Scientists have discovered a new iguanodontian dinosaur from the early Cretaceous Isle of Wight. The 125 million year old Istiorachis macarthurae sports an unmistakable “sail” along its back and tail.
Iguanadonts are a family of large herbivorous dinosaurs that could walk on two or four feet. We used to consider Istiorachis to be another run-of-the-mill Iguanodon relative. But a new study published last month revealed it had neural spines far taller than usual.
Because of this sail, the species is named after Ellen MacArthur. She is an English sailor who set a world record for the fastest solo non-stop voyage around the world, in 2005. She did that on her first attempt/
What was the sail for?
“It is difficult to explain hyperelongation of neural spines purely in terms of their primary physiological function” Dr. Jeremy A. F. Lockwood et al wrote in their paper. “The function of elongate neural spines was probably pluralistic and differed in different taxa. No single explanation fully supports the variation seen throughout the Cretaceous.”
Istiorachis macarthurae gen. et sp. nov. holotype (MIWG 6643). The anterior-most seven caudal vertebrae in left lateral view with neural spines reconstructed. Abbreviations: Cd, caudal vertebra; SC, sacrocaudal. Scale bar represents 50 mm. Image Credit: Jeremy A. F. Lockwood et al
Researchers believe they most likely served as visual signals, possibly linked to sexual selection or species recognition. These display structures are echoed in modern reptiles, where sails often appear more pronounced in males. “Most iguanodontian taxa are represented by single specimens, and neural spines are often incomplete, leaving us with insufficient material to assess whether neural spine height may have played a sexually dimorphic role in some species“, the researchers said.
This hyperelongation of spines in iguanadonts in the region has coincided with increasing biodiversity among similar looking species in the area. Researchers believe that this could indicate that the presence of a big sail “may have contributed to species recognition, even if only as a secondary function“.
“Additionally, tall neural spines could have made individuals appear larger, which may have helped to ward off predators or intraspecies rivals”
The Sail-backed dinosaur inspired the creation of a research database
The researchers built a vast database of iguanodont spine heights from Jurassic through early Cretaceous fossils. They used available published photographs, drawings and reconstructions.
This dataset powered an ancestral state reconstruction and helped prepare a newly calibrated iguanodontian evolutionary tree. It showed that modest spine elongation began with Ankylopollexia in the late Jurassic, intensified through the early Cretaceous. Spine elongation spiked during the period when Istiorachis lived.
The study picked out a handful of dinosaurs (like Rhabdodon priscus, Hypselospinus fittoni, and Ouranosaurus nigeriensis) that exhibited hyper-elongated spines, well above the ancestral norms. Istiorachis clearly belongs in this rare elite.The analysis also confirmed that, although neural spine elongation began in late Jurassic iguanodonts, none were as exaggerated as Istiorachis. Making Istiorachis macarthurae a most fascinating and significant discovery.