Kulindadromeus is a type of early neornithischian dinosaur that lived during the Middle Jurassic period. The first fossil of Kulindadromeus was discovered in Russia. Its feather-like skin covering shows that protofeathers may have been present in the Ornithischia group and possibly in all dinosaurs, not only in Coelurosauria, as scientists had previously believed.

Discovery and naming

In 2014, the main example of the species Kulindadromeus zabaikalicus was named and described by Pascal Godefroit, Sofia Sinitsa, Danielle Dhouailly, Yuri Bolotsky, Alexander Sizov, Maria McNamara, Michael Benton, and Paul Spagna. The genus name comes from the Kulinda sites and the Greek word dromeus, meaning "runner." The species name refers to the Zabaykalsky Krai region.

The holotype, labeled INREC K3/109, was discovered in a layer of the Ukureyskaya Formation from the Middle to Late Jurassic period, specifically the Bajocian to Tithonian stages. The specific layer is likely from the Bathonian stage of the Middle Jurassic, about 168 to 166 million years ago. It includes a partial skull with lower jaws. Volcanic ash layers in the formation helped preserve fossils exceptionally well, creating a Konservat-Lagerstätte.

Since 2010, the area has produced many additional fossils, including subadult and juvenile remains. In 2014, two new genera, Kulindapteryx and Daurosaurus, were named based on fossils from this site. However, Godefroit stated that these names were based on stolen specimens and should not be recognized. The material includes six skulls and hundreds of disarticulated skeletons found in two bone beds. Each bone shows the same type of structure, and Godefroit and others concluded that the bone beds contain only one species.

Description

Kulindadromeus was a small dinosaur that belonged to the ornithischian group. It was about 1 to 1.5 meters (3.3 to 4.9 feet) long and weighed 2 kilograms (4.4 pounds). Like other early neornithischians, Kulindadromeus walked on two legs. It had a short head, short front legs, long back legs, and a long tail.

Scientists who studied Kulindadromeus identified several unique features. The upper part of the maxilla (a bone in the upper jaw) had a front section that was much shorter than the back section. The fenestra maxillaris (a specific opening in the upper jaw) was larger than the antorbital fenestra (a larger opening in the snout). The jaw bone near the eye socket had a notch. The postorbital bone (a bone near the eye) had a back section that was wide vertically. The rear part of the ilium (a bone in the hip) was thin when viewed from the side. The second, third, and fourth metatarsals (bones in the foot) had deep grooves for the tendons of the extensor muscles.

Several fossils of Kulindadromeus show parts of its skin. This includes overlapping rows of scales on its tail and a covering of scales that branch into feather-like structures. These feather-like structures were once thought to be found only in theropod dinosaurs (a group of carnivorous dinosaurs). The feathers found on Kulindadromeus are of three types, showing complexity in feather evolution. The first type consists of hair-like filaments covering the body, neck, and head. These filaments are up to 3 centimeters long and resemble the "dino-fuzz" seen in theropods like Sinosauropteryx. The second type includes groups of six or seven short, downward-pointing filaments (up to 1.5 centimeters long) growing from a base plate. These are found on the upper arm and thigh and are similar to type 3 feathers in theropods. The base plates are arranged in a hexagonal pattern but do not touch. The third type is unique and found on the upper lower legs. It consists of bundles of six or seven ribbon-like structures, up to 2 centimeters long. Each ribbon is made of about ten thin, parallel filaments, each less than 0.1 millimeters wide.

There are also three types of scales. Overlapping hexagonal scales, up to 3.5 millimeters wide, cover the lower legs. Small, round, non-overlapping scales (less than 1 millimeter in size) cover the hands, ankles, and feet. The top of the tail has five rows of arched rectangular scales, each up to 1 by 2 centimeters. These scales overlap slightly, with the front edge of each scale covering the back edge of the one in front. In the middle of the tail, a small bump on each scale covers the back edge of the previous scale, forming overlapping rows. The scales are smooth and thin, less than 0.1 millimeters thick. Scientists believe these were not bony structures like osteoderms. Toward the end of the tail, the scales become smaller and rounder, and they no longer overlap.

Fossils of Daurosaurus and Kulindapteryx also show preserved skin on the upper arm. These include "oval platy scales" and bristles. The bristles are reported to be longer than the scales.

Classification

According to a classification study by the researchers who described it, Kulindadromeus is an early member of the Neornithischia group, positioned just above Hexinlusaurus in the evolutionary tree. It is the close relative of the Cerapoda group.

Godefroit and colleagues concluded that the filament-like structures found in Ornithischia, such as in Psittacosaurus and Tianyulong, may be similar to the "protofeathers" seen in non-avian theropods. Since feather-like structures are also known in pterosaurs, there is evidence that these traits may have appeared early in the evolutionary history of Ornithodira. A study attempting to determine the likely appearance of early dinosaurs suggested that both Ornithodira and Dinosauria may have originally had scales. However, this result was only found when assuming that early pterosaurs had scales.

A comment on Godefroit et al.'s work argues that the filament-like structures in Kulindadromeus do not look like protofeathers but instead resemble broken-down collagen fibers. Godefroit and his team responded, "If the filaments are collagen fibers, as Lingham-Soliar suggests, it is surprising that they are not preserved in the many (more than 20) tails of Kulindadromeus, since the tail should have had a lot of collagen for support. Also, Lingham-Soliar does not explain why scales would be broken down or moved near the arm and leg bones but not near the lower leg or tail." They added that the structure of the skin features does not match broken collagen fibers, as the filaments are spread far apart.