Mamenchisaurus

Kingdom: Animalia

Phylum: Chordata

Class: Reptilia

Order: Saurischia

Family: Mamenchisauridae

Genus: Mamenchisaurus

Species: Mamenchisaurus constructus

Introduction

The Jurassic period (201–145 million years ago) was a time of high sea levels, warm climate, and no polar ice. By the Late Jurassic (163–145 million years ago), the Pangaean supercontinent had already divided into the massive landmasses of Gondwana and Laurasia. It was a time when such giant sauropods as Mamenchisaurus, Camarasaurus, Diplodocus, Apatosaurus, and Brachiosaurus roamed the earth and were widely distributed across a number of continents.

First discovered from a partial postcranial skeleton in 1952 in China, Mamenchisaurus is one of the largest Asian sauropods ever uncovered. Its neck, in particular, is one of the longest ever recorded for any species and reached up to 13 meters long (43 feet) and contained up to 19 vertebrae.

Classification

Scientists use two primary methods to organize animals into different groups. Linnaean taxonomy, also called “systematics,” groups organisms in a hierarchical fashion according to overall physical similarity, while an alternate system called “cladistics” attempts to create groups that better reflect the evolutionary relationships between species. The more shared evolutionary novelties they have, the more likely it is that they also shared a common ancestor, placing them in the same clade.

A Linnaean classification places Mamenchisaurus under the Saurischia (“lizard-hipped”) order and further under the Sauropodomorpha group of long-necked herbivores. Mamenchisaurus also falls under the Sauropoda infraorder of large, quadruped dinosaurs. The first true sauropods appeared in the fossil record as early as the Late Triassic more than 201 million years ago, and by the Middle Jurassic (174–163 million years ago), the sauropod dinosaurs had begun to diversify. Despite this diversity, all sauropod dinosaurs are classified as herbivorous species, typified by their long neck and tail and small head and brain.

Despite continuing debate as to whether the sauropod Mamenchisaurus should be grouped within Diplodocidae or Camarasauridae (sometimes referred to as Bothrosauropodoidea), Chinese paleontological theory states that this species is actually a basal eusauropod. Although Mamenchisaurus resembles the body form of giant dinosaurs such as Diplodocus, skull fragments have shown that Mamenchisaurus differed from Diplodocus in terms of head shape. Research has also shown that Mamenchisaurus more likely resembled the blunter snout of Omeisaurus and should be classified as in the euhelopodid clade.

Anatomy

Mamenchisaurus was an extremely large, four-legged herbivore that roamed the forests of the Asian landscape during the Early Cretaceous period. At between 25 and 26 meters (82–85 feet) in length and 3.5 meters (11.5 ft) tall at the hip, Mamenchisaurus was an impressive dinosaur. Although estimates vary, it may have weighed up to 40,000 kilograms (53,000–88,000 lbs). It possessed many physical traits typical of sauropod dinosaurs, including a small box-shaped head, very long neck, and powerful legs ending in thick club-like feet and clawed toes. Its front legs were also shorter than its hind legs, similar to species such as Brachiosaurus and Diplodocus.

Typical of herbivorous sauropods, Mamenchisaurus was a quadruped. Despite its huge size, however, it was a slow mover and would rarely have moved faster than a plodding walk. It is also hypothesized that this species may have been able to stand up on its strong hind legs to reach high foliage in the tall forest trees from which it fed.

At more than 13 meters (43 ft) long and making up half the length of its enormous body, the most notable Mamenchisaurus feature was its exceptionally long neck, one of the longest of any dinosaur ever found. Despite the length, its neck was comparatively light as many of the bones were extremely thin and hollow. Like all long-necked sauropods, each neck vertebrae possessed a pair of rod-like, overlapping spines underneath to maintain and support their rigid neck. In addition, elongated muscles connected to the central backbone helped keep their neck in the correct position. Problems with high blood pressure (a frequent issue in long-necked species) were likely combated by Mamenchisaurus holding its neck parallel to the ground.

The weight of its long neck was balanced by its large tail. Like several other sauropod species, Mamenchisaurus possessed a club at the tip of its tail, which it likely used as a defensive structure by swinging its tail side-to side to scare threatening predators and possibly to protect its young.

Intelligence

Measurement and comparison of animal intelligence is most often determined by the encephalization quotient (EQ). This technique is calculated as a ratio between body and brain size. With the exception of the troodontid species (EQ of 5.8) and the dromaeosaurid species (EQ of 5.7), all dinosaur species have an EQ below 2.0. (The scale extends to an upper limit of 8.0, the approximate value calculated for the human brain.) Although this figure compares poorly to most modern-day animals, techniques that assume dinosaurs more closely resembled the neural anatomy of birds than that of reptiles have indicated that dinosaurs may have higher EQs than previously thought.

Based on their very large bodies yet tiny heads, sauropod species were among the least intelligent of dinosaurs. Mamenchisaurus also possessed a small head and tiny brain and, as such, falls at the lower end of dinosaur intelligence with an EQ of 0.2. Current evidence has shown that carnivorous dinosaurs were more intelligent than their herbivorous prey. This is thought to be a reflection of a hunter's need for greater cognitive capability and behavioral flexibility.

Historically, it was thought that species such as Mamenchisaurus possessed a second brain located in the hip region of the spinal cord, used to control its tail and rear-body movements. Later theory states, however, that this enlarged oval space located at the base of the spinal cord is similar in form and function to a “glycogen body” (found in extant bird species and containing glycogen).

Reproduction and Population

Given the rarity of organisms being fossilized and the incompleteness of the fossil record, it is very difficult to determine past population numbers of dinosaur species. Current paleontological evidence shows, however, that with several species and many fossil finds, Mamenchisaurus was one of the most representative dinosaur genera to have existed during the Early Cretaceous period in China. Although its habitat was restricted to what is now East Asia, it had the widest distribution and the most endemic features of any Asian sauropod during the Late Jurassic.

Most dinosaurs are believed to have been oviparous, including Mamenchisaurus. Females laid a number of eggs per clutch, although the method differed for different dinosaur species. Although some sauropod species appeared to drop eggs in a linear pattern while walking, other species used nests, buried eggs underground, or exhibited incubation behavior.

Diet

Mamenchisaurus, like all sauropods, was an herbivorous species. Although its spatulate, peg-like teeth were small, they were well suited to its diet of soft plants. Given its size and exceptionally long neck, it is likely that Mamenchisaurus foraged and plucked foliage from the tall trees at the edge of forests, such as Ginkgoes and conifers, or fed on soft pteridophytes that were found in their habitat, such as horsetails, mosses, and ferns.

Like many of today's largest herbivore species, Mamenchisaurus would have spent a considerable amount of time feeding every day to obtain enough nutrients to power its massive body. Some palaeontologists believe that Mamenchisaurus would have used its small teeth to tear foliage from plants, swallowing much of it whole, and may have required gastroliths to aid digestion of the huge quantity of plant material it consumed every day.

Although debate continues regarding whether dinosaur species were cold-blooded or warm-blooded, some palaeontologists argue that such species as Mamenchisaurus would have to be cold-blooded, or they would have to consume ten times more food than they did to maintain their body temperature and regulate their metabolism. However, most researchers claim that more definitive evidence is required before this debate is settled.

Behavior

Paleontological evidence from bone beds and trackways has suggested that certain sauropod species traveled in herds. Herding behavior is often a mechanism used for the defense and protection of offspring. Although an adult Mamenchisaurus was very large and safe from most predators, fossil evidence of a healed wound in a Yangchuanosaurus scapula bone provided tantalizing evidence of conflict behavior. Chinese scientists believe Mamenchisaurus may have inflicted such wounds with its club-like tail, which indicates probable parental protection or personal defense behavior from Mamenchisaurus.

Though it seems that Mamenchisaurus herding behavior would certainly have benefited immature individuals of smaller size, fossil remains of several other sauropod dinosaurs has indicated that some herds were segregated by age.

The explanation for juveniles and adults traveling in their own separate herds remains unclear. One theory has suggested that resource partitioning occurred, with adult and juvenile sauropod dinosaurs having different feeding strategies.

Habitat and Other Life Forms

Mamenchisaurus inhabited the temperate and tropical forests and rich river valleys of what is now central China. Grass was yet to evolve so gymnosperms (such as ferns, horsetails, cycads, and mosses) dominated the low-lying landscape, while tall trees (such as Ginkgoes and conifers) dominated the temperate and sub-tropical forests. With its exceptionally long neck, it is likely that Mamenchisaurus foraged and plucked foliage from the tall trees in an effort to minimize resource competition with the other large herbivores at the time, a feed strategy known as “resource partitioning.”

Mamenchisaurus lived alongside many other dinosaurs in these warm humid conditions, including other herbivorous long-necked sauropods such as Omeisaurus, several back-plated dinosaurs including Chialingosaurus, Chungkingosaurus, and Tuaijangosaurus, the tiny Xiaosaurus herbivore, and efficient theropod predators such as Yangchuanosaurus.

Given their relatively wide distribution across East Asia, Mamenchisaurus may also have been a migrating species. Much like modern-day elephants who move to new areas once food resources are diminished, paleontologists believe that Mamenchisaurus would have consumed so much vegetation that it would have needed to travel to new areas to find food and survive. It is also likely that such feeding and migration behavior would have significantly influenced the vegetation, and giant sauropod species such as Mamenchisaurus may have been instrumental in shaping and opening up the forest landscape (again, much like modern-day elephant species).

Research

Mamenchisaurus is one of the largest Asian sauropods ever uncovered. A partial postcranial skeleton was discovered in Sichuan, China, during the construction of a highway in 1952. It was described and named Mamenchisaurus constructus in 1954 by one of China's preeminent paleontologists, Prof. Yang Zhongjian (also referred to as C. C. Young), from Peking University.

Mamenchisaurus constructus was not the only species recovered. A second, smaller species was unearthed in the Upper Shaximiao Formation of Sichuan in 1972 and was named M. hochuanensis by Prof. Yang. Although this species of Mamenchisaurus was first discovered in the early 1970s, it was the recovery of a second specimen in 2001 that clarified certain anatomical features that were previously not known, specially its defensive tail club.

A third species, M.sinocanadorum, was discovered in 1993 by Dale Russell, a Canadian paleontologist, and his Chinese colleague, Prof. Zheng, in the Junggar Basin, Xinjiang, China. This particular species is renowned for possessing the largest rib bones of any sauropod species ever discovered. Since this discovery, a further three species have been found and described in Sichuan China: these include M. youngi in 1996, from the Upper Shaximiao Formation; M. anyuensis in 1996, from the Penglaizhen Formation and the Suining Formation; and M. jingyanensis in 1998, from the Upper Shaximiao Formation.

Over the years, another question that scientists sought to resolve regarded how sauropods were able to get enough blood to their brains up their especially long necks. In 2015, paleontologist Michael Habib reported that an investigation that he had conducted into the matter indicated that the necks of the sauropods included springy cervical ribs that, when flexed, likely impacted nearby air sacs in a way that would have aided the heart in pumping blood. He further hypothesized that as Mamenchisaurus had a particularly long neck, it would have been equipped with even more skeletal muscle contributing to the blood-pumping process. Continued interest in such large dinosaurs was illustrated when the natural history museum at Nottingham's Wollaton Hall began displaying exhibits that included a skeleton of Mamenchisaurus in 2017. These exhibits, which were composed of fossils and specimens shipped from China, were being shown outside of China for the first time.

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