes show that they have thicker walls than the plates, and the interior spongy hollows are smaller. Each spike had a single artery that ran along the longitudinal axis. Skin impressions from Hesperosaurus give us a glimpse of what it looked like enfleshed: a part of the lower flank preserves rows of small, hexagonal, non-overlapping convex scales; higher on the flank are rosette structures with large central scales. Impressions of the back plates show no scales but a smooth surface with low parallel ridges; these likely represent the horn sheath covering the plate, and if so, these represent the first direct proof of such plate sheaths in stegosaurs. The horn sheath on the plates indicate that they had a defensive function, as the horn layer would not only strengthen the bony plate but give it sharp cutting edges. Simultaneously, the display function seems plausible, as these plates would likely be brightly colored in real life. Horn sheaths are a blow to those who believe the plates served a thermoregulatory function, as the sheaths would’ve hampered the plates’ ability to thermoregulate; however, the theory cannot be wholly rejected, for some cattle and ducks use horns and beaks to dump excess heat despite horn coverings. 

Dryosaurus

a family group of Dryosaurus drink at a river beside a Stegosaurus
(never mind the presence of grass, which had yet to evolve)

Type Species: Dryosaurus altus  

Classification: Dinosauria – Ornithischia – Ornithopoda – Iguanodontia – Dryomorpha – Dryosauridae 

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore

Dryosaurus was an iguanodont ornithopod, though it was formerly classified as a hypsilophodont. Its name means ‘tree lizard,’ referring to the forested environment it inhabited (some people erroneously believe the name refers to its vague oak-leaf shaped cheek teeth). Dryosaurus could grow between eight to fourteen feet in length and could weigh up to two hundred pounds; because all known specimens are of creatures in various stages of growth, the maximum adult size is unknown. Dryosaurus had a long neck, slender legs, and a long, stiff tail that acted as a counterbalance when running. Its short arms had five fingers on each hand. Dryosaurus had a horny beak and cheek teeth, and some paleontologists believe it had cheek-like structures to prevent the loss of food while chewing. Living in the Morrison Formation, Dryosaurus was dwarfed by its titanic herbivorous brethren, sauropods such as Diplodocus, Brachiosaurus, and Supersaurus; larger ornithopods such as Camptosaurus; and massive stegosaurs such as Stegosaurus. It likely used speed as its best defense against large predators such as Allosaurus and Ceratosaurus, and it’s speculated that with its small size, it stuck to the overgrown conifer forests that would be difficult for larger animals to navigate. Thus if we were to step back in time to the Late Jurassic western United States and tour a deep conifer forest cut by streams with overgrown banks and littered with deep ponds teeming with crocodylomorphs and early amphibians, we might spy small herds of Dryosaurus roaming like deer through the trees and munching on ferns and cycads, safely tucked away from the giant theropods roaming the neighboring floodplains and valleys. 

Camarasaurus

Type Species: Camarasaurus supremus  

Classification: Dinosauria – Saurischia – Sauropoda - Gravisauria - Eusauropoda - Neosauropoda – Macronaria – Camarasauridae 

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore

Camarasaurus was the most common sauropod of the Morrison Formation; vast herds roamed the floodplains and valleys of Late Jurassic North America. Herding behavior is attested in the fossil record: a group of two adults and a forty-foot-long juvenile died together around 150 million years ago in what is now Wyoming. It appears their bodies were washed by a flooding river and buried in sediment. This find implies that Camarasaurus traveled in herds or at least in family groups, though the discovery of Camarasaurus eggs laid in lines rather than nests suggests that Camarasaurus didn’t tend to its young. It’s possible that Camarasaurus embraced an R-strategy for survival, in which overwhelming numbers of young were hatched in the hope that some of them made it to adulthood. Juveniles, having survived the many predators of the Morrison Formation, may have ‘linked in’ with roving herds for safety. The size of herds is pure speculation, for unlike some ceratopsians of the Cretaceous, vast Camarasaurus bone-beds haven’t been discovered. 

Camarasaurus could grow up to sixty feet in length, though some specimens reached up to seventy-five feet. Skeletal studies indicate that these sauropods reached sexual maturity around twenty years of age. Camarasaurus had a blunt, box-like head that, despite its bulldog appearance, was filled with lots of air cavities (called fenestrae) which helped lighten the skull’s weight. Its teeth were seven and a half inches long, and they were stronger than those of most sauropods. These sturdy, spoon-like, deep-rooted teeth extended nearly into the cheeks, in contrast with the front-only teeth of some sauropods such as Diplodocus. Studies on the plentiful teeth have revealed that they were used vigorously in chewing and were replaced on average every two months; the lack of gastrolith fossils in the numerous specimens indicate that Camarasaurus chewed its food down to a pulp prior to swallowing. When scientists performed clumped isotope thermometry on the enamel covering the teeth of several genera of Jurassic sauropods, including Camarasaurus, it revealed temperatures of 90.3-98.4 degrees Fahrenheit, comparable to that of modern mammals. Its neck was held up high so that the head were at a level where Camarasaurus could eat from the tree canopy like its contemporary cousin Brachiosaurus; the numerous fenestrae in its skull would’ve lightened the weight, enabling it to hold it higher off the ground. 

Camarasaurus’ legs were about equal length (the forelegs were only slightly shorter than the hind-legs), unlike Brachiosaurus’ long forelegs and Diplodocus’ longer hind-legs. The fact that its forelegs were slightly shorter than the hind-legs would’ve made its back slope forward, but its high shoulders off-balanced this so that its body ran nearly parallel to the ground. Its neck and tail were relatively short for sauropods, and like most sauropods its inner toe bore a large, sharp claw. Camarasaurus’ name comes from the Greek word for ‘chamber,’ referring to the chambers discovered in its vertebrae. These air-filled cavities made its bones pneumatic, and their purpose has been approached in two ways: the air-filled cavities (pleurocels) may have served to reduce the weight of its skeleton, or they may have even been air sacs that were part of the sauropods’ respiratory system. These features are found in plenty of other sauropods as well as in theropods; it may very well be a common feature of the saurischian lineage that leads to the avian respiratory system in which birds use similar air sacs to increase their breathing potential. If this is the case for Camarasaurus, the air sacs would’ve provided a super-efficient airflow down the long neck so that the fresh air went down one network of sacs whilst oxygen-depleted air went up another. This would result in a constant supply and exchange of breathable air despite the distance of the lungs from the mouth and nostrils. 

Haplocanthosaurus

Type Species: Haplocanthosaurus priscus 

Classification: Dinosauria – Saurischia – Sauropoda – Gravisauria - Eusauropoda - Neosauropoda - Diplodocoidea – Flagellicaudata – Diplodocidae

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore

Haplocanthosaurus is known from four specimens belonging to two species, and none of these specimens has provided a skull. It was one of the smallest sauropods of the Morrison Formation, reaching only fifty feet in length. Its position in the sauropod family tree is debated, as it is seemingly a primitive form of either the diplodocids with their long and thin build or of the taller, shorter, and stockier macronarians (it's currently classified as a diplodocid, but the debate continues). That this primitive sauropod lived side-by-side with far more advanced diplodocids and macronarians is bewildering, and scientists speculate that it was a type of sauropod that appeared much earlier in the Jurassic – perhaps during the ‘paleontological black hole’ of the Middle Jurassic – but continued into the Late Jurassic. 

Haplocanthosaurus was originally named Haplocanthus by paleontologist John Bell Hatcher, but he later found out about a fish genus that sounded just like this, and thinking that the name was already taken, he changed it to Haplocanthosaurus. Ironically, he had no need of changing the name: the fish was actually named Haplacanthus (spelled with an ‘a) rather than Haplocanthus (spelled with an ‘o’). This wasn’t noticed until years later when the name Haplocanthosaurus was in regular use, and the ICZN – the scientific body which oversees the naming of animals – granted a Green Light to the Haplocanthosaurus designation because of its wide usage. 

Coelurus

Type Species: Coelurus fragilis 

Classification: Dinosauria – Saurischia – Theropoda – Tetanurae – Coelurosauria – Tyrannoraptora – Maniraptoromorpha  

Time Period: Late Jurassic 

Location: United States 

Diet: Carnivore

Coelurus was the first small theropod from the Morrison Formation to be named, and it was named after the numerous hollow spaces – called pleurocels – oddly distributed among its vertebrae. These pneumatic vertebrae lightened the skeleton, making Coelurus more swift of foot and lightweight. It grew up to eight feet in length, stood two feet tall at the hips, and weighed around forty pounds. Its elongated neck vertebrae gave it a longer neck than is seen in most theropods, and its long, slender hind legs indicate that it was a lightly-built swift runner, certainly faster than its similar but shorter-footed contemporary Ornitholestes. It shared its environment with much larger dinosaurs, and it may have needed to use its speed to avoid being preyed upon by larger theropods (or from being trampled by hapless sauropod herds). It likely hunted lizards and early mammals, though it may have preyed upon smaller ornithopods such as Dryosaurus and Othnielosaurus, particularly if they were weak or juvenile. While there’s no indication that Coelurus hunted in packs, it’s possible that it utilized pack-hunting behavior in taking down smaller-sized ornithopods. 

Coelurus lends its name to the clade Coelurosauria, which in the early days of dinosaur taxonomy included all small theropod dinosaurs. Our knowledge of dinosaur relationships has greatly increased, so now Coelurosauria includes all theropod dinosaurs more closely related to birds than to carnosaurs. Coelurosauria branches off into well-known dinosaur groups such as the dromaeosaurs, the ornithomimosaurs, and the tyrannosaurs. Coelurus belongs to the Tyrannoraptorans,  members are all descendants of the last common ancestor of Tyrannosaurus rex and the common house sparrow; within that clade, Coelurus is a member of the smaller clade Maniraptoromorpha, which is a ‘transition’ clade between the tyrannoraptorans and the ‘New Coelurosaurs’ (Neoceolurosauria), which includes all compsognathids and the maniraptoriformes. Coelurus’ placement indicates that it’s somewhat of an oddball in theropod taxonomy, and its only concrete neighbor was its Morrison Formation contemporary Ornitholestes. 

Camptosaurus

Type Species: Camptosaurus dispar 

Classification: Dinosauria – Ornithischia – Ornithopoda – Iguanodontia – Dryomorpha – Ankylopollexia

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore 

‬The Late Jurassic ornithopod Camptosaurus has gone through a ‘winnowing’ over the last several decades, as many assumed species have turned out to be different genera altogether. Nonetheless, Camptosaurus remains a steadfast citizen of the low-lying floodplains of the Morrison Formation, which stretched from New Mexico to Canada in western North America. This low-browsing ornithopod could grow up to twenty-six feet in length and could stand up to six and a half feet tall at the hips; it lived among other herbivores such as Stegosaurus and the sauropods Apatosaurus, Brachiosaurus, and Diplodocus. The largest predators of its environment were Allosaurus, Ceratosaurus, and Torvosaurus; to defend against these predators, Camptosaurus may have traveled in herds, utilized its grisly thumb-spike as a defensive weapon, or may have even attempted to flat-out run from its hunters (scientists speculate it could reach up to fifteen miles per hour running speed). It had powerfully built legs and may have spent much of its time in a bipedal stance, though its front legs were still able to bear its weight. Its fingers weren’t packed together, giving them some ability at movement, implying that it could feed from medium-height foliage by grasping food and bringing it to its mouth. Its skull was triangular with a pointed snout: when feeding, it would crop food with its beak, and it tightly-packed teeth chewing teeth wore down the food before swallowing it. Some scientists speculate that it fed on tough plant material that other low-browsing herbivores couldn’t handle. A nine-inch-long fossilized embryo, thought to belong to Camptosaurus, has been discovered in Utah. 

Diplodocus

Type Species: Diplodocus longus 

Classification: Dinosauria – Saurischia – Sauropoda – Gravisauria - Eusauropoda - Neosauropoda - Diplodocoidea – Flagellicaudata – Diplodocidae – Diplodocinae

Time Period: Late Jurassic 

Location: United States 

Diet: Herbivore   

Diplodocus is an infamous dinosaur, a childhood icon, for at least two reasons: for a long time it was thought to be the largest dinosaur, and it’s one of the earliest dinosaurs about which scientists could say a lot. Its remains were first discovered in the late 19th century, and Othniel Charles Marsh studied it immensely during the ‘bone wars.’ He named it Diplodocus longus – meaning ‘long double beam’ – after a pair of long bone growths on the underside of the caudal vertebra (while Marsh thought these features were unique to this sauropod, they’re actually fairly common among sauropods). Diplodocus had the standard sauropod body plan: a long tail, a large body supported by four pillar-like legs, and a long neck. Because its hind legs were longer than its front legs, its body sloped slightly forward. While it could reach up to eighty feet in length, most of that length was accomplished via its long neck and whip-like tail. Its whip-like tail may have served as a defensive weapon or as a tool by which it could ‘crack’ the sound barrier to communicate to other of its kind or ward off predators. Diplodocus may have had keratinous spines running the length of its back; evidence of such spines has been found in later specimens, and they’re known to be present in other closely-related sauropods. 

Herds of Diplodocus roamed the low-lying drainage basins that swallowed runoff from the emergent Rocky Mountains in western North America. This lowland environment – which stretched from New Mexico to Canada and is known as the Morrison Formation – was scarred by crisscrossing streams and rivers and was dotted with swamplands, lakes, and floodplains. Diplodocus’ contemporaries included other sauropods such as Apatosaurus, Brachiosaurus, and Camarasaurus; ornithischians like Camptosaurus, Dryosaurus, and Stegosaurus; and carnivorous theropods such as Stokesosaurus and Ornitholestes. The largest predators were Ceratosaurus, Allosaurus, and Torvosaurus. Though a full-grown Diplodocus would’ve likely been nigh invulnerable to these predators, juvenile sauropods or the aged, weak, and sick would’ve been easier prey; some scientists believe that the vulnerability of young sauropods implies that Diplodocus reproduced with an R-strategy of having large nests with lots of young in the hope that even just a few survive into adulthood. If this is the case, it could very well be the case that young sauropods were a prime staple of the predators’ diet in this Late Jurassic ecosystem. Crocodylomorphs such as Hoplosuchus prowled the lowlands, and pterosaurs such as Harpactognathus and Mesadactylus roamed the skies. The aquatic elements of the lowlands were inhabited by ray-finned fishes, frogs, salamanders, and turtles; bivalves and aquatic snails were plentiful. Plant life included green algae, fungi, mosses, horsetails, cycads, ginkgoes, and several kinds of conifers. The nature of the vegetation changed upon ones location in the Morrison Formation: there were river-hugging forests of tree ferns, gallery forests of ferns, and fern savannahs dotted with the occasional conifer. Paleontologists believe Diplodocus stuck mostly to the floodplains and sparsely-wooded areas on the fringes of denser forests that may have been too overgrown for them to navigate; though older interpretations put Diplodocus and its sauropod contemporaries in swamps and lakes, it’s now known they were fully terrestrial creatures. 

Diplodocus was likely a low-browser, carrying its long neck above the ground to graze on ferns and cicads; however, some scientists speculate that it could raise its neck up to a forty-five degree angle without putting too much stress on its heart or vertebrae. It may even have been able to ‘rear up’ on its hind legs by using its tail as a tripod. Diplodocus’ peg-like teeth pointed forward, and in adults toothwear is limited to the forward portion of its mouth; this indicates that it was a ‘leaf-stripper’ who fed by closing its mouth around vegetation, capturing the stems of the plant between its teeth, and then pulling back so that as the stems ran through the peg-like teeth, the leaves were stripped off and swallowed. Its snout was longer than those of other sauropods, so that it could fit more plants in its mouth. Interestingly, toothwear of juvenile Diplodocus had teeth in the backs of their mouths, indicating they may have fed by moving their head side-to-side as they stripped stems of leaves. This may have been an easier method of feeding in their young age, a method they rejected as they grew older. As a low-browser, Diplodocus would’ve competed with its contemporary Stegosaurus, but if it could indeed raise its neck higher off the ground, it could feed on plants unavailable to stegosaurs. Some scientists believe Diplodocus may have fed on aquatic plants by standing on the sides of a lake or river and using its long neck to reach out over the water and dip its head beneath the surface to pull up softer water weeds. It likely fed at multiple points during the day and night, as its scleral rings indicate it was cathemeral, active for periods throughout both the day and night; this feeding schedule may have been necessary to keep up with its incredible caloric needs.