Devonian traces in Morocco reveal ancient roots of complex arthropod behaviour

In the stony deserts of southeastern Morocco, some 16 kilometres southeast of the fossil-trading town of Erfoud, a small ridge called Hamar Laghdad has long been famous among palaeontologists for its eerie conical limestone mounds and its spectacular Devonian cephalopods. Now the ridge is making headlines for something far subtler: the footprints, burrows and feeding galleries of animals that left no bones at all. A new study in the Swiss Journal of Palaeontology documents more than 200 invertebrate trace fossils from Middle Devonian strata at Hamar Laghdad, and among them are records that push the known history of sophisticated arthropod behaviour back by tens of millions of years.

The research team, working at outcrops between 2021 and 2024, photographed, analysed and measured the specimens directly in the field, because the lithology and consistency of the host rock made collecting them impracticable. The traces are preserved in concave and convex epirelief, hyporelief and full relief within marly limestones of Eifelian and Givetian age, roughly 393 to 383 million years old. Two distinct ichnoassemblages emerged from the analysis. The Eifelian strata yielded Arenicolites isp., Crescentichnus antarcticus, Selenichnites hundalensis and Osculichnus cf. labialis, while the Givetian layers contain Arenicolites isp., Diplocraterion isp., Sinusichnus sinuosus and Thalassinoides paradoxicus.

The most eye-catching of these fossils are the crescentic and horseshoe-shaped impressions attributed to xiphosurans, the group of arthropods that includes modern horseshoe crabs. Remarkably, no xiphosuran body fossils have ever been found in Devonian deposits anywhere in Africa, yet the traces at Hamar Laghdad show exactly the kind of morphology that living limulids produce when they dig into the sea floor. Selenichnites hundalensis from the lower Eifelian consists of suboval depressions with paired crescentic lobes, faint segmentation, occasional scratch traces and, in some specimens, an elongate telson impression trailing behind. The new Moroccan material represents the oldest occurrence of Selenichnites in Gondwana and one of the oldest in the world, previously known only from Late Triassic strata.

Crescentichnus antarcticus, preserved as elongated crescentic depressions measuring 5 to 8 centimetres long and 7 to 11 centimetres wide, is interpreted as a prod trace made when a horseshoe crab’s prosoma touched the sediment at an angle, perhaps as the animal swam up from the sea floor or made rapid contact to catch prey. The study draws on decades of work on modern limulid burrowing, including Eldredge’s classic four-stage description of burial by downward flexion of the prosoma and promotor-remotor swings of the prosomal legs. Intriguingly, the co-occurrence of Crescentichnus, Selenichnites and the fish-jaw trace Osculichnus with dwelling burrows of Arenicolites on the same surfaces hints at a hunting and feeding relationship between the tracemakers and the burrow-dwelling worms or crustaceans below.

Osculichnus cf. labialis itself is a curiosity: a hypichnial bilobate mound whose two lip-like lobes reflect the outline of upper and lower jaws. Such traces are usually attributed to jawed fish that plunged vertically or obliquely into the sediment to seize prey. The Hamar Laghdad specimen, with a larger lobe 20 millimetres wide and a smaller one 10 millimetres wide, separated by an undulate furrow with a small central knob, is the first record of this poorly known ichnogenus from the Middle Devonian of Africa, adding a predatory fish to the cast of characters roaming the Devonian shelf.

The Givetian assemblage tells a different story, one of complex engineering. Sinusichnus sinuosus consists of regular to slightly irregular sinusoidal tunnel systems with smooth, unlined walls, predominantly horizontal, and branching at triple junctions in Y-shaped and occasionally T-shaped patterns, with branch angles between 112 and 125 degrees. Tunnel diameters range from 1 to 2 centimetres, meander amplitudes from 4 to 8 centimetres and wavelengths from 10 to 20 centimetres, with individual systems locally exceeding 60 centimetres in length. The new Moroccan material is currently the oldest known occurrence of this ichnospecies in Gondwana, and the first evidence of the ichnogenus in Africa.

Even more striking are the burrow networks of Thalassinoides paradoxicus, which in the upper Givetian carbonates surround and converge upon the shells of dead cephalopods, including oncocerids, orthocerids and multilobate pharciceratid and beloceratid ammonoids. The burrow fill, a brown, very fine-grained clayey to marly carbonate, contrasts sharply with the grey to black host rock. The researchers propose an exceptional feeding strategy: after the cephalopods died and their soft parts decayed, the carcasses became nutrient sources, microbes colonised the organic compounds, and the Thalassinoides tracemakers exploited the biofilm inside the body chambers and around the conchs through conveyor-belt feeding. To the authors’ knowledge, no similar association of body fossils surrounded by such complex burrow systems has been reported before.

Because Palaeozoic burrow systems assigned to Thalassinoides predate the callianassid shrimps that construct similar burrows today, the Devonian tracemakers must have been non-callianassid crustacean arthropods or trilobites. Likewise, the Sinusichnus producers are proposed to be eumalacostracan crustaceans, whose segmented abdomens would have permitted the sinuous movements recorded in the burrows. The study observes gradual transitions between Sinusichnus and Thalassinoides in the same strata, suggesting that the tracemakers shifted between feeding strategies, from fodinichnia to domichnia. Taken together, these traces suggest that the complex burrowing, navigational and scavenging behaviours of modern arthropods have far more ancient roots than previously thought.

The trace fossils also encode a detailed record of environmental change. During the Eifelian, the Tafilalt Platform was a pelagic submarine high with very slow carbonate sedimentation, producing condensed nodular limestones and marls rich in ammonoids such as Fidelites, Foordites and Pinacites. The ichnofauna of that interval, combining fodinichnial traces like Crescentichnus and Selenichnites with vertical dwelling burrows of Arenicolites, belongs to the Skolithos-Cruziana ichnofacies and points to a moderately shallow inner shelf with high hydrodynamic energy, well-oxygenated water and abundant organic matter. By the Givetian, deposition took place in somewhat deeper settings, still on the shelf, with alternating calm and energetic conditions recorded by the co-occurrence of horizontal and vertical burrows.

This sea-level rise on the inner shelf of an epicontinental sea at the northwestern margin of Gondwana was shaped by peculiar local conditions. The more than 40 conical Kess-Kess mud-mounds of late Emsian age, distributed over 7 kilometres, created a palaeotopography that controlled sedimentation and ecosystem conditions long after the mounds stopped growing, generating diverse microenvironments with varying water depth, energy, sediment accumulation and light availability. The region’s famous palaeobiodiversity, it turns out, includes not only the skeletal record of ammonites, bivalves and brachiopods but also this hidden archive of behaviour. The abundance and variety of both trace and body fossils strongly encourage further systematic prospecting at Hamar Laghdad, where the Devonian sea floor may yet surrender more stories written not in bone but in burrow.

Subject of Research: Middle Devonian ichnofossils from Hamar Laghdad (eastern Anti-Atlas, Morocco)

Article Title: Middle Devonian ichnofossils from Hamar Laghdad (eastern Anti-Atlas, Morocco)

Article References: Bel Haouz, W., Lagnaoui, A., Klug, C., Baidder, L., & Maanan, M. (2026). Middle Devonian ichnofossils from Hamar Laghdad (eastern Anti-Atlas, Morocco). Swiss Journal of Palaeontology, 145, 403-431. https://doi.org/10.3897/sjp.145.188799

Image Credits: AI Generated

DOI: 10.3897/sjp.145.188799

Keywords: Devonian, ichnology, trace fossils, Morocco, Anti-Atlas, paleontology, xiphosurans, horseshoe crabs, arthropods, Hamar Laghdad, Middle, ichnofossils