Eomaia

Eomaia is a genus of extinct fossil mammals. Its name means "dawn mother."

This tiny creature lived during the Barremian Age of the Lower Cretaceous. It lived approximately 125 million years ago. Scientists discovered its remains in the Yixian Formation in China's Liaoning Province. The fossil specimen is nearly complete and is very well preserved. Even though the skull is squashed flat, researchers can see the teeth. They can also see tiny foot bones, cartilages, and even the animal's fur.

Scientists use specific anatomical features to study how Eomaia relates to other animals. Eomaia scansoria possessed several traits found in placental mammals. Placental mammals are a group that includes animals that give birth to more developed young. One such trait is an enlarged malleolus at the bottom of the tibia. The tibia is the larger of the two shin bones. A malleolus is a small, hammer-shaped part. Eomaia also had a specific joint in its foot. In this joint, the first metatarsal bone is offset from the medial cuneiform bone. In metatherians, which include modern marsupials, these joints are level with each other.

Despite these similarities, Eomaia was not a true placental mammal. It lacked several specific features that define the placental group. For example, it did not have a malleolus at the bottom of the fibula. The fibula is the smaller of the two shin bones. It also lacked a complete mortise and tenon upper ankle joint. This is a type of joint where the foot bones fit into a socket. Additionally, Eomaia had an atypical dental formula. It had five upper and four lower incisors. This tooth pattern is more typical for metatherians. Placental mammals usually have up to three incisors on each side. Eomaia had five premolars and three molars. While the tooth proportions were similar to placentals, the specific count was different.

Physical structures in the pelvis also tell us about how Eomaia lived. Eomaia had epipubic bones that extend forward from the pelvis. These bones help to stiffen the body during locomotion, or movement. While these bones are found in many early mammals, they are not found in placentals. This stiffness would be difficult for a pregnant placental mammal. This is because a placental mammal's abdomen must expand as the fetus grows. Eomaia also had a narrow pelvic outlet. This suggests that it gave birth to small, undeveloped neonates. These young animals would have required extensive nurturing, much like modern marsupials.

The history of how we classify Eomaia involves significant scientific debate. The original discoverers studied 268 different characters to place the animal on the family tree. They claimed Eomaia was at the root of the eutherian family tree. Eutherians are the group that includes placental mammals. They suggested it was related to other fossils like Murtoilestes and Prokennalestes. However, a much larger study was published in 2013 by O'Leary and colleagues. This study was much more detailed than previous research.

The 2013 study by O'Leary et al. changed our understanding of this species. The researchers examined 4,541 anatomical characters across 86 different mammal species. They found 100% jackknife support that Eomaia falls outside of Eutheria. This means Eomaia is a stem taxon to Theria. A stem taxon is an early relative that sits outside a specific group. Because of this, Eomaia is not considered a true placental or eutherian. This finding is part of a larger debate regarding the age of origin for placental mammals.

Since that major study, several scientists have offered different views. In 2014, Meng referred to Eomaia as a eutherian. However, Meng did not provide any analytical evidence to support this claim. Other scientists, including Gheerbrant and Sole, also mentioned Eomaia in 2014. They suggested it represented a primitive eutherian condition. Like Meng, they provided no analytical evidence for these statements. More recently, a 2023 cladistical study recovered Eomaia as a basal eutherian. This shows that the exact position of Eomaia on the mammalian tree remains a topic of study.