Istiorachis macarthurae

Istiorachis macarthurae was a medium-sized, sail-backed iguanodontian ornithopod that lived in what is now the Isle of Wight, England, during the Early Cretaceous period, approximately 129–125 million years ago (Barremian stage). It was discovered in the Wessex Formation, a fossil-rich sedimentary deposit known for its diverse dinosaur fauna from the Wealden Group.

The name Istiorachis combines “istion,” Ancient Greek for “sail,” and “rachis,” meaning “spine,” alluding to its distinctive sail-like structure. The species name macarthurae honors Dame Ellen MacArthur, a renowned British sailor and founder of a cancer trust for young people on the Isle of Wight.

As a member of the Ankylopollexia group within Iguanodontia, Istiorachis was closely related to other stiff-tailed ornithopods, showcasing the diversity of neural spine morphologies among these dinosaurs.

Etymology

The genus name “Istiorachis” is derived from the Ancient Greek words “istion,” meaning “sail,” and “rachis,” meaning “spine” or “backbone,” referring to the probable sail-backed appearance of the dinosaur. The species name “macarthurae” honors Dame Ellen MacArthur, an English sailor who set a world record in 2005 for the fastest solo non-stop voyage around the world and founded the Ellen MacArthur Cancer Trust for young people on the Isle of Wight.

Physical Characteristics

Istiorachis macarthurae was a relatively medium-sized iguanodontian, estimated to reach 6–7 m (20–23 ft) in length. While this is substantial by modern standards, it is comparable to contemporaries like Mantellisaurus atherfieldensis, though exact comparisons are limited by the incomplete skeleton.

Key anatomical features include:

• Holotyp: MIWG 6643
• Hyperelongated neural spines on dorsal and caudal vertebrae, suggesting a sail-like structure for display or thermoregulation.
• A single posterior cervical vertebra with an anteroposteriorly elongate centrum (length-to-height ratio of 1.4), a broad ventral keel, and robust transverse processes.
• Eight dorsal vertebrae showing variation: anterior ones with ventral keels and posterolaterally directed transverse processes; posterior ones lacking keels, with hyperelongated neural spines (Nh/Ch ratio up to 4.3).
• Partial sacrum with a smooth ventral surface on the sacrodorsal and keels on subsequent vertebrae, indicating fusion typical of iguanodonts.
• Seven caudal vertebrae, including a possible sacrocaudal, with transversely compressed centra, ventral sulci, and posterodorsally inclined neural spines decreasing in height posteriorly.
• Incomplete pubes with a dorsoventrally expanding prepubic process, predominantly dorsally.
• Robust ischia with a sinusoidal shaft in lateral view and a strongly developed distal boot (3.3 times midshaft diameter).

These features indicate a lightly built but robust structure, adapted for quadrupedality with potential display elements from the elongated spines.

Diet and Feeding Habits

As a herbivorous ornithopod, Istiorachis macarthurae fed on a variety of plants that thrived in the Early Cretaceous ecosystems of Europe. Its diet likely included:

• Ferns and cycads, which were abundant in its floodplain environment.
• Conifers, the dominant trees of the time.
• Early flowering plants (angiosperms), which were emerging during the Barremian.

Like other iguanodontians, Istiorachis probably relied on its long neck to browse vegetation at different heights, allowing it to access food sources that smaller herbivores could not reach.

Habitat and Distribution

Istiorachis macarthurae lived in what is now the Isle of Wight, England, specifically in the Wessex Formation of the Wealden Group, a geologic unit that preserves a rich array of Early Cretaceous dinosaurs.

Its environment was characterized by:

• Floodplains and river systems, creating a humid, fertile landscape.
• Seasonal climate fluctuations, with periods of wet and dry conditions.
• Diverse dinosaur fauna, including other iguanodonts, theropods, and ornithopods.

The Wessex Formation has yielded fossils of other iguanodontians, but Istiorachis macarthurae appears to be one of the distinctive sail-backed representatives of this group.

Paleoenvironment

The discovery site was a plant debris bed in an ancient floodplain ecosystem. During the Early Cretaceous, temperatures were warmer than today, with water sources like rivers concentrating biodiversity. The fossil assemblage includes diverse species such as tyrannosauroids, ornithopods, and other vertebrates, providing a comprehensive snapshot of the ecosystem in which Istiorachis macarthurae thrived.

Behavior and Social Structure

Although no direct evidence of social behavior has been found for Istiorachis macarthurae, many iguanodontians are thought to have exhibited herding behavior for protection and migration. Possible behaviors include:

• Living in small herds or family groups, as seen in some iguanodontian trackways.
• Migrating seasonally in search of food and water.
• Using size and sail structures as a defense or display, though Istiorachis was medium-sized and may have relied on quadrupedality for agility.

As an ankylopollexian, it likely adopted a facultative quadrupedal gait, supporting bulk herbivory in a diverse ecosystem.

Discovery and Research

The fossils of Istiorachis macarthurae were discovered in the Wessex Formation of the Isle of Wight, England. The holotype consists of partial vertebrae, ribs, sacrum, pubes, and ischia, which were used to identify it as a new genus and species of iguanodontian.

This discovery provides important insights into the diversity of neural spine morphologies in European iguanodontians, showing that sail-backed forms evolved multiple times.

Discovery Context

The fossils of Istiorachis macarthurae were unearthed in the “Black Band” plant debris bed near Grange Chine on the Isle of Wight, dating back approximately 129–125 million years. This site, once a floodplain ecosystem, was excavated by Nicholas Chase but unfortunately poached, resulting in the loss of parts of the skeleton.

Significance and Interesting Facts

• One of the sail-backed iguanodontians, highlighting the sporadic evolution of neural spine hyperelongation.
• Discovered in the Wessex Formation, which has also yielded fossils of other iguanodontians, theropods, and ornithopods.
• Provides evidence of ecological diversity among iguanodontians, suggesting some species developed display structures amid rising diversity.
• Named in honor of Dame Ellen MacArthur, recognizing her contributions to sailing and charity.
• Demonstrates that iguanodontians were not uniform in morphology, with some taxa adapting elongated spines for potential biomechanical or signaling roles.

Conclusion

Istiorachis macarthurae was a medium-sized, sail-backed iguanodontian from the Early Cretaceous of England, showcasing the remarkable diversity of neural spine adaptations within this group. While many iguanodontians developed elongated spines at the Jurassic–Cretaceous boundary, Istiorachis represents a hyperelongated form, likely for pluralistic functions including display and support. Its discovery enhances our understanding of the ecological roles played by ornithopods in prehistoric Europe.