A kangaroo joey continues to use its mother's pouch to feed after it is able to walk
meat eating dinosaurs such as allosaurs dominated the landscape Archaeopteryx a bird that had teeth and a long tail flew through the air At night in the cover of the trees the tiny J. sinensis could hide from predators and seek its own prey Juramaia sinensis which means Jurassic mother from China is the name scientists gave this animal It weighed less than 0.5 ounces 15 g Its skull was 0.87 inches 22 mm long It looked like a shrew and could climb into the trees Juramaia sinensis was also a placental mammal Scientists know that placental mammals often called eutherian mammals and today's marsupials evolved from a common ancestor Marsupials are mammals that give birth to immature young that often shelter and feed in the mother's pouch Kangaroos and opossums are marsupials Placental mammals have a longer period of growth in the womb so they give birth to more mature young These young receive nutrients in the womb through a complex organ called the placenta Dogs horses and human beings are placental mammals The J. sinensis specimen is the oldest known fossil of a placental mammal It was discovered in China's Liaoning province in 2011. The fossil consists of only a partial skull and skeleton in addition to impressions of soft tissue including hair in the surrounding rock This fossil
is evidence that placental mammals and marsupials split apart in their evolution more than 160 MYA Before discovering 1. sinensis scientists had estimated when this split occurred They did this by comparing marsupial DNA and placental mammal DNA based on the molecular clock hypothesis The molecular clock hypothesis says that the order of proteins and the DNA sequences that they make up evolve at a constant rate that is unchanged over time and between organisms Scientists compare DNA samples from two or more organisms Based on the number of differences they can estimate how long ago the organisms shared a common ancestor The comparison between marsupial and placental mammal DNA suggested that the split had occurred approximately 160 MYA but there were no fossils old enough to support this date The previous oldest known placental mammal fossil was from 125 MYA The discovery of J. sinensis helped confirm when these two mammalian branches diverged or split into two distinct groups Understanding how this took place requires a basic understanding of evolution EVOLUTION When scientists speak of biological evolution they are talking about the physical changes that take place in animal populations and species over time Evolution doesn't mean a group of
animals is getting better but it doesn't mean the group is getting worse either It simply means a plant or animal species is changing often in response to its environment One of the first people to try to explain how species change over time was French naturalist Jean Baptiste Lamarck In 1809 he theorized that animals could gain a trait over a lifetime and pass it down to their offspring He used giraffes as an example explaining that giraffes reach up to feed on tree leaves stretching their necks and front legs Their necks and front legs got longer and these traits were passed down to their offspring It didn't take long for his fellow FOSSILIZATION Fossilization is a process through which a plant or animal or an animal's tracks is preserved after it dies Certain conditions must be met for fossilization to take place although it can occur in several different ways Sometimes water buries a plant or animal in sediment such as sand silt or gravel That is when the most common form of fossilization called petrification can occur Buried soft tissues decay but the bone is left behind Water carrying minerals seeps into the bones and deposits the minerals which crystallize Then the bones harden along with the sedimentary rock In another process called an external mold when an organism completely dissolves it can leave a space shaped just like itself in the sediment When this shape fills with another mineral a cast is formed A similar process makes what is known as an internal mold This is created when minerals fill the inside of a skull or shell and the remains then dissolve
