Jurassic and Cretaceous

The class Mammalia evolved in the Early Jurassic and diverged into several subclasses during the Jurassic and Cretaceous, which included the ancestors of monotreme, marsupial, and placental mammals (Figure 11-17). Although controversial, the fossil record indicates that three nonplacental ancestors of modern placental mammals survived the End Cretaceous extinction and became the placental ancestors of the three major groups of placental mammals on northern continents: (1) carnivores (Ferae), (2) horses and cows (Euungulata), and (3) rodents and primates (Euarchontaglires).

The position of Placentalia in the Mammalia cladogram (Figure 11-17) is misleading since there were no placental mammals in the fossil record until after almost all of the divergences in Figure 11-17. Minimum divergence times are established by the timing of the drifting apart of continents in the Cretaceous Period. The timing of the origin of the placental condition is one of the major controversies in paleontology.

Several orders evolved within the Mammalieformes in the Late Triassic and Early Jurassic, including the Morganucodontidae, Docodonta and Trithelodonts (Figure 11‑14). The Docodonta looked like beavers and had a semi-aquatic lifestyle. In 2001, scientists discovered the trithelodont Hadrocodium, “large head,” in an early (195 Ma) Jurassic formation in China. Its entire body was the length of a paper clip (3 cm long) and it weighted 2 gm, one of the smallest mammals ever. It had large eyes, indicating a nocturnal lifestyle. A sense of hearing that detected higher frequencies must have been important for hunting insects. In Hadrocodium, the remaining bones at the back of the jaw in Morganucodon had evolved to form the finely tuned mammalian inner ear. Hadrocodium developed a large brain in order to rapidly process complex signals from whiskers, eyes, and ears as it hunted at night. Hadrocodium is now considered the first of the crown mammals in the fossil record. Several mammal subclasses evolved in the Early and Mid Jurassic (Figure 11-18).

One of the early subclasses of mammals, Australosphenida, survived to the present. They include the monotremes (Figure 11‑19): the modern platypus and four species of echidna (Figure 11‑19) in Australia. The Australosphenida have not changed as much as other modern mammal subclasses. For example, rather than give birth to young, the monotremes lay eggs. The transition to the formation of the bones in the jaw for use in the ear is seen in the fetal development of the modern monotreme ear.

Eutriconodonts were an early mammal subclass. They were first thought to be exclusively insectivores, but there were many eutriconodont body plans and lifestyles in the Cretaceous. They are primarily classified as eutriconodonts by a pattern of three main cusps on the crown of their molar teeth. They had long canines, which indicates that they were carnivores. They had epipubic bones, which indicates that they gave birth to very small and undeveloped young or laid eggs.

Another early mammal subclass, the multituberculates, were named after multiple nodules on teeth. Scientists recently discovered a 120 Ma multituberculate with the complex bone structure of the mammalian ear. The multituberculata were the most successful mammals in that they survived for 166 million years until the Late Eocene (40 Ma).

Marsupial mammals (Metatherians) include animals such as kangaroos and Koala bears. They give birth to extremely immature young. The first metatherian in the fossil record is 110 Ma.[2] Although marsupial mammals look like placental mammals, they are inferior in many ways; for example, they bear extremely immature young after several weeks, which then attach to the mother or live in a sack and nurse through the skin of the mother for several months to a year. Placental mammals (eutherians) grow a placenta that enables them to nourish the young in the womb and give birth to relatively large and mature young.

Eomaia (130 Ma) was a eutherian (ancestor of placentals) insectivore (Figure 11‑23). As with modern marsupials, it had an epipubic bone, which means it laid eggs or gave birth to extremely immature young; however, it had several eutherian characteristics, particularly, 3 molars on the upper part and lower part of the jaw. It had 5 premolars on the upper and lower parts, which is one more than the eutherian mammals; however, in mammal evolution, teeth are always lost over time so this does not disqualify it as a eutherian. Wrist and ankle bones also had eutherian characteristics.

The Boreoeutherians inhabited the northern continents after the End Cretaceous extinction, the Xenarthra inhabited South America, and the Afrotherians inhabited Africa. The Boreoeutherians became dominant in Africa and South America after the continents joined back together with northern continents. Many paleontologists think that the Boreoeutherians (animals on northern continents) descended from three eutherian clades (Zhelestidae, Zhalamdastestilidae, and Cimolestes) in the Late Cretaceous. They had characteristics that link them with orders of placentals mammals in the next era of Earth’s history, the Cenozoic, the Age of Mammals.

The zhelestids were successful eutherian (nonplacental) mammals distributed across Europe, Asia, Africa, and North America in the Late Cretaceous. Their earliest remains are in the Cenomian (100 Ma to 93 Ma) in Central Asia. They were the most common Late Cretaceous mammal fossils. Zhelestids are viewed as stem ungulates (horses, cows). Rose linked them with ungulates because of the shapes of their molars. [3] Halliday conducted an extensive morphological (skeleton and teeth) and molecular (DNA) study of Late Cretaceous and Early Cenozoic mammals and grouped the Cretaceous zhelestids with the Cenozoic ungulates. [4]

Rose, Halliday, and others link the Cretaceous zalambdalestids (Figure 11-24) with the placental Glires mammals (include rodents and rabbits) of the Cenozoic Era; [5] [6] Although they were eutherians, they had epipubic bones and were not placental mammals.[7] They were hopping and running animals that probably hunted insects in the forest underground. Zalambdalestid limbs were long with fused fibula and tibia. Rabbits have similar behavior and morphology. [8] The most primitive zalambdalestid was Kulbeckia, from the Turonian (93.9 to 89.8 Ma) of Uzbekistan. Zalambdalestes (Figure 11-24), Ukhaatherium (84-72 Ma), and Maelestes gobiensis (71 to 75 Ma) were Late Cretaceous zalambdalestids of the Gobi Desert in Mongolia. There are many dental and skeletal links between rodents and zalambdalestids.

Prior to the discovery of epipubic bones after the Cenozoic, Cimolestes had been assumed to be ancestral to Cenozoic creodonts and carnivorans because of its carnassial teeth.[9] Halliday and others still conclude that the ancestor or close sister group to carnivores was Cimolestes. Carnivores with placental characteristic appeared almost immediately after the End Cretaceous extinction. The creodonts and carnivorans were not in Africa or South America in the early Cenozoic, which makes sense since Cimolestes appeared in the Late Cretaceous fossil record. However, they later migrated to Africa and South America when Africa and South America reattached to the northern continents. Cimolestes (68 – 56 Ma) crossed the Cretaceous/Cenozoic boundary and still was not a placental mammal on the Cenozoic side of the boundary.

Other than the origin of the groups of Boreoutherians, there are other cretaceous mammals that might be at the base of Afrotheria and Xenarthra groups. For example, Paranyctoides lived 70.6 to 66 Ma, and it may have been ancestral to the Afrosoricida although some disagree.[10]

Positions of the continents in the Jurassic and Cretaceous played a major role in eutherian evolution and diversification. All of the modern continents originated from one continent called Pangaea (Figure 11-26) in the Early Jurassic (200 Ma).

Pangaea (Figure 11‑25) split apart in 175 Ma into northern (Laurasia) and southern (Gondwana) halves (Figure 11‑26). Although controversial, modern simulations based on plate tectonics show a seaway between Africa and Europe by 150 Ma (Figure 11‑26). Gondwana split apart in the early Cretaceous (150-140 Ma) into Africa and South America. As with the plate tectonics simlations, molecular clock studies indicate that Afrotheria (elephants, aardvarks, manatees, hyraxes) and Xenarthra (armadillos, sloths, and anteaters) diverged from other eutherian clades and from each other over 100 million years ago. The clades, the Laurasiatheria (includes ungulates, carnivores, and others) and Euarchontaglires (primates and rodents) formed within the Boreoeutheria on the northern continent of Laurasia.

Laurasia divided into Eurasia and North America approximately 100 Ma (Figure 11‑27). Africa and South America were completely separated from each other and from the northern continents at the time of the End Cretaceous extinction (Figure 11‑28); however, there were periodic land bridge connections in the Bering Sea between Asia and North America.

References

[1] Bi, Shundong, Xiaoting Zheng, Xiaoli Wang, Natalie E. Cignetti, Shiling Yang, and John R. Wible. "An Early Cretaceous eutherian and the placental–marsupial dichotomy." Nature (2018): 1.

[2] Bi, Early Cretaceous.

[3] Rose, Beginning, p. 92

[4] Halliday, Thomas, Paul Upchurch, and Anjali Goswami. Resolving the relationships of Paleocene placental mammals. Biol. Rev. (2015), 92, pp. 521–550. 521 doi: 10.1111/brv.12242

[5] Rose, Kenneth D. The beginning of the age of mammals. JHU Press, 2006, p. 91

[6] Halliday, Resolving.

[7] Novacek, Michael J., Guillermo W. Rougier, John R. Wible, Malcolm C. McKenna, Demberelyin Dashzeveg, and Inés Horovitz. "Epipubic bones in eutherian mammals from the Late Cretaceous of Mongolia." Nature 389, no. 6650 (1997): 483.

[8] Rose, Beginning, p. 92

[9] Halliday, Resolving

[10] Rose, Beginning, p. 93

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Zalambdalestes (Mid to Late Cretaceous). Credit: Nobu Tomura. Used here per CC BY-SA 4.0.