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<\/div>Krusatodon kirtlingtonensis<\/em>, a small mammal that lived during the Jurassic Period<\/p>\n Maija Karala<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n In the middle of the Jurassic Period, small mammals had much longer lives than their modern counterparts, and spent years instead of weeks being cared for by their parents. This suggests that at some point, small mammals underwent a significant shift in how quickly they grow \u2013 but it is unclear precisely what caused it.<\/p>\n The finding is based on two fossilised skeletons of an extinct mouse-sized creature called Krusatodon kirtlingtonensis,<\/em> which lived on the Isle of Skye in Scotland around 166 million years ago. The fossils were unearthed decades apart: the first in the 1970s, and the second in 2016.<\/p>\n Together, they are a rare find \u2013 two fossils from the same species, one an adult, one a juvenile. The research team could compare the specimens to see how the animals grew and developed. \u201cIt meant that we could start asking questions we couldn\u2019t have dreamt of with just one specimen,\u201d says Elsa Panciroli<\/a> at the National Museum of Scotland.<\/p>\n First, the scientists used X-ray imaging to count the annual growth rings in the specimens\u2019 teeth, which, similar to rings in a tree trunk, can be used to estimate age. They found that the adult specimen was around 7 years old and the juvenile was between 7 months and 2 years old.<\/p>\n <\/p>\n Because the youngster still had its baby teeth, Panciroli says she expected the juvenile fossil to be much younger. \u201cThat was quite surprising because this animal is about the size of a squirrel or shrew,\u201d says Panciroli. \u201cWe would expect it to be replacing its teeth much earlier \u2013 within weeks or months. So we knew immediately that it must have grown very differently [than modern species].\u201d<\/p>\n The finding suggests K. kirtlingtonensis <\/em>took up to two years to wean from its mother, a notable leap from the handful of weeks it takes most modern small mammals. Analysis of the fossils\u2019 bone lengths and size revealed that the animals \u201cwere also growing throughout their whole lives\u201d, says Panciroli. Today, small mammals like mice grow rapidly when young, but stall once they reach adulthood.<\/p>\n \n