Microscopic structure of bones provide clues about how ancient mammals grew

By: Luke Weaver (5–10 minute read)

Ever wondered why it takes so much longer for a human to grow up and grow old compared to a dog? Or why elephants are pregnant for 22 months but a mouse is only pregnant for 22 days? Questions like these are a major focus for biologists who study animal life history, the changes an organism goes through from the time it is a developing embryo to the time it dies. Biologists can follow an organism around throughout the course of their life and carefully document all of the changes that occur. But for me, as someone who studies our earliest mammal ancestors that lived alongside the dinosaurs, I want to know when different mammalian life histories evolved and in which ancient mammal groups… and to do that, we need to chop tiny mammal bones to pieces and look at them under a microscope! *insert evil Tim Curry laugh here*

Life history strategies are different among the three major groups of mammals. Monotremes lay eggs and the babies that hatch from them are tiny and poorly developed, growing for a long time outside of the egg. Marsupials give live birth, but their babies are extremely tiny—the young crawl into their mother’s pouch where they suckle and develop for a long time. Placentals also give live birth, but their babies are more well developed compared to monotremes or marsupials. Placental babies spend less time suckling and become independent from their mother more quickly.

Monotremes (like the platypus), marsupials (like kangaroos, koalas), and placentals (most mammals you’d think of, humans, dogs, mice, deer) all have very different life history strategies—they are born and grow up differently. There are ongoing debates about when these different life history strategies evolved, so we need fossils to help answer these questions.

Although these life history differences among the three major groups of modern mammals (Monotremes, Marsupials, and Placentals) are well established, determining when these different strategies evolved and whether one strategy is more “primitive” or “advanced” has remained a topic of major debate in mammal biology research. In very general terms, there are two competing camps: (1) Placentals are advanced, marsupials retain more primitive life history traits vs. (2) Marsupials are advanced, placentals retain more primitive life history traits. (Most everyone agrees that monotremes probably retain the most primitive life history strategy because they still lay eggs). My research is focused on using the fossil record to fill in the gaps between modern marsupials and modern placentals using 66 million year old mammal fossils.

OK, what does this have to do with chopping up bones?

Life history traits—such as how fast an animal grows, it’s reproductive strategy, or how old it was when it died—may not be preserved very well on the outsides of bones, but they actually leave a pretty recognizable mark on the inside. Paleontologists use bone histology, or the study of bone tissues, to infer life history traits of extinct animals. They do this by cutting slices of fossil bones, grinding them down really thin, and looking at the microscopic patterns under a microscope. The microscopic structure of bones is governed (in large part) by how fast an animal grows, and those growth rates change throughout an animal’s life. Bone growth can also pause during times of stress and leave distinct lines, like tree rings, that can give clues about how old the animal was, when it was born, or whether it lived in a harsh environment. When you hear people talking about how old T. rex was, this is how they know!

Sketch of a mammal femur (A) showing where a cut would be made along the shaft of the bone. Once a chunk is cut out, it is ground down until light can pass through (B). We can then look at the microscopic structure of the bone tissue (C), which preserves clues about the animal’s life history.

OK, what does this have to do with chopping up tiny mammal bones?

Most ancient mammals were really small (< 1 kg) and could probably sit in the palm of your hand. Because I am interested in understanding the evolution of mammal life history strategies, and since bone histology is the best way to figure out the life history strategies of extinct mammals, I need to cut up ancient mammal bones, which are really tiny. First I need to understand the “mark” that different life history strategies leave in the bone microstructure of living mammals (in which we can observe and measure their life histories) so that I can interpret the microscopic patterns we see in the bones of extinct small mammals.  Since 2016 I have been cutting up the bones of modern and extinct small mammals and creating bone histology slides. Not all by myself, thank goodness—shout out to DIG Instructors Henry Fulghum and Dr. Megan Whitney! I’ve been working to take detailed measurements from all of our modern mammal slides and compare the patterns I see to the life history variables we already know about the different species.

My desk setup for working from home and doing bone histology research. One perk: I could watch football at the same time!

Much to my delight, we are finding that the life history strategies of modern mammals leave a pretty distinct signature in their bone tissues! Plus, these signatures are different than what we expected with what people have seen in larger animals (such as dinosaurs). We are cautiously optimistic that these differences are meaningful and we think they will shed new light on whether marsupial or placental life history strategies are more advanced. Beyond the specifics of the results, we are now building a new framework for investigating mammal life history evolution. By ground-truthing the histology of modern mammals and integrating them with the fossil data, we are creating a foundation upon which future studies can build.

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