How do we know that? What a small Cretaceous mammal can teach us about preservation and evolution.

Whenever I teach paleontology, or whenever I interact with members of the general public, one of the burning questions out there is how do we know what extinct animals looked like? Related to this question is often a large dose of skepticism – after all, isn’t most of what paleontologists do conjecture? No one has a time machine, and we often infer what should be there in terms of skin, hair, or feathers based on skeletal remains. The inferences are often robust even if there is no direct evidence.

For example, all amniotes (tetrapods like reptiles, birds, and mammals) have land-viable eggs and embryos that produce four life-supporting membranes. (If you’re wondering, the embryonic membranes allow reptiles and birds to breathe through their egg shells and retrieve yolk, whereas in most mammals the membranes have been adapted to implant the embryo into mother’s uterine wall and form the umbilical cord.) As it turns out, all amniotes have key skeletal features that no other tetrapods (such as amphibians) have. As one example, amniotes all have a distinct atlas and axis neck vertebra, whereas frogs and other amphibians do not. So, if you find a fossil with a distinct atlas and axis, you not only know you have an amniote on your hands, but you can infer that it produced amniotic eggs.
Sadly, a persistent myth lingers among those who are not well-acquainted with the science of paleontology that whereas we may make inferences, there is no way to adequately test them. A case in point has to do with predicting the presence of hair and external ears (pinnae) in early mammals. You know when you have a mammal skeleton for several reasons, chief among them being the presence of a single lower jaw bone (the dentary) rather than many (as in reptiles and birds) and a distinct section of lumbar (rib-less) vertebrae between the thoracic (rib-bearing) vertebrae and the sacrum (bone which connects the pelvis to the vertebral column). But often, such fossils do not come sporting hair, pinnae, or any number of traits the public readily identifies as mammalian. Yet, a paleontologist seeing such a fossil can already envisage the furry coat and pricked ears that must have been there when the mammal was alive.
What many non-paleontologists are unaware of is that there are cases (rare but increasingly more common) where fossils are preserved under such environmental conditions that traces of soft tissues are present. Often, these specimens are part of a fossiliferous rock unit called a Lagerstattë, where a series of fortunate coincidences (fine mud, rapid burial, oxygen-depleted environment) preserve more than just the bones. A recent small but spectacular find from a Spanish locale, the La Hoyas Conservat-Lagerstattë, shows that our inferences about animal appearance in paleontology can be tested against nature’s record.
An Early Cretaceous (~125-120 Ma) theriimorph mammal named Spinolestes xenarthrosus was described by Martin et al. (2015) earlier this year that preserved hair and other soft tissues in exquisite detail, allowing us to test how accurate predictions based on skeletal remains are. Theriimorph mammals are more derived than monotremes (such as the duck-billed platypus), and were a diverse group of fur-bearers in the Mesozoic Era. It is from the theriimorphs that therian mammals (the so-called marsupials and placentals) are descended. Spinolestes is not on the direct line to therian mammals, but is part of an outgroup called the eutricondonts.
What does Spinolestes show us about the inference of fur and pinnae with mammalian skeletal hallmarks? Quite a lot. First, the hair preservation of Spinolestes is so exquisite that not only can we see the furry coat of this little animal (estimated mass of 52-72 g, in the range of small opossums or mice), but the structure of its follicles are preserved (Martin et al., 2015)! Your hairs grow out of specialized skin structures called follicles that generate the hair itself as well as connect the base of the hair to smooth muscles and a nerve supply. In many living mammals, the hairs are arranged in compound follicles, and most mammals have a coarser coating of guard hairs underlain by softer secondary hairs. That is precisely what was observed in the preserved skin patches of Spinolestes (Martin et al., 2015). Moreover, second, we know that Spinolestes was not just furry but spiny: like hedgehogs, the skin preserves evidence of these spines in the form of multiple, merged follicles (where spines arise from in hedgehogs and other spiny mammals) (Martin et al., 2015).
But, third, perhaps most fascinating of all is that we have in Spinolestes a perfectly preserved and detailed tracing of the outer left ear or pinna (Martin et al., 2015). Intriguingly, in this specimen, much of the scalp and the left pinna are separated from the skull in such a way that the head skeleton is opposite that of its soft tissues. How could this be? Ingeniously, Martin and colleagues placed a dead rat in water and documented how the skeleton fell apart from the soft tissues as it decayed. Remarkably, the skull of the deceased rat detached and sank in the water while the lighter scalp and pinnae remained floating (Martin et al., 2015), which is precisely what seems to be preserved in the fossil!
Finally, fourth, branching, tube-like structures were found within the ribcage, whereas a reddish-brown stain was preserved posterior to these tubes. Both of these soft tissues are distinctly separated from one another. Martin et al. (2015) have interpreted these as the decayed remains of the lungs and liver, respectively. And their orientation within the body cavity of Spinolestes is what would be predicted for mammals generally where the lungs lie anterior to the liver and are separated by the diaphragm muscle. In mammals, the diaphragm is main driver of respiration (Perry et al., 2010). The presence of lumbar vertebrae as a key mammal trait has often been inferred to be associated with the presence of a diaphragm. Here, in Spinolestes, we seem to have strong evidence to support this hypothesis.
This little Mesozoic mammal, preserved in the Early Cretaceous vegetated wetlands of Spain, is but one example of how our understanding of what extinct animals looked like is not only rigorously testable but far from being simply guesswork. Whereas the fossil record is incomplete and imperfect, so is a crime scene. Just as the forensic anthropologist’s inferences of a past event are based on a rigorous and thorough exploration of the data, so, too, is the paleontologist’s reconstruction of life that once was.
Submitted by Matthew F. Bonnan, Stockton University
Martin, T., J. Marugán-Lobón, R. Vullo, H. Martín-Abad, Z.-X. Luo, and A. D. Buscalioni. 2015. A Cretaceous eutriconodont and integument evolution in early mammals. Nature 526:380–384.
Perry, S. F., T. Similowski, W. Klein, and J. R. Codd. 2010. The evolutionary origin of the mammalian diaphragm. Respiratory Physiology & Neurobiology 171:1–16.
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