A 105-million-year-old monotreme fossil discovered in Victoria's Dinosaur Cove in 1996 has been re-examined, revealing new insights into the evolutionary history of platypuses and echidnas. The bone, identified as the humerus of a prehistoric echidna-like creature named Kryoryctes cadburyi, suggests a semi-aquatic ancestor for modern monotremes and was linked to the fulfillment of a long-standing chocolate promise made to volunteers.
The re-examined fossil, Kryoryctes cadburyi, provides crucial evidence for a semi-aquatic ancestor of platypuses and echidnas, shedding new light on monotreme evolution.
Discovery and Initial Identification
In 1996, palaeontologists Dr. Thomas Rich and Professor Pat Vickers-Rich unearthed a mammal bone at Victoria's Dinosaur Cove. The pinky-length humerus was initially misidentified as a turtle's arm bone and was later found among leftover material from the dig. Upon subsequent re-examination, it was reclassified as the 105-million-year-old humerus of a Cretaceous monotreme, an ancient relative of modern echidnas and platypuses. This fossil represents the oldest monotreme limb bone discovered in Australia.
The "Chocolate Promise"
During the excavation period, Dr. Rich had pledged chocolate to volunteers who discovered a mammal bone. Reports of the promised quantity vary, describing it as a cubic meter or a one-tonne block of chocolate. Volunteer Helen Whitelaw is noted as initiating this promise.
In 1998, Cadbury's chocolate factory, with the assistance of a volunteer's connection, hosted an event that provided thousands of chocolate bars to all volunteers who had participated in the Dinosaur Cove excavations, thereby fulfilling the commitment. The prehistoric creature was later scientifically named Kryoryctes cadburyi, in reference to the chocolate company.
Re-examination and Evolutionary Insights
The Kryoryctes cadburyi humerus was recently analyzed using the DINGO neutron radiography device at the Australian Nuclear Science and Technology Organisation (ANSTO). The analysis indicated that while the bone's exterior structure resembles that of an echidna, its internal structure is consistent with that of a platypus.
The bone's density strongly suggests it belonged to a semi-aquatic animal, potentially one that swam in rivers and dug through mud.
This characteristic is observed in modern water-dwelling species such as hippos and sea otters, which utilize dense bones for underwater ballast. Dr. Joseph Bevitt, a co-author of the research paper, stated the animal was likely semi-aquatic.
Implications for Monotreme Evolution
This finding supports the hypothesis that platypuses and echidnas may have evolved from a common aquatic ancestor. Professor Suzanne Hand, the lead author of the research, highlighted that this would imply echidnas transitioned from a water-dwelling existence to a terrestrial one.
This evolutionary pathway, from water to land, is considered unique for a mammal, as most mammalian evolution involves adaptation from land to water.
This aquatic ancestry could also provide an explanation for certain features observed in echidnas today, such as their backward-turned hind feet, which can function as rudders for swimming, and their oxygen-saving dive reflex.