A Small Arthropod from the Waukesha Lagerstätte Illuminates Myriapod Evolution Before the Transition to Land

All living myriapods—including centipedes, millipedes, pauropods, and symphylans—are terrestrial. Their elongate bodies consist of numerous segments bearing walking limbs. Yet the origins of this body plan, and the stage at which myriapods left the sea, have remained poorly documented in the fossil record. A small arthropod discovered in the Waukesha Lagerstätte of Wisconsin, USA, now provides part of the missing evidence. The species, represented by 35 specimens, has been named Waukartus muscularis. The genus name Waukartus combines Waukesha, the locality where the fossils were found, with the Latin artus, meaning limb or joint. The specific name muscularis refers to the preservation of the animal's muscles. It lived in a Silurian sea approximately 436.2–437.4 million years ago, about 15 million years before the earliest known crown-group myriapods.

Fossils of Waukartus muscularis（Image source：Briggs DEG et al. (2026)， CC BY 4.0 ）

Fossils of Waukartus muscularis（Image source：Briggs DEG et al. (2026)， CC BY 4.0 ）

The fossils were originally collected during the 1980s, but their identity remained uncertain for decades. They were variously described as a "myriapod-like animal" or a "caterpillar" animal, and were at one point interpreted as lobopodians. By re-examining the specimens using macrophotography, scanning electron microscopy, elemental analysis, and phylogenetic analysis, the researchers were able to reconstruct the animal's anatomy and evolutionary position in much greater detail.

Waukartus muscularis was small. The more complete specimens range from approximately 11 to 28.8 millimetres in length, with an average of about 21 millimetres. The body was only around one-tenth as wide as it was long. It consisted of a head, a trunk of 11 segments, and a short terminal telson. Beneath the telson was a pair of blade-like projections that may have been controlled by muscles. The head is poorly preserved, but the researchers identified the probable outline of an eye and four pairs of head appendages. The anteriormost appendage was extremely small, while the others increased in size towards the rear. These head appendages were shorter than the trunk limbs and may have served sensory or feeding functions, although their precise roles remain unknown.

Reconstruction of Waukartus muscularis, with the head at the bottom and the telson at the top（Image source：Briggs DEG et al. (2026)， CC BY 4.0 ）

Each of the 11 trunk segments bore a single pair of appendages. Earlier studies suggested that each segment was divided into a long and a short section, giving it an appearance somewhat reminiscent of the diplosegments of millipedes. The new investigation showed that this apparent division resulted from the overlap between successive segments. After death, minerals filled the interior of the body and formed an internal mould, which was subsequently compressed within the sediment. This combination of overlap, mineral infilling, and flattening produced the illusion that each segment consisted of two separate divisions. Each trunk unit was therefore a single segment bearing one pair of limbs. The segments could overlap to different degrees, indicating that the trunk was flexible, an interpretation also supported by specimens in which the body is strongly curved.

The limbs of Waukartus had broad bases and gradually narrowed towards their tips. Each consisted of at least six podomeres and ended in a short terminal claw. Their structure indicates that they were suitable for walking across the seafloor. The researchers found no evidence of a second limb branch corresponding to the exopod commonly present in the appendages of many aquatic arthropods. Each appendage consisted of only a single main ramus and was therefore uniramous. In most specimens, the walking limbs are flexed posteriorly in a posture incompatible with normal locomotion. This position probably resulted from burial, post-mortem muscle contraction, or passive forces acting on the joints, and should not be interpreted as the animal's living gait.

The preserved tissues reveal longitudinal muscles running along both the dorsal and ventral sides of the trunk, together with dorso-ventral muscles extending across the body. Muscle fibres arranged horizontally and vertically were also preserved within the broad bases of the limbs. These muscles may have moved the appendages forwards, backwards, and upwards. The clearly defined podomeres confirm that the appendages were genuinely segmented, as expected in an arthropod. At the same time, the muscular arrangement within the limb bases resembles certain features of the lobe-like limbs of onychophorans. The researchers therefore concluded that the musculature of Waukartus was, in some respects, intermediate between that of onychophorans and euarthropods.

Several specimens also preserve traces of a dark, carbon-rich endoskeleton, probably composed of chitin and protein. Ring-like structures occur on either side of each trunk segment at the points where the limbs joined the body. The paired rings were connected by transverse bars extending across the ventral side of the trunk, producing an overall ladder-like arrangement. This internal cuticular framework would have provided structural support around the limb bases and may also have served as an attachment system for the muscles.

The Waukesha Lagerstätte formed in a shallow marine environment influenced by tides. After death, organisms were transported into an anoxic sediment trap and rapidly buried in fine-grained mud near microbial mats. There is no reliable evidence that terrestrial organisms were washed into the deposit, making a marine mode of life the most plausible interpretation for Waukartus. It probably crawled across the seafloor on its short, segmented limbs, while its flexible trunk allowed it to bend and turn. Because its appendages lacked exopods that could have assisted in respiration, the researchers proposed that gas exchange may have occurred through a relatively thin ventral cuticle, as is thought to have occurred in some marine lobopodians.

Phylogenetic analyses place Waukartus muscularis within the myriapod stem group, branching below the clade comprising euthycarcinoids and crown-group Myriapoda. It should therefore not be regarded simply as a direct ancestor of modern centipedes or millipedes. Its combination of characters instead provides evidence for the morphology present during an early stage of myriapod evolution.

Phylogenetic position of Waukartus muscularis（Image source：Briggs DEG et al. (2026)， CC BY 4.0 ）

The absence of exopods has often been interpreted as an adaptation associated with terrestrial locomotion in myriapods, partly because insects and arachnids independently evolved similar uniramous limbs. Yet Waukartus was clearly marine and had already lost the exopod. This shows that uniramous limbs appeared in the myriapod lineage before the transition to land. The loss of the exopod may initially have evolved under a different set of selective conditions in an aquatic environment and was only later co-opted for terrestrial locomotion, making it an example of exaptation.

The transition to land did not require every anatomical feature to evolve simultaneously as part of a single coordinated transformation. Changes in body form and ecology can occur at different times. A structure that first evolves under one set of environmental conditions may later become advantageous in a completely different setting. In the case of myriapods, at least one feature that would eventually support terrestrial life—the uniramous walking limb—had already evolved while their ancestors were still moving across the seafloor.

Author: Shui-Ye You

Reference:

Briggs DEG et al. (2026). A marine stem-myriapod from the Silurian Waukesha Lagerstätte, Wisconsin, USA: terrestrial traits pre-date the transition to land. Proc. R. Soc. B.