20 Mysterious Fossils That Paleontologists Still Can’t Explain

The Tully Monster looks like a hodgepodge of several unrelated animals, as if it had been born out of the strangest committee meeting imaginable. Discovered in the Mazon Creek deposits in Illinois, it had stalked eyes, a long, flexible proboscis, fins, and a morphology that has been the subject of debate among scientists for decades.

Dickinsonia is one of the most iconic fossils of the Ediacaran Period, but it remains difficult to classify. Its oval, ribbed body has been subject to various interpretations, and researchers are still debating its exact place among the earliest animals. Some evidence suggests that it is an animal, but its precise place in the animal family tree remains difficult to determine. 

Charnia resembles a delicate frond, but it lived long before plants colonized the Earth as we know it today. This means that it was not algae in the conventional sense, even though it undeniably resembles a plant at first glance. Scientists are still debating how many Ediacaran organisms, resembling fronds, fed, grew, and were related to later life forms. 

Hallucigenia was once reconstructed upside down, which gives an idea of just how strange this little Cambrian creature was. It had spines, legs, claws, and a body so unusual that early interpretations struggled to distinguish its top from its bottom. Scientists now understand it much better, but its exact relationship to other primitive molting animals and its cousins, the velvet worms, is still the subject of extensive debate. 

Opabinia had five eyes and a long, beak-like appendage with a prehensile tip, which seems a bit excessive, even by Cambrian standards. Its reclassification in the 1970s helped change the way scientists understood the strange aspects of early animal evolution. Its overall place among the ancestors of the earliest arthropods is now better understood than before, but its unusual body plan continues to raise questions.

Anomalocaris was long misunderstood because its various body parts had been discovered separately and classified as belonging to distinct organisms. Its mouth, prehensile appendages, and body looked so unusual that it took time to correctly reconstruct the animal as a whole. Today, scientists recognize it as a major predator of the Cambrian period, but details regarding its diet, ecology, and relationships with other species continue to be refined.

Palaeospondylus is a tiny Devonian vertebrate native to Scotland that has puzzled researchers for over a century. Its unique anatomy made it difficult to compare it to known fish, the ancestors of amphibians, or other groups of vertebrates. Recent high-resolution images suggest that it may be closely related to the ancestors of the first four-limbed animals, but its strange characteristics still make it an enigma. 

Typhloesus has been dubbed the “alien goldfish,” a term that paleontologists do not use lightly. Discovered in Carboniferous rocks in Montana, it has a torpedo-shaped body, a caudal fin, and internal features that have led to conflicting interpretations. A 2022 study suggested that it might have been a mollusk-like animal, but even this hypothesis remains open to question. 

Prototaxites was a giant, trunk-like organism that lived at a time when forests were nothing like they are today. For more than 150 years, scientists have wondered whether it was a plant, a fungus, a lichen-like organism, or something even stranger. Recent research challenges the old “giant fungus” theory, suggesting that it may not fit clearly into any known fungal groups.  

Saccorhytus is a tiny Cambrian fossil with a wrinkled, sac-like body and a mouth that has sparked a great deal of interest. It was initially thought to be a very ancient ancestor of humans and other deuterostomes, but subsequent research has shown that it actually belongs to another group, likely the Ecdysozoa, which includes molting animals. Its identification has evolved so much that it serves as a reminder to us all that even tiny fossils can spark heated debates. 

Vetulicolids resemble segmented, tailed swimming sacs—and even that description is very rough. Scientists have wondered whether they were primitive deuterostomes, ancestors of chordates, or a form even harder to classify. Their fossils indicate a genuine body plan—not merely a coincidence of preservation—but exactly how they fit into the evolutionary history of early animals remains controversial. 

Since its discovery, Nectocaris has been the subject of various interpretations, partly because the available fossils are small and unusual. Some researchers have suggested that it might be related to the earliest cephalopods, while others have expressed doubts about the reliability of this classification. Its body appears to combine characteristics that do not clearly correspond to those of any known animal groups. 

Wiwaxia was covered in scales and spines, giving it an appearance that suggested it should not be approached lightly. Its exact phylogenetic relationships are a matter of debate, with scientists sometimes comparing it to mollusks, sometimes to annelid worms, and sometimes to a broader group of primitive lophotrochozoans. The problem is that Cambrian animals often exhibited combinations of characteristics that later lineages classified differently. 

Spriggina is an Ediacaran fossil with a segmented body that once led researchers to compare it to early arthropods or worms. The problem is that many Ediacaran organisms do not clearly correspond to animal groups that appeared later. Its body symmetry, structure, and evolutionary significance are still the subject of debate. 

Tribrachidium exhibits a strange three-part spiral symmetry, unlike most animals we know of. This simple fact makes studying it difficult, as most modern animals follow different patterns of symmetry. Researchers have hypothesized that it may feed by passing streams of water through its body, but the exact nature of its feeding mechanism remains unclear.

Parvancorina is another Ediacaran fossil that has prompted comparisons to arthropods because of its shield-like shape. These comparisons are appealing, but not everyone agrees that it should actually be classified as an arthropod. Its state of preservation and age make it difficult to interpret the fossil with absolute certainty. 

Cloudina was one of the very first animals to develop a rigid, shell-like structure, which makes it of considerable importance. It lived toward the end of the Ediacaran Period, just before the Cambrian Explosion revolutionized animal life. Scientists are still debating its exact biological identity and its place among the earliest animals.

Namacalathus had a chalice-shaped skeleton with openings that gave it a surprising delicacy for such an ancient organism. It coexisted with other organisms of the late Ediacaran period and has been linked to the earliest attempts at skeletal formation. Its exact phylogenetic relationships are a subject of debate, with some studies linking it to certain animal groups with greater certainty than others. It is tiny, elegant, and continues to pose a challenge to paleontologists regarding its evolutionary tree.

Helicoprion is famous for its strange spiral tooth, which is so unique that scientists have spent decades debating its exact location in the animal’s mouth. It was a cartilaginous fish related to sharks, but this spiral tooth has given rise to some very fanciful reconstructions over the years. Modern imaging techniques have largely solved the mystery of its location, but this animal remains wonderfully strange nonetheless. 

Paleodictyon is not a fossil of an organism, but a mysterious honeycomb pattern found in ancient rocks and even on today’s seafloor. Scientists wonder whether these are burrows, feeding traces, a form of microbial “gardening,” or something else entirely. The fact that similar patterns appear over immense spans of time makes this mystery all the more fascinating.