Ichnological terms define key concepts for interpreting trace fossils and understanding the evidence they preserve. Knowledge of anatomy, locomotion, and behavior enhances the study of tracks and trackways. This article clarifies fundamental terms in ichnology, providing a foundation for accurate interpretation and effective trackway analysis in paleontological research.
Anatomical Ichnological Terms
Because the study of tracks directly relates to understanding the structure of the animals that made them, some anatomical terms are indispensable. Such ichnological terms specifically describe the characteristics of tracks associated with the anatomy of limbs, feet, hands, and particularly fingers.1
Autopodium. The distal part of a limb, i.e., pes or manus. A Greek-derived word meaning “foot,” which consists of three parts: the sum of basipodium (the carpus or tarsus—wrist or ankle), metapodium (the metacarpus or metatarsus—palm or instep), and acropodium (the phalanges, forming the toes or fingers). This ichnological term has a general meaning; therefore, one should not use it to refer to a footprint.
Manus. The anterior autopodium—hand, or if used for locomotion, forefoot (or front foot). In mammals, people refer to it as the “front paw.” The term “hand” serves as a synonym for “manus” in both bipedal and quadrupedal animals. For a track left by the manus, the most commonly used terms are “manus track,” “manus print,” and “manus impression.”
Pes. The posterior autopodium. Less technical synonyms include “hind foot” and “rear foot.” In mammals, people refer to it as the “rear paw.” The term “foot” applies to any autopodium used for locomotion. To describe a track left by the “pes,” commonly used terms include “pes track,” “pes print,” and “pes impression.”
Plantar. This term refers to the surface of the “pes” that makes contact with the ground during movement.
Palmar. This term refers to the surface of the “manus” contacting the ground while moving.
These ichnological terms can also denote the corresponding impression in a track. Additionally, people use “plantar” to refer specifically to the sole of the foot (pes) and “palmar” to describe the palm of the hand (manus), excluding the fingers.
Heel. The term refers to the tarsal region of the pes or the rear part of a track. In plantigrade trackmakers (animals that walk on the soles of their feet), the mark left by the heel is referred to as the “heel impression” or “heel pad.”
The ichnological “heel” does not necessarily correspond to the anatomical heel. It generally describes the rear part of pes tracks, but one can also apply it to manus tracks. The rear part of tracks made by three-toed dinosaurs should refer to a heel only when the complete metatarsus shows an imprint.
Pads, also known as phalangeal or digital nodes, are the soft, fleshy structures on the underside of hands and feet, including toes and fingers. These pads act as natural shock absorbers, helping to cushion movement and reduce impact during locomotion. When an animal leaves a footprint, these pads leave visible marks called pad impressions. The spaces between individual pads are known as interpad grooves, which can sometimes function as flexion creases when the joint bends.
Different animals have unique pad structures:
Birds often have finger pads that are spaced apart rather than touching. Carnivores (like dogs and cats) have plantar pads, which can merge into a single large pad with lobed edges. Dinosaurs sometimes left behind a heel pad impression, though this term usually refers to a larger pad behind the toe marks, not the actual heel. Frequently, it represents a metatarsophalangeal pad—a cushion where the toes connect to the foot. Elephants, camels, and sauropods have large, thick pads known as cushions, which support most of their weight by extending beneath the foot.
Basal Pad. This term refers to a significantly enlarged area at the proximal end of a digit impression or at the posterior part of it. For example, the basal pad in the track of the seymouriamorph Amphisauropus is associated with digit I and corresponds to the carpal (and tarsal) region. Similarly, in chirotherian digit impressions, the basal pad corresponds to the joint area of the V finger.
Pternion. The rearmost point, or apex, of the heel. Similarly, acropodion (not to be confused with acropodium) refers to the foremost point of the foot (i.e., the tip of the most protruding digit). These terms apply to both tracks and actual feet.
A non-plantigrade track does not have an impression of the anatomical heel. In such cases, the pternion is the rearmost point of the plantar/palmar surface—that is, the rearmost point of the fully impressed track.
Gleno-acetabular Distance (Body Length, Trunk Length). We can measure this parameter from animal footprints to indicate body size (the actual body length, excluding the tail and head). It is measured as the distance between the midpoint of a line connecting two consecutive pes tracks and the midpoint of a line connecting the corresponding manus tracks.
Anatomically, this corresponds to the distance between the glenoid (shoulder joint socket) and the acetabulum (hip joint socket).
Digit and Foot Structure
Hypex. (Plural: hypexes or hypices). The most proximal point of the gap between two digits; the “apex of the inward angle between the digits.”
Divarication of Foot from Midline or Footprint Rotation. It refers to the obtuse angle formed between the longitudinal axis of the foot and the trackway midline. The apex of this angle can point either behind or ahead of the track’s direction, depending on whether the foot faces outward or inward.
Nail, Claw, Hoof. A keratinous covering of the terminal phalanx. A nail is a blunt structure at the tip of a digit; a claw is a pointed structure. A hoof is a significantly enlarged and blunt structure, broadly rounded and often featuring a flat underside. A hoofprint typically does not include impressions of digital pads.
Semiclaw. Semiclaws describe claws that are pointed but have a flat underside, as seen in most ornithopods before the Cretaceous period. This term applies exclusively to dinosaurs.
Webbed Track, Interdigital Web. A webbed track is a footprint that also imprints the interdigital web (interdigital space—the area between digit impressions). It indicates the presence of extensive webbing (membranes between the digits) adapted for swimming.
This feature is characteristic of partially or fully aquatic animals and serves as an indicator of their habitat. A small interdigital web is also present between the thumb and the second digit in primates (apes) and humans.
Digitation. The arrangement of digits. In ichnology, digitation refers to the presence of well-defined, separate digits on the hands or feet in fossilized tracks.
The ichnological term “digitation” refers to the presence and arrangement of finger-like structures or impressions in trace fossils, particularly in footprints and trackways. It describes how these digit-like structures appear in fossilized tracks left by organisms, providing insight into the positioning and movement of their limbs or appendages.
Dactyly. The number of fingers or toes in a footprint or the corresponding autopodium: monodactyly (one digit), didactyly (two digits), tridactyly (three digits), tetradactyly (four digits), and pentadactyly (five digits). Scientists commonly use these ichnological terms to categorize various animal limb morphologies.
For example, stegosaurs had three toes on their feet, ceratopsians had four, and ankylosaurs had either three or four.
The number of digits in a footprint can help determine the group of animals to which it belongs. For instance, a track with four toes may indicate a quadrupedal mammal, while a three-toed track is often characteristic of a dinosaur. Early amphibians could have had more than five digits (polydactyly). For example, Acanthostega had 8 digits on the hind limbs, Ichthyostega—7, and Tulerpeton—6. By comparing the digit characteristics in footprints with known animal groups, it becomes easier to identify the type of animal that left the track.
When referring to a track, these terms indicate the number of visible digit impressions, which does not necessarily reflect the actual number of digits in the autopodium. For example, most non-avian theropods used only digits II through IV for locomotion, and these digits regularly touched the ground, while digit I (and ancestrally digit V) was a dewclaw. Although these trackmakers left tridactyl footprints, their hind limbs were actually tetradactyl or even pentadactyl, and only functionally tridactyl.
Phalangeal Formula
Phalangeal Formula.A schematic way of representing the number of phalanges in the digits of the autopodium. For example, the basic phalangeal formula for the manus of reptiles is represented as 2-3-4-5-3. But in many reptiles, it is 2-3-4-5 for the first four digits, while the number of phalanges in digit V is variable. In theropod dinosaurs, the pes typically have the formula 0-3-4-5-0, meaning that digits I and V are absent (0 phalanges).
In amphibians, the phalangeal formula is highly variable. Ancient amphibians had a phalangeal formula of 2-3-4-5-4, while modern amphibians typically have variants such as 1-2-3-2 and 2-2-3-3. The phalangeal formula of seymouriamorphs (transitional forms toward reptiles) is 2-3-4-4-3.
In some cases (e.g., many Therapsida), the number of phalanges follows the basic reptilian formula: 2-3-4-5-3(4). However, functionally, many phalanges reduce to thin bony discs, indicating a significant reduction. In mammals, the typical formula is 2-3-3-3-3.
If the manus and pes formulas differ significantly, scientists provide them separately. However, if the only difference is in digit V, the number of phalanges in the fifth digit of the pes is placed in parentheses at the corresponding position. For example: 2-3-4-5-3(4).
Toe Configurations
The classification of bird, dinosaur, and certain other animal feet is determined by the arrangement and orientation of their digits, along with their functional adaptations.
Anisodactyly. Three toes (II, III, and IV) point forward, while one one toe (digit I, the hallux) points backward. It is the ancestral and most common foot structure among all birds, inherited from their archosaur ancestors. Here are some examples of extinct archosaurs (with anisodactyly) from the group Avialae:
Ichthyornis dispar (Late Cretaceous), Eocypselus rowei, Primobucco mcgrewi, and Pumiliornis tessellatus (Early Eocene).
Zygodactyly. Two toes (II and III) point forward, while two (I and IV) point backward. In this arrangement, digit IV is rotated backward to facilitate perching. This toe configuration is most characteristic of arboreal species (e.g., parrots, owls). Zygodactyl tracks have been found in rock layers dated to 120–110 million years ago (late Early Cretaceous), approximately 50 million years before the first identified fossils of animals with zygodactyl feet. Example: Shandongornipes muxiai (Lower Cretaceous, China), Zygodactylus luberonensis (Oligocene–Miocene).
Semi-zygodactyly (ectropodactyly). The ability to rotate the outer toe (IV) both forward and backward (e.g., owls, ospreys). Example: Presbyornis pervetus (Late Paleocene to Early Eocene).
Pamprodactyly. Typically, all four toes point forward, but digits I and IV remain mobile, allowing them to rotate either backward or forward. This configuration of digits is associated with climbing or grasping, especially in vertical environments such as trees or cliffs. Among modern birds, this includes swifts and some species of mousebirds; among extinct species, Eocypselus and Scaniacypselus (Early Eocene).
Syndactyly. A keratinous covering may partially or fully fuse the third and fourth toes together, while the first toe remains very small and points backward. This adaptation is primarily associated with perching, climbing, or burrowing behaviors. Syndactyly occurs exclusively in Avialae. Among modern birds, it is found in kingfishers, hornbills (partially), and rollers. Among extinct species, it is known or suspected in Primobucco mcgrewi and Halcyornis toliapicus (Early Eocene).
Heterodactyly. Toes III and IV point forward, while toes I and II point backward, meaning digit II takes on a reversed position instead of digit IV. The enantiornithine bird Dalingheornis (Early Cretaceous) had this toe arrangement, which now occurs only in trogons. Dalingheornis stands out as the only confirmed fossil that exhibits a heterodactyl toe arrangement.
Axony: Types of Foot Arrangements
Axony. Axony is the orientation of the axis in a foot or track. This ichnological term refers to the arrangement of the digits in relation to the main axis of the most important (dominant or principal) toe. Typically, it corresponds to the axis that bears the greatest load. It is usually the most deeply impressed digit.
Entaxony refers to a footprint where the dominant toe is the medial one (II or I). The entaxonic condition is present in the feet of sauropods and humans. It is very rare in other animals.
Mesaxony refers to a footprint where the dominant toe is the central one, typically III. This condition is quite common in most bipedal three-toed dinosaurs (archosaurs). In perissodactyls (odd-toed ungulates), weight is transferred onto the middle toe, meaning they have mesaxonic feet and hands.
Paraxony. A track with two or four toes where digits III and IV are equally dominant. Neither half of the hand nor foot is more dominant than the other. This condition is rare in reptiles (Isochirotherium is nearly paraxonic). However, it is universal in even-toed ungulates and evolved in late mesonychids.
Ectaxony. Describes a track where the dominant toe is the outer or lateral one (most commonly digit IV). Lepidosauria and, for instance, the extinct kangaroo Protemnodon frequently show this condition.
Paraxonic tracks are symmetrical, mesaxonic tracks are nearly symmetrical, and ectaxonic tracks are noticeably asymmetrical, with the toes increasing in length from I to IV. In the entaxonic condition, the inner or medial toes predominate, making these tracks asymmetrical as well.
Biomechanical Ichnological Terms
These ichnological terms directly or indirectly relate to how an animal moves, its posture, and the forces involved in locomotion.2
Bipedal and Quadrupedal Locomotion
Locomotion. An animal is considered quadrupedal or bipedal if it moves (or stands still) on all four limbs or only on its hind limbs, respectively. It is a facultative biped (semibipedal) if it is generally bipedal but occasionally places its forelimbs on the ground while walking slowly (e.g., Postosuchus).
Quadrupedal Trackways. Tracks imprint on both sides of the midline. There are prints of both hands and feet, usually with a variable rhythm. When the pace is very long, especially during a gallop, the tracks appear closely grouped together.
Semibipedal Trackways. Tracks of the forelimbs appear on the ground only during slow walking, when the animal stops, or sometimes when it changes direction.
Bipedal Trackways. Left and right hind foot impressions alternate, one in front of the other, on either side of the midline. The ancestors of dinosaurs were fully bipedal animals (closely related to Eoraptor). All theropod dinosaurs are bipedal, as are their descendants—the birds.
Size of Front and Hind Footprints and Bipedality
In modern mammals, there are often differences between the surfaces of the forelimbs and hind limbs. Typically, the manus has a larger surface area than the pes. It is the case for most terrestrial carnivores (Fissipedia), as well as artiodactyls and perissodactyls. This difference indicates that the center of gravity in these animals is closer to the shoulder girdle than to the pelvic girdle.
In contrast, reptiles and some other mammals (such as bears, rabbits, and kangaroos) have larger hind limbs compared to their forelimbs. This means that their center of gravity is located closer to the pelvic girdle, indicating that these animals are not only capable of assuming an upright posture but also of walking on two legs. Bipedalism depends on the position of the center of gravity within the body.
Any reptile with a long, heavy tail, a short neck, and a small head is well adapted for bipedal locomotion.
Heteropody. A condition in which the manus and pes differ in size and morphology. It is resulting in a difference in surface area between their tracks. This is the most common condition (e.g., in dinosaurs, humans, frogs, and rabbits). Such biomechanical differences typically reflect functional distinctions between the limbs.
Homopody. A condition in which the forelimbs and hindlimbs are similar in size and morphology (e.g., all being plantigrade or digitigrade). This is rare among reptiles (e.g., in Therapsida) but more common in mammals, such as artiodactyls, some perissodactyls, and carnivores, among others.
Gait and Movement Analysis
Grady.The placement of the hand or foot. The foot or manus assumes a specific position, either resting more or less on the ground or being lifted off the ground during progression or at rest. As locomotion transitions from plantigrady to unguligrady, an animal’s ability to achieve greater speeds increases.
Plantigrade. In a plantigrade foot, both the toes and the anatomical heel (i.e., the entire sole of the foot) make contact with the ground, with the phalanges and metatarsal/metacarpal bones positioned approximately horizontally. The track represents an impression of the entire autopodium. It was the positioning of dinosaur feet during movement, where the metapodium fully contacted the substrate, leaving elongated tracks.
Semiplantigrade. In a semiplantigrade foot, the basipodium remains elevated above the ground, a trait seen in primates except for African apes and humans, which exhibit a fully plantigrade stance. The footprint includes the entire foot except for the heel.
Digitigrade. In a digitigrade foot, all the weight-bearing phalanges (toe bones) are in full contact with the ground, including the metatarsophalangeal joints where the main bending occurs. The footprint shows the entire toes. This is the common foot position during movement for most bipedal dinosaurs: the toes spread across the ground, while the metapodia make no or only partial contact with the ground. As a result, footprints often contain impressions of the toes and the most distal part of the metapodium. The phalanges have increased flexibility and resistance to twisting and loads. Examples include Allosaurus and Camptosaurus.
Semi-digitigrade. An intermediate state between “plantigrade” and “digitigrade.” In a semi-digitigrade foot, the metapodium partially touches the ground or rests on a metapodial pad. This pad directly transfers weight from the metapodium to the ground. When walking, the heel does not touch the ground, but it does not rise as high as it does in a digitigrade foot. Only the front part of the sole or palm leaves an impression on the substrate. When referring to the forelimbs, the terms “palmigrade” and “semi-palmigrade” are sometimes (but very rarely) used. Synonym: mid-digitigrade.
Subdigitigrade. In a subdigitigrade foot, most of the phalanges make contact with the ground, but the metatarsophalangeal joints remain elevated above the ground. As a result, the toes appear only partially in the track. For example, it occurs in Saurolophus.
Unguligrade. Tracks of animals that rely solely on the tips of the terminal phalanges for support, which usually have a nail or hoof covering them. Only the ungulate phalanges make contact with the ground (for example, in horses), while the other phalanges and metatarsals (or metacarpals) are positioned subvertically.
Subunguligrade. An intermediate position between unguligrade and digitigrade. In a subunguligrade foot, both the ungulate and the penultimate phalanx make contact with the ground. The phalanges have less flexibility, resistance to twisting, and load-bearing capacity compared to digitigrade.
Calcigrade. The heel makes the deepest imprint in such footprints. These tracks typically appear when the trackmaker stands still. This ichnological term describes the morphology of the tracks rather than referring to foot anatomy.
Locomotor Posture
The terms “unguliportal,” “digitiportal,” and “plantiportal” (e.g., Pentasauropus) refer to the parts of the foot that bear the body weight, rather than just the parts that touch the ground while standing. These terms describe how the foot functions during movement (dynamic postures) rather than when it is stationary (static postures).
In most animals, the dynamic foot postures correspond to their static postures. However, there are exceptions, such as the hippopotamus and the tapir, which have unguligrade (hoofed) feet but function in a digitiportal manner (using their toes), and the elephant, which is digitigrade (walking on its toes) but functions in a plantiportal manner (using the flat part of its foot).
Manners of Gait
Gait. The progression of an animal that results from a succession of paces or bounds made successively in a determined direction. Generally, the animal moves forward; only rarely are trackways that represent a retrocession registered.
Symmetrical and Asymmetrical Gait. The ichnological term “symmetrical gait” describes a ladder-like trackway pattern of arthropods, where the left and right tracks sit next to each other. In contrast, moving tetrapods display a zigzag pattern known as “asymmetrical” or “diagonal” gait.
Sprawling and Erect Gait
Sprawling Gait. A gait in which the legs are positioned far apart from each other with a wide base, and the pace angulation is typically low.
A trackway with these characteristics indicates an animal resembling a lizard or a heavy, slow-moving animal of primitive structure with horizontal or transverse limb disposition. Basal tetrapods and many reptiles exhibit this condition. During movement, the body bends from side to side. It may drag along the ground (e.g., salamanders) or be raised above the ground. This gait is also characteristic of many arthropod animals.
Erect Gait.A gait in which the legs are positioned beneath the body and move in the parasagittal plane. The base is narrow, and the pace angulation is high, with angles reaching up to 180°. This gait is characteristic of agile and fast walkers, such as Triassic Rauisuchia, dinosaurs, as well as birds and mammals.
Semi-erect Gait. It is an intermediate posture. It represents a significantly raised, stretched position. We observe this gait in non-mammalian synapsids, monotremes (Platypus), certain marsupials (Monodelphis domestica), moles, sea lions when on land, and large lizards, including monitor lizards and tegus.
Modern crocodiles can use both a stretched and semi-erect gait. Some of their extinct Mesozoic relatives, such as the terrestrial crocodile Erpetosuchus, had a fully erect gait.
Ichnological Terms for Behavior Traces
Crawling Traces. Some groups of amphibians and reptiles have highly reduced or lost limbs, including caecilians and several groups of salamanders and squamates. Crawling traces are tracks formed during locomotion without the use of limbs, such as “the trail of a snake.” This ichnological term also describes the traces of invertebrates, such as snails and arthropods, and is not limited to limbless locomotion.
Swimming Track (swim trace, paddling trace). A general term for tracks made by swimming organisms, whether they are punting (partially submerged and pushing off the substrate) or fully buoyant (suspended) while using different swimming gaits. These gaits include “paddling” (movement using limbs) and “undulation” (movement using the tail or the entire body).
Takeoff Trace (take-off trace). A trace recording the takeoff of a flying animal, such as a bird or a pterosaur. Conversely, a landing trace records the landing of a flying animal. Such traces are very rare in the fossil record.
A—Overhead, slightly oblique angle photograph of resting trace. Note the normal Eubrontes track cranial to the resting traces (top center) made by the trackmaker during the first step upon getting up. B—Schematic of trace to scale with A. The scale bar equals 10 cm. First resting traces (manus, pes, and ischial callosity) in red; second (shuffling, pes only) traces in gold; final resting traces (pes and ischial callosity) in green; and tail drag marks made as the trackmaker moved off in blue. Note long metatarsal (“heel”) impressions on pes prints.
Abbreviations: ic = ischial callosity, lm = left manus, lp = left pes, rm = right manus, rp = right pes, td = tail drag marks. ©Milner et al.
Resting Trace (Crouching Trace, Sitting Trace, Cubichnia). A trace that records the resting position of the trackmaker, usually in a crouching or lying posture. Resting traces are typically characterized by the side-by-side arrangement of tracks rather than a zigzag pattern. They also feature impressions of body parts that do not normally contact the ground. These can include tail prints, manus impressions (in bipeds), metatarsal impressions (in digitigrade trackmakers), or ischial callosity impressions.
All known theropods are perceived as obligate bipeds; no known theropod habitually adopted a quadrupedal posture for locomotion. Theropod trackways therefore do not typically exhibit hand imprints. Only when the trunk was lowered toward a substrate, as in a crouched posture, could the hands potentially create impressions.
Gregariousness. The grouping of trackmakers into social units. People refer to herbivorous dinosaur groups as herds, theropod groups as packs, and bird groups as flocks. Multiple subparallel trackways can serve as evidence of gregarious behavior. Features like consistent track depth might suggest that the tracks originated around the same time. Additional evidence of gregariousness includes rarely intersecting lateral tracks or a consistent intertrackway spacing (the distance between parallel trackways). Interaction between trackmakers may also be evident, such as when tracks change direction in unison.
Environmental Context
Subaqueous Track. A track formed beneath the water’s surface. Two methods can produce a subaqueous track. The first is during half-swimming in shallow water, with the digits touching the bottom while the body and tail float. The second method for creating subaqueous tracks is swimming close to the bottom with the whole body submerged.
Subaerial Track. A subaerial track forms when the tracking surface is exposed to the open air.
Movement Traces and Surface Marks
Trace fossils reflect the behavior or activity of organisms rather than the actual body parts of the organism itself. Some extinct soft-bodied organisms are primarily known only from their traces. Certain accumulations of trace fossils indicate specific depositional conditions, making it possible to reconstruct the surrounding paleoenvironment. People occasionally use the German synonym Lebensspuren for this ichnological term.
©Marchetti et al.
Drag Mark (Drag Trace, Smear Mark, Smears). A trace that records the dragging of a body part across or through the substrate. Unlike scratch marks, drag marks typically form in the direction of movement. Common examples include digit drag marks, tail drags, and belly drag marks. Drag marks of digits or an entire foot can form when both entering and exiting the substrate and may extend between individual tracks.
Tail Traces (Tail Drag).A very common feature in the tracks of animals with a sprawling gait. It mainly occurs in small animals and is extremely rare in large reptiles, such as bipedal or even quadrupedal dinosaurs. Even in sauropod tracks, tail drag impressions appear only in exceptional cases.
It is likely that the bodies of bipedal animals remained parallel to the ground while moving, with their tails held elevated. Similarly, quadrupedal dinosaurs also appear to have kept their tails raised.
There are two types of tail traces:
Tail impressions—these do not indicate forward movement (e.g., in resting traces); tail drag marks—formed by the tail dragging along the substrate.
Tetrapod tracks can sometimes preserve impressions of the skin or the covering integument. Some birds (e.g., willow ptarmigan) and mammals have feet densely covered with feathers or fur, leaving feather and fur impressions in their tracks. Fossilized tracks have also revealed possible feather and hair impressions.
Skin Impression (Skin Texture). An impression that reflects the texture of bare skin or squamous (scaly) skin. Skin impressions can be left by the plantar or palmar surfaces of the feet or by other body parts that touch the ground, particularly in resting traces.
Bare skin impressions may appear as folds, ridges, and grooves, with some of the most noticeable being impressions of flexion creases. Flexion creases occur at bending points, such as beneath the metacarpophalangeal joints of the hand. Depending on their location, we can distinguish between the creases of finger flexion (or “interpad creases”) and plantar or palmar flexion creases. Fossilized footprints sometimes display flexion creases.
Paleontologists discovered this skin impression in the village of Vallcebre, near Barcelona, within the Tremp Formation, dating to the end of the Cretaceous. The rocks at this site contain dinosaur trackways and detailed skin impressions, some of which are up to 10 inches (ca. 25 cm) wide. The scales are preserved in a natural cast. The size and shape of these scales resemble those of giant sauropods found in North America, and they likely belong to a titanosaur, which is the only type of sauropod known to exist in this region.
Scale impressions can document either overlapping (imbricated) or non-overlapping scales. Researchers distinguish different types of scales on the feet of dinosaurs and birds.
The plantar surface of birds and dinosaurs features small, non-overlapping, round or polygonal scales, known as reticulate scales. A skin impression consisting of numerous reticulate scales is also referred to as a reticulate array.
Birds and at least some theropod dinosaurs have a specialized layer of scales covering their legs, known as the podotheca. The podotheca was also present in the non-avian theropod Concavenator corcovatus.
Wrinkle Structures (Wrinkle Marks, Wrinkles, Crinkle Marks). Grooves and folds located inside or outside a track. Several different mechanisms can lead to the formation of these structures, including microbial mats, erosion, and impressions of the integument. In most cases, described “wrinkle marks” are actually broken layers of penetrative tracks.
Striations (Scratch Lines). Narrow grooves on a track, usually parallel to each other, that record the movement of the foot as it contacts the substrate. Scales, other foot structures, or sediment particles attached to or dragged by the foot can create striations.
Striations formed by scales are also known as “scale striations” or “scale scratch lines.”
Associated Traces
Posterior Mark. Any extension behind the footprint. Posterior marks include drag marks, retro-scratches, metatarsal marks, and traces of uncertain origin. Tracks of tridactyl dinosaurs with long metatarsal marks are called elongate tracks. Researchers interpret such tracks as evidence of facultative plantigrady, but deep foot penetration into soft sediment, followed by sediment collapse, creates them in most cases.
Scratch Marks (also known as Scratches, Scratch Traces, or Scrape Marks) are grooves made by a digit when a limb is pulled back. This can happen during activities like digging, swimming, climbing, or kicking off the ground. People sometimes treat “scratch mark” as a synonym for “drag mark.”
The term “retro scratches” refers to scratches that go beyond the back edge of a track, indicating that the foot slipped back during kick-off. Many modern birds, especially shorebirds, create nest scrapes—simple depressions in the ground. For example, male plovers make several nest scrapes as part of their courtship rituals. Researchers have found fossil scrapes linked to tracks of non-avian theropods, suggesting that dinosaurs may have had similar nesting behaviors.
Slip Mark (Slide Mark). A track indicating the sliding of the foot on a wet surface. It usually occurs as a posterior mark merging with the track itself. Slip marks are often wide with an indistinct posterior edge, asymmetrically curved, sometimes at an angle to the long axis of the track, and may have striations oriented according to the direction of sliding.
Traction. Friction between the foot and the substrate. Some tetrapods may have adaptations to increase traction in order to reduce slipping on wet ground. For example, furrows covering the keratinous pads of elephants or scales on the soles of turtles; such adaptations may be preserved in fossil tracks.
