Apatosaurus

• 1. Introduction

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  Apatosaurus means "deceptive lizard," and the name has lived up to its promise for nearly 150 years. Othniel Charles Marsh chose it in November 1877 because of one strange feature: the chevron bones beneath its tail looked nothing like those of other dinosaurs. They resembled instead the bones of mosasaurs, the great marine reptiles. Marsh had been deceived by a fragment, and so would generations of scientists and museum-goers after him. This was a herbivore of the Late Jurassic, a quadruped that stretched 21 to 23 meters and carried a mass measured in tens of tons. It walked North America roughly 152 to 151 million years ago. Yet for most of the twentieth century, the public knew it by the wrong name and pictured it with the wrong head. How does a creature this enormous lose its own skull for sixty years? Why did a rival paleontologist mount it under a name that science had already rejected? And what does its triangular neck reveal about how these animals lived and fought?

• 2. Robust By Design

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  The femora of Apatosaurus are among the most robust of any sauropod that ever lived. This stockiness runs through the whole skeleton and separates it from its close relative Diplodocus. The cervical vertebrae are less elongated and more heavily built, and the leg bones are stouter even though they are longer. The result was a more powerfully constructed animal. Its forelimbs sat slightly shorter than its hindlimbs, and a single large claw tipped each forelimb, with the first three toes of each hindlimb bearing claws as well.

  The neck tells its own story of engineering. Its vertebrae were deeply bifurcated, carrying neural spines split by a large central trough that produced a wide, deep neck. Cervical ribs reached farther toward the ground than in diplodocines, and the bones narrowed toward the top, making the neck nearly triangular in cross-section. An extensive system of weight-saving air sacs filled this great structure, leaving the bones internally full of holes. In the resting posture, Apatosaurus would have held its neck angled upward with the head pointing downward.

  The tail breaks the diplodocid pattern. It is comparatively slender, because the height of the vertebral spines falls away rapidly with distance from the hips. The holotype of A. louisae records 15 cervicals, 10 dorsals, 5 sacrals, and 82 caudals, though the number of tail vertebrae could vary even within a species. Toward its end, the tail narrowed into the whip-like form shared by its relatives, a shape that would later spark a debate about loud noises and combat.

• 3. The Missing Skull

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  For sixty years, no one knew what an Apatosaurus skull truly looked like. Its head had been confused with those of Camarasaurus and Brachiosaurus until 1909, when the holotype of A. louisae was found with a complete skull lying just a few meters from the front of its neck. That skull, designated CM 11162, looked very much like the skull of Diplodocus, not the heavy head everyone had imagined.

  Adam Hermann oversaw the first mounted sauropod skeleton at the American Museum of Natural History and could find no Apatosaurus skull at all. He was forced to sculpt one by hand. Henry Fairfield Osborn wrote that the result was "largely conjectural and based on that of Morosaurus," the animal now called Camarasaurus. The sculpted head drew on the biggest, thickest, strongest skull bones, lower jaws, and tooth crowns from three different quarries.

  William H. Holland accepted that the Diplodocus-like skull belonged with specimen CM 3018, and defended his view in 1914 before the Paleontological Society of America. Osborn rejected it. Rather than fight, Holland left the Carnegie Museum mount headless. After his death in 1934, museum staff placed a cast of a Camarasaurus skull on the body anyway.

  Vindication came slowly. In the 1970s, John Stanton McIntosh and David Berman redescribed the skulls of Diplodocus and Apatosaurus and found that Holland, who never published his opinion, had almost certainly been right. On the 20th of October 1979, the first true skull of Apatosaurus was finally mounted on a skeleton, at the Carnegie.

• 4. The Brontosaurus Problem

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  Brontosaurus is one of the best-known dinosaur names in the world, and for most of the past century it was not supposed to exist. Marsh named Brontosaurus excelsus in 1879 from a larger, more complete specimen at Como Bluff, Wyoming. In 1903, Elmer Riggs studied a diplodocid skeleton from near Fruita, Colorado, and realized that the holotype of A. ajax was merely immature. The features once thought to separate the two genera were not valid. Because Apatosaurus was the older name, Riggs recombined the species as Apatosaurus excelsus.

  Osborn ignored him. A strong opponent of Marsh, Osborn labeled the American Museum's mount Brontosaurus, and the museum's popularity carried the discarded name into common speech. So Brontosaurus thrived in public life while remaining invalid through nearly all of the twentieth and early twenty-first centuries.

  The argument never fully died. In the 1990s, Robert T. Bakker maintained that A. ajax and A. excelsus were distinct enough to deserve separate genera. Then in 2015, Emanuel Tschopp, Octavio Mateus, and Roger Benson published a study comparing 477 morphological characters across 81 dinosaur individuals. They proposed that genera could be diagnosed by thirteen differing characters and species by six. By that measure B. excelsus stood apart, including in its narrower neck, and they returned it to its own genus, Brontosaurus.

  Not everyone agreed. Michael D'Emic called the chosen criteria arbitrary, warning the name might have to be abandoned again if new analyses shifted. Donald Prothero said he would keep "Brontosaurus" in quotes and not treat it as valid.

• 5. How A Giant Breathed

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  A 30-ton Apatosaurus carried an estimated 184 liters of dead-space air, the unused air left in the mouth, trachea, and tubes after each breath. The sheer scale of the body and neck made breathing a genuine puzzle for physiologists. Paladino calculated the tidal volume, the air moved in a single breath, at 904 liters with an avian system, 225 liters if mammalian, and only 19 liters if reptilian.

  The numbers point toward birds. An avian respiratory system would need a lung of about 600 liters, while a mammalian one would demand 2,950 liters, more than the body could hold. The total thoracic volume has been estimated at 1,700 liters, leaving room for a 500-liter, four-chambered heart, a 900-liter lung capacity, and roughly 300 liters of necessary tissue. The pneumaticity of the vertebrae supports this picture, and Wedel argued in 2003 that those hollows likely connected to air sacs, as in living birds.

  Heat and blood posed further problems. James Spotila and colleagues concluded in 1991 that such a large body could not shed enough heat to sustain high metabolic rates, assuming a reptilian system. Wedel countered that an avian system would dump more heat. As for the brain, the near-horizontal posture of the head and neck may have eased the strain of pumping blood upward, since the head would not have been raised high. James Farlow estimated in 1987 that a 35-ton animal would hold 5.7 tons of fermentation contents, while a 1997 estimate suggested it needed to drink only about 262 liters of water per day.

• 6. Crack Of The Tail

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  Over 200 decibels, comparable to a firing cannon: that was the sound a diplodocid tail might have produced, according to a 1997 study by Nathan Myhrvold and paleontologist Philip J. Currie. Their computer modeling treated the tail as a tapering bullwhip. The tail makes up roughly 54 percent of the total body length, narrowing toward its tip in a way that invited the comparison.

  Not all evidence cooperates with that romantic image. The tails of diplodocids were quite light and narrow compared with those of Shunosaurus and the mamenchisaurids, so striking another animal hard enough to wound it would have severely injured the tail itself. In 2020, Baron judged the bullwhip idea unlikely, arguing that such whip speeds could cause catastrophic muscle and skeletal damage. He proposed instead that the tail worked as a tactile organ, brushing against neighbors during migration to keep a group together and to ease communication.

  The bones carry their own scars. One pathology shows two caudal vertebrae fused seamlessly along the entire articulating surface, including the arches of the neural spines. The defect may have stemmed from a lack or inhibition of the substance that forms intervertebral disks or joints, a frozen joint preserved in stone.

• 7. Life In The Morrison

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  Allosaurus accounted for 70 to 75 percent of theropod specimens in the Morrison Formation and sat at the top of its food web. This was the world Apatosaurus inhabited, a sequence of shallow marine and alluvial sediments dated by radiometric methods to between 156.3 and 146.8 million years ago. The basin stretched from New Mexico to Alberta and Saskatchewan, formed as the precursors to the Rocky Mountains pushed up to the west. The setting was locally semiarid, with distinct wet and dry seasons.

  Apatosaurus was the second most common sauropod in this ecosystem, after Camarasaurus, yet it may have been more solitary than its neighbors. Its fossils appear only in the upper levels of the formation. Remains of A. ajax come exclusively from the upper Brushy Basin Member, about 152 to 151 million years ago, while the rare A. louisae is known from a single site of late Kimmeridgian age, about 151 million years ago.

  The surrounding cast was rich. The theropods Ceratosaurus, Ornitholestes, and Torvosaurus shared the land, alongside sauropods such as Brachiosaurus and Diplodocus and ornithischians including Camptosaurus, Dryosaurus, and Stegosaurus. Ray-finned fishes, frogs, salamanders, turtles, lizards, crocodylomorphs, and pterosaurs filled out the fauna. The flora ran from gallery forests of tree ferns to fern savannas dotted with the Araucaria-like conifer Brachyphyllum. The same genera echo across the world in Portugal's Lourinha Formation, where Apatosaurus finds a close counterpart in Dinheirosaurus.