Feather-like Structures on the Back of Mirasaura: Evidence from the Middle Triassic

Life reconstruction of Mirasaura grauvogeli（Courtesy of Rick Stikkelorum）

A fossil of a small diapsid reptile discovered in the Vosges Mountains of northeastern France has revealed an unusual body structure dating to roughly 247 million years ago in the Middle Triassic. The specimen is remarkable for a series of dorsal skin appendages arranged along its back. These structures resemble feathers in overall appearance, yet they are neither true feathers nor comparable to the scales or other skin derivatives known in modern reptiles. Researchers named the species Mirasaura grauvogeli. The genus name combines the Latin words mirus (“marvellous” or “wonderful”) and saura (“reptile”), while the species name honours Louis Grauvogel, whose work in excavating the Voltzia Sandstone sites contributed significantly to the discovery of the fossil material.

The animal itself was extremely small. Its skull measured only about 17 millimetres in length, and the skeleton overall was slender and lightly built. The fossil shows an elongated snout, a slightly domed skull roof, and forward-facing eye sockets. These features give the skull a superficial resemblance to those of birds or pterosaurs, yet phylogenetic analyses indicate that the species was only distantly related to those groups. Instead, it belongs to Drepanosauromorpha, an unusual lineage of reptiles known exclusively from the Triassic period. Members of this clade are widely interpreted as arboreal specialists.

Image source：Spiekman SNF et al， CC BY-NC-ND 4.0 

a: photograph of the holotype specimen SMNS 97278 under ultraviolet light; b: skull of SMNS 97278 under ultraviolet light; c: skull reconstruction of SMNS 97278 based on a synchrotron radiation microcomputed tomographic scan; d: specimen SMNS 97279; e: interpretative anatomical drawing of SMNS 97279.

Anatomical abbreviations:

cav (caudal vertebra); cv (cervical vertebra); dv (dorsal vertebra); fe (femur); fl (forelimb); fr (frontal); hu (humerus); il (ilium); j (jugal); l. (left); la (lacrimal); mand (mandibular ramus); mx (maxilla); na (nasal); pa (parietal); pmx (premaxilla); po (postorbital); pof (postfrontal); prf (prefrontal); q (quadrate); r. (right); sc (scapula); sq (squamosal)

Researchers examined two well-preserved skeletons together with more than eighty isolated specimens of the integumentary appendages. These appendages formed a vertical crest along the midline of the animal's back. Each structure was elongated and unbranched, and the series gradually decreased in height toward the rear of the body. The foremost appendage was approximately four times taller than the last one. Importantly, the structures do not correspond one-to-one with individual vertebrae, and there is no evidence that their bases were directly attached to bone. This indicates that the crest consisted of skin-derived appendages rather than skeletal projections.

Closer examination reveals that each appendage consisted of a narrow basal section and a distally expanded portion. The proximal part was formed by three parallel bands, with the central band being the widest. Toward the distal end, the structure broadened into two laminae separated by a distinct medial element. The overall form therefore resembles a symmetrical leaf-like expansion with a central axis, somewhat reminiscent of the vane and shaft of a feather. However, the structural details differ fundamentally from true feathers. In particular, the appendages lack branching barbs, which are essential components of feather architecture.

Another notable feature of the appendages is the presence of irregular surface wrinkles, known as rugae. These folds appear in various orientations across the structures. Their exact origin remains uncertain. They may reflect the surface texture of the living tissue, but they could also result from post-mortem processes such as dehydration, decay, or compression during fossilization. Similar surface patterns are also known from the Triassic reptile Longisquama, suggesting that both species may share a comparable integumentary structure inherited from a common ancestor.

One of the most intriguing discoveries involves microscopic pigment structures preserved within the appendages. Under scanning electron microscopy, researchers identified numerous elongated, oval microbodies consistent with melanosomes. These pigment-bearing organelles closely match the geometry of melanosomes found in bird feathers, yet they differ from those observed in reptile skin or mammalian hair. Statistical comparisons show that the shape diversity of the melanosomes in Mirasaura overlaps extensively with that of feathers. This similarity suggests that the integumentary appendages possessed pigmentation systems comparable to those in avian feathers.

Despite these similarities, the structures cannot be considered homologous to feathers. Their lack of branching architecture demonstrates that they developed through a different structural pathway. In birds, the central shaft of a feather forms through the fusion and reorganization of branching barbs during development. Because the appendages of Mirasaura lack these barbs entirely, their medial axis cannot represent a true feather rachis. Consequently, the feather-like appearance reflects convergent evolution rather than shared ancestry with bird feathers.

The skeletal anatomy of Mirasaura provides additional insight into its lifestyle. The vertebral column included seven cervical vertebrae and twenty-four dorsal vertebrae. Gastralia and cervical ribs were absent, features typical of drepanosauromorph reptiles. Although the tail was incompletely preserved, the preserved portion indicates a deep vertical profile. The forelimb anatomy includes an ulna with a distinct olecranon process, a feature absent in Longisquama. The pelvis shows a tall, blade-like ilium extending upward and forward, another trait characteristic of the clade.

The limbs also suggest an arboreal lifestyle. The feet bear large, curved claws suited for grasping branches, and the limb proportions are consistent with climbing behaviour. The skull structure further supports an insectivorous diet. The anterior portions of the jaws lack teeth, whereas the posterior regions bear slender pointed teeth. Combined with the elongated snout, these features would have allowed the animal to probe into narrow crevices in tree bark to capture insects and other small invertebrates.

Phylogenetic analyses place Mirasaura firmly within Drepanosauromorpha, alongside Longisquama and related reptiles. Prior to this discovery, drepanosauromorph fossils were known primarily from Late Triassic deposits. The Middle Triassic age of Mirasaura extends the evolutionary history of the group by roughly twenty million years. This finding indicates that the lineage had already diversified earlier than previously recognized.

Phylogenetic tree（Image source：Spiekman SNF et al， CC BY-NC-ND 4.0 ）

The function of the dorsal crest remains uncertain, but several possibilities can be ruled out. Because the appendages form only a single row along the back and show no structural adaptation for aerodynamic surfaces, they are unlikely to have played a role in flight. Similarly, their limited distribution on the body makes a thermoregulatory function improbable. Instead, their prominent size and vertical orientation suggest a role in visual display. Such structures could have been used for communication within the species, perhaps in courtship displays or territorial signalling, or as a deterrent to predators.

Although drepanosauromorph reptiles did not survive beyond the Triassic, their diversity demonstrates that reptile evolution during this period was highly experimental. The discovery of Mirasaura reveals that complex skin appendages were already evolving in multiple reptile lineages long before the appearance of true feathers in dinosaurs or hair in mammals. In the dynamic ecosystems of the Triassic, animals explored a wide array of anatomical innovations. The enigmatic crest of Mirasaura stands as a reminder of this evolutionary experimentation, representing a distinctive branch of reptilian history that ultimately left no living descendants.

Author: Shui-Ye You

Reference:

Spiekman SNF et al. (2025). Triassic diapsid shows early diversification of skin appendages in reptiles. Nature.

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