RESEARCH:: Celebrating Darwin
Evolution, Environments, and the Earliest Squid Relatives
By Dr. Ed Landing
Specimen of living Nautilus at Suma Aqualife Park, Kobe, Japan. Picture shows color-banded shell, tentacles, and leathery protective hood above the large eye. Image used with permission from creative commons.
One group that appeared only after the Cambrian radiation is the class Cephalopoda (squids, octopuses, and their ancient relatives)—a mollusk group with a fossil record extending only c. 491 million years ago into the latest Cambrian. Cephalopods are high-level predators in marine food chains. Their late appearance may mean that a smaller variety of predators existed earlier or that cephalopods replaced predators that dominated older ecosystems.
New insights into early cephalopod evolution have resulted from work with Dr. Björn Kröger of the Humboldt Museum (Berlin) on eastern New York specimens. Our results are detailed in four scientific papers published or accepted for publication since 2007.
Early cephalopods were shelled forms related to the modern Nautilus. They first appeared and diversified in tropical, shallow-marine habitats. The great variety of early cephalopods reflects eastern New York’s latitude of 35° S in the Cambrian and Ordovician periods.
Thrombolite reefs outlined in yellow in Late Cambrian rocks; NYSM Scientific Reservation, Lester Park, Saratoga County, New York. The reefs are up to 1.5 m (5 feet) high and surrounded by wave-deposited limestone.
Our work emphasizes that the Early Ordovician featured a second cephalopod evolutionary radiation. The oldest Ordovician fauna includes small, straight to gently curved shells. These forms belong to a single “hold-over” cephalopod group that escaped the end-Cambrian extinction. The Early Ordovician rocks of eastern New York were deposited during three successive, global sea-level rises that allowed accumulation of limestones that now form three geological formations. Each formation has distinctive cephalopods that do not change through the formation—the implication being that relatively little time is represented by each formation, and there is no evidence for evolution of the cephalopods. By comparison, the sea-level falls and the breaks in sedimentation between the formations bracketed far longer time intervals. With extinctions of the older faunas, new cephalopods evolved in the seas that covered only the margin of ancient North America during low sea-level times.
The later Early Ordovician shows an increasing variety of cephalopods with larger, often strongly curved shells. These include coiled shells up to 30 centimeters (10 inches) wide and straight shells a meter (39 inches) long. The coiled shells are from cephalopods that swam as well as Nautilus, and were efficient predators. The long shells show that cephalopods were the largest Early Ordovician animals.
Coral reefs have the highest variety of animals in modern oceans. However, coral reefs first appeared in the Late Ordovician, and algae and blue-green bacteria produced the Late Cambrian to Early Ordovician reefs. These mound-like structures, called thrombolites, have internal cavities that suggest Swiss cheese. Our research shows that the most abundant and diverse cephalopods occur in the thrombolite reefs in each formation. These data have led us to conclude that cephalopods first appeared and diversified in the tropical, thrombolitic reef habitat.
Dr. Ed Landing curates the paleontology collection of the New York State Museum. His research interests include the evolution of the oldest animals and the relationship of extremely high sea levels to ancient and future global warming.