Dino of the Week: Anchiornis

Type Species: Anchiornis huxleyi

Classification: Dinosauria – Saurischia – Theropoda – Coelurosauria – Maniraptora – Paraves – Avialae - Anchiornithidae

Time Period: Late Jurassic

Location: China

Diet: Carnivore 

The feathered paravian Anchiornis became the first Mesozoic dinosaur species for which almost its entire ‘life appearance’ could be determined, and it was an important source of information on the early evolution of birds. Anchiornis is one of many paravian theropods discovered from the Tiaojishan Formation of China. This bone-bed gives us a snapshot of the Oxfordian stage of the early Late Jurassic in southern Laurasia. Anchiornis and its ilk lived in a subtropical to temperate climate that was warm and humid. The mountainous landscape was overshadowed by brooding volcanoes that had a penchant for erupting, and the area was cut by mountain streams and deep sapphire lakes choked by dense gymnosperm forests. The jungle-like forest consisted of ginkgoes and conifers, lycopsids and horsetails, cycads and ferns. Many creatures called this place home: small feathered dinosaurs, numerous pterosaurs, salamanders and insects and arachnids. There were early mammals – including the earliest gliding mammal Volaticotherium and an aquatic protomammals Castorocauda. Interestingly, there are no ‘large’ animals present in the fossilized record of the Tiaojishan Formation.

Though we know that southern Laurasia was host to large dinosaurs – for other Chinese bone-beds have preserved sauropods, stegosaurs, early ornithopods and ceratopsians, and medium- to large-sized theropods – there are none present in this part of China (the largest creature preserved at Tiaojishan is the two-foot-long heterodontosaur Tianyulong!). Researchers have posited two main theories to explain the absence of larger organisms. The first explanation is that maybe these larger creatures simply didn’t live in this environment. Maybe the mountainous terrain was too difficult to navigate, or maybe the jungle-like forest, lush from the common nutrient-rich volcanic eruptions, was too difficult for larger animals to penetrate? These factors would be no match for smaller animals such as the panoply of paravian dinosaurs, pterosaurs, amphibians, and mammals that Tiaojishan does preserve. Another explanation is that larger dinosaurs were present in the environment, but they escaped the periodic volcanic eruptions that deposited fine dust and ash. The exquisite preservation of the Tiaojishan fossils is due to the unique volcanic sedimentary rock in which they’re preserved; this has allowed us to examine them closely and see features – such as feathers – that are usually destroyed. That these animals were entombed in volcanic ash is a given. Perhaps when the volcanoes erupted, the larger organisms – the stegosaurs and larger theropods – were able to flee whereas smaller animals were doomed, buried quickly as the ash layer built up. Marine organisms would be choked as the waterways and lakes turned to ashen mush; extreme heat and volatile air currents would damage pterosaur membranes or send them careening to the ground; and the ash would cake the feathers of the feathered paravians, weighing them down and consigning them to their deaths. In a very real way, the Tiaojishan Formation may be a series of snapshots of recurring Oxfordian apocalypses.

The handful of Anchiornis specimens preserved met their demise in volcanic ash, but their particular fossilization has given us a wealth of information about them. Anchiornis were crow-sized, four-winged, feathered paravians; they grew a little over a foot in length, weighed around half a pound, and had a wingspan of up to twenty inches. Their triangular-shaped skulls shared characteristics in common with dromaeosaurs, troodontids, and more primitive avialans. They had long, wing-bearing arms, long legs, and a long tail. Like all paravians it was covered in feathers, though it also had scales on some parts of its body. It was a carnivore, and we know this because some specimens had ‘poop pellets’ in inside their bodies when they died (making Anchiornis the earliest theropod known to have produced pellets). These pellets contained lizard bones and fish scales. 

Anchiornis’ feathers were exquisitely-preserved, so we know a lot about what it looked like. The wings, legs and tail supported long but relatively narrow vaned feathers. Two types of simpler, downy (in science-speak, plumaceous) feathers covered the rest of the body. Long, simple feathers covered almost the entire head and neck, torso, upper legs, and the first half of the tail. The rest of the tail bore contour (in science-speak, pennaceous) feathers that resembled retrices (larger feathers that most modern birds use for steering in flight; but Anchiornis, as we’ll see, likely didn’t fly). Its head bore long crown feathers that may have formed a crest. Anchiornis’ body plumage consisted of short quills with long and independent flexible barbs; these barbs stuck out from the quills at low angles on two opposing blades. This arrangement gave each feather a forked shape and resulted in Anchiornis having a softer textured and ‘shaggier’ plumage than is seen in modern birds. ‘Shaggy’ contour feathers probably served to help with thermoregulation and water repellance abilities; combining these shaggy feathers with the open-vaned wing feathers, and Anchiornis had decreased aerodynamic efficiency. 

Anchiornis’ wings were composed of eleven primary feathers and ten secondary feathers. The wing feathers had curved, symmetrical central quills; these were small and thin with rounded tips, which would’ve hindered aerodynamic abilitiy. In the paravians Microraptor and Archaeopteryx, the longest wing feathers were those nearest the wrist, making the wings appear long and pointed; Anchiornis had a different arrangement in which the longest wing feathers were those nearest the wrist, making the wing broadest in the middle and tapering near the tip for a more rounded (and less flight-adapted) profile. Anchiornis had a propatagium – a flap of skin connecting the wrist to the shoulder and rounding out the wing’s front edge – and this part of the wing was covered in covert feathers (the shorter feathers covering the bases of the longer feathers) that smoothed wing and covered the gaps between the larger primary and secondary feathers; but unlike modern birds, these feathers weren’t arranged in tracts or rows. In some Anchiornis specimens, several layers of wing feathers extend down to cover most of the wing’s surface, so that the wing is essentially composed of multiple layers of feathers. This multi-layered wing arrangement may have helped strengthen the wing. Anchiornis had three clawed fingers; the longest two fingers were bound together by the skin and other tissue forming the wing, so Anchiornis was functionally two-fingered. These bound fingers were incorporated into the post-patagium that supported the bases of the main wing feathers. Tiny, rounded scales covered the bottom of the fingers. 

Anchiornis’ legs were covered in long, vaned feathers. This has led some researchers to view Anchiornis as a four-winged theropod, much like Microraptor and Sapeornis; however, the feathers on the hind legs didn’t have the shape or arrangement suspected from flight feathers, so their role was likely display rather than aerodynamics. Anchiornis’ long legs indicate that it was a fast runner, but the extensive leg feathers would’ve prevented it from reaching any decent speeds; thus the long legs may be a vestigial feature rather than an adaptation to its environment. Anchiornis had four toes on its foot, with the third and fourth toes being the longest. The first toe – the hallux – wasn’t reversed, as we see in perching paravians. The hind-wings were shorter than those of Microraptor and were composed of twelve to thirteen feathers anchored to the lower leg and ten to eleven on the upper foot. The hind wing feathers were longest closer to the body while the short foot feathers directed outward, almost perpendicular to the foot bones. Anchiornis’ feet were covered in feathers (except for the claws). Some specimens have preserved scales on the toes, tarsus, and even lower leg, suggesting that scales were present beneath the feathers. The underside of the toes were fleshy pads with distinct creases at the joints. These pads were covered in small, pebble-like scales. 

The ultimate question, of course – the one everyone’s asking – is, “Did Anchiornis fly?” Feathers don’t necessitate flight ability (they likely evolved for thermoregulation or display; flight was a happy byproduct in some paravian lineages). We’ve already seen that Anchiornis has several features that dampen flight ability, but can we know for sure? In 2016, scientists set out to answer whether Anchiornis could fly. They determined that juvenile specimens may have been able to use their wings to assist running up an incline (a nice advantage in the mountainous terrain of their habitat), and they may have achieved flapping flight if a very high-angle flapping wing stroke was used. This would’ve been an ungainly flight and nothing pretty to see. Adult Anchiornis lacked the ability to gain any aerodynamic benefit from their wings, for they were simply too heavy. However, by flapping the wings while running, they could have increased their running speed by twenty percent. Flapping the wings while leaping would result in a fifteen to twenty percent increase in height and distance. All this to say that while juvenile Anchiornis may have been able to enjoy something like powered flight, adult Anchiornis didn’t have this ability. Their feathered wings may have helped them make ‘leaps and bounds’ as ground-runners, but they would’ve remained terrestrial. Another factor against flight is the fact that Anchiornis seems to have lacked a breastbone; some argue that it may have been made out of cartilage rather than bone (as in some more primitive theropods), but they’re at a loss to explain why the cartilage would not be preserved whilst feathers were. 

In 2010 scientists studied the distribution of Anchiornis’ melanosomes (the pigment cells that give feathers their color). By studying the types of melanosomes Anchiornis had and comparing them with those modern birds, scientists were able to map the specific colors and patterning present on the specimen when it was alive. The scientists found that most of Anchiornis’ body feathers were gray and black; the crown feathers (on the head) were rufous (reddish-brown) with a gray base and front; the face had rufous speckles among predominantly black head feathers; the forewing and hindwing feathers were white with black tips; the shorter feathers covering the bases of the long wing feathers (in science-speak, coverts) were gray, contrasting the mainly white main wings. The larger coverts of the wing were white with gray or black tips, forming rows of darker dots along mid-wing. These took the form of dark stripes or even rows of dots on the outer wing but a more uneven array of speckles on the inner wing. The shanks of the legs were gray other than the long hindwing feathers; and the feet and toes were black. In 2015, a second specimen was studied similarly, but only gray-black type melanosomes were found. Even the crown feathers were gray-black. This difference has been explained in several ways: perhaps the melanosomes were damaged or not preserved the same way; perhaps this Anchiornis was another species than the one previously studied; or maybe this was due to gender (it could very well be that males were brightly colored to attract mates and compete with other males for the ‘blander-looking’ females). Below is a portrait of the 2010 Anchiornis, a fairly accurate representation of what it looked like “in real life.”