Cretaceous Creatures Live at the Zoo
About 65 million years ago, at the tail end of the Cretaceous period, in a colossal example of what it means to be in the wrong place at the wrong time, a 7.5-mile-wide asteroid (probably the largest of a series) collided with what is now the Yucatán Peninsula. The impact created the Chicxulub crater and triggered titanic earthquakes, tsunamis, volcanic eruptions, and massive wildfires that resulted in dramatic climate changes. This catastrophic chain of events ultimately wiped out the large dinosaurs that dominated the planet.
For a short time, a collection of those Cretaceous creatures has taken up residence at the L.A. Zoo. You can experience them up close at Dinosaurs: Unextinct at the L.A. Zoo. The exhibit includes 14 dynamic animatronic dinosaurs that bring these animals back to life. But dinosaurs are just the beginning of your time-traveling adventures at the Los Angeles Zoo.
The Cretaceous-Tertiary extinction event (or K-Pg boundary) wiped out roughly half of all the species on Earth. Among the survivors were crocodilians; the small dinosaur ancestors of birds; invertebrates; and early mammals—all of which are alive and well at the Zoo. At the end of the Cretaceous period, crocodilians had been in existence for some 150 million years, and scientists are still uncertain why they overcame the radical climate changes that followed the K-Pg boundary. Some believe their slow metabolisms (as compared with the massive, endothermic dinosaurs) enabled them to survive with little food and in harsh conditions. Others believe that their reliance on freshwater habitats, which were less drastically effected than saltwater environments by the K-Pg boundary aftermath, was advantageous. In any case, their living descendants are relatively unchanged in form. Notice the similarities between the Zoo’s tomistoma pair, the dwarf caiman, and Reggie the American alligator. These reptiles share a semi-aquatic lifestyle, a body shape that is long and low to the ground, plus jaws lined with many sharp teeth that pack a powerful bite force.
Monotremes and marsupials are the oldest lineages of mammals. Monotremes share with their reptile antecedents many internal physical structures, and they lay eggs. However, like all other mammals, they feed their young milk. The surviving monotremes include the platypus and four echidna species. The Zoo’s short-nosed echidnas can be seen in the Zoo’s Australia section. Marsupials diverged from the same line of early mammals as the monotremes, about 166 million years ago. Though they do not lay eggs, their young are tiny and altricial at birth and complete their development in pouches of skin on the mothers’ bellies. Opossums are among the most primitive living marsupials. A group of gray short-tailed opossums, native to South America, are among the outreach animals at Australia House.
The first land animals appeared in the fossil record about 150 million years before the dinosaurs, and the included early spiders. Spiders’ distinctive ability to spin silk has become very sophisticated over time. The beautiful geometric orb webs we associate with spiders developed during the Jurassic period along with the rise of flying insects, but not all spiders net their prey in this way. Some of the oldest surviving groups of spiders, including tarantulas, live in burrows that they line with silk and from which they ambush prey.
Birds are the dinosaurs that never went extinct, the descendants of theropod (“beast footed”) dinosaurs that began branching off during the Triassic period more than 200 million years ago. So the tiny, fierce black phoebes that snatch insects so adroitly out of the air in your backyard are the living legacy of Tyranosaurus rex. It’s tempting to seek out the most primitive of dinosaurian bird species. Certainly ratites such as the cassowary and the ostrich closely resemble the theropod body type found in fossils, but that similarity is misleading. These flightless birds are the latest in a long line of animals that began with theropods, developed flight as other birds did, took to the air, colonized distant regions, then gradually lost the ability to fly. Raptors such as eagles and hawks, despite the semantic connection with velociraptors, developed their specialized hunting abilities more recently. These predatory adaptations take time to evolve. Ducks, geese, and pheasants are among the most ancient bird groups, but evolutionary biologists find that all living birds, no matter how diverse their lifestyles, have traveled essentially the same evolutionary distance from the theropods.
So your Jurassic journey may begin with Dinosaurs: Unextinct at the L.A. Zoo, but it will lead you back to the future throughout the Zoo.
Originally published article, “Cretaceous Creatures,” can also be seen in Zooscape, May-June 2016.