Isasicursor: several individuals, one incomplete record

Isasicursor santacrucensis is an ornithopod from the Late Cretaceous of southern Patagonia. It was named from a concentration of bones in Argentina’s Chorrillo Formation, not from one articulated skeleton. The sample includes overlapping limb and vertebral elements from at least four individuals. It records a distinctive dinosaur, but the skull, teeth and complete body proportions remain unknown.

Quick facts

A fossil concentration near El Calafate

The material comes from the Chorrillo Formation in Santa Cruz Province, roughly 30 kilometres southwest of El Calafate. The formation preserves a diverse assemblage of land and freshwater organisms, including plants, invertebrates, fish, amphibians, turtles, snakes and dinosaurs. The fossil-bearing beds are generally discussed as Upper Campanian to lower Maastrichtian, near the final part of the Cretaceous.

Fernando Novas and colleagues described Isasicursor in their 2019 account of the Chorrillo fauna. The genus name honours Marcelo Pablo Isasi, the field technician who discovered the remains; the Latin element cursor means runner. That word is a name, not a measured speed. The species epithet refers to Santa Cruz Province.

The type locality includes a dense spread of disarticulated bones. Its layer is only several metres long and less than a metre thick, and the sample contains repeated limb elements and bones of different sizes. The authors interpreted it as representing at least four individuals. This is a minimum count from the duplicated anatomy, not proof that the animals formed a herd or died in one event.

What the type specimen is

The holotype, MPM 21525, is the upper portion of a left tibia. It is the name-bearing specimen and carries anatomical features used in the diagnosis. The broader hypodigm includes a partial neck vertebra, dorsal vertebrae, a sacrum, many tail vertebrae, parts of the shoulder and pelvis, several femora and lower leg bones, metatarsals, toe bones and foot claws.

These elements provide a much wider anatomical sample than the holotype alone, but they are not all parts of one connected animal. Some bones occur more than once, and some are from smaller individuals. They are assigned to the same species because of their locality and anatomical comparison. A reconstruction can use the full sample while still showing that it combines individuals.

No skull, jaw or teeth have been assigned to Isasicursor. The head in an illustration therefore comes from comparisons with other ornithopods. A beak and plant-eating diet are consistent with its placement among ornithopods, but details of the snout and tooth rows cannot be read from this fossil concentration.

Leg and foot features

The original diagnosis identifies several features in the pelvis and hind limb. The sacrum curves downward in side view, and the first two sacral vertebrae have an unusual peg-and-socket contact. The tibia has a prominent crest at its upper end where muscles attached. Its outer ankle-side condyle also bears an additional projection. On the second metatarsal, a ligament pit lies unusually far toward the upper end of the bone.

Those details help distinguish the type from other small and medium ornithopods. They are more useful than a broad resemblance to a “runner” because they can be checked directly on the fossils. Robust foot bones suggest a strong support structure, but they do not yield a precise top speed. Speed depends on muscle, tendon, body mass and stride length, most of which are not preserved.

Some of the tail vertebrae have tall neural spines. They show that the tail was structurally important, as in other dinosaurs, but do not by themselves fix its complete length or exact posture. The hands are incompletely known, and no trackway has been securely attributed to this genus.

Relationship to other southern ornithopods

Novas and colleagues placed Isasicursor among Elasmaria, a group of Gondwanan ornithopods, and recovered it within basal Iguanodontia. Elasmaria has included South American and Antarctic forms such as Anabisetia. Its precise membership and position have shifted among phylogenetic analyses as researchers add fossils and revise character coding.

The fossils are younger than many better-known South American elasmarians. Their presence in southern Patagonia shows that ornithopods outside the classic northern hadrosaur lineages persisted in high-latitude ecosystems late in the Cretaceous. The result adds a branch to the regional fauna; it does not establish that Isasicursor was ancestral to later hadrosaurids such as Hadrosaurus.

Nearby marine-reptile teeth and evidence of aquatic environments belong to the broader Chorrillo record. They do not indicate that this ornithopod lived in water. The leg and foot bones are consistent with a terrestrial animal. Which habitats it preferred within the region is not recorded by the type material.

Size, growth and movement

The sample includes bones from animals of different sizes, including immature individuals. It does not provide one adult skeleton that can be measured from snout to tail. Published body lengths are therefore comparative estimates. A careful description should identify Isasicursor as a small-to-medium ornithopod rather than imply a precise measured length.

Repeated bones in one deposit can represent a group, but they can also accumulate through water transport, scavenging or more than one depositional episode. The fossil bed needs its own sedimentological and taphonomic analysis before a social interpretation is justified. Multiple ages in the sample may be compatible with a breeding population, but they do not demonstrate parental care or a family group.

The long tail and hind-limb anatomy fit a bipedal ornithopod body plan. Some ornithopods could also use all four limbs at slower speeds, but the incomplete forelimb sample cannot establish how Isasicursor moved. The name’s “runner” meaning should not be converted into an unsupported speed claim.

Living near the end of the Cretaceous

The Chorrillo Formation contains large titanosaurs, megaraptorid theropods, other dinosaurs and a broad freshwater and terrestrial community. The mixture of remains allows researchers to compare different ecological roles across the formation. It does not mean every animal lived together at exactly the same time, because the unit spans a substantial interval and fossils may have been transported.

Plant fossils and pollen record vegetation in the wider depositional system, but no stomach contents or diagnostic coprolite is tied to Isasicursor. Herbivory follows from its ornithopod anatomy and evolutionary position. Particular plants, feeding height and seasonal diet remain unknown. Skin, feathering, colour, calls and nesting are not preserved in the material assigned to the genus.

In the dinosaur catalogue, Isasicursor is best understood as a taxon recognised from a multi-individual bone concentration. Keeping the holotype, referred sample and ecological interpretation distinct lets the fossils support a meaningful account without turning the deposit into a complete animal or an observed herd.