Most people would class the Cenozoic (the period of time spanning from 66 Ma to the present) as the Age of Mammals. Certainly the diversity of mammals exploded and the majority of modern groups evolved after the demise of the non-avian dinosaurs at the end of the Cretaceous. However, what a lot of people don’t realise when they think about mammals is that they have been around for a lot longer than 66 Ma. The oldest known definite mammals date to around 165 Ma but the actual origins of the group would have been some time previous to that but remains uncertain, primarily due to the fact that most early mammals are known only from isolated teeth.
Two remarkable new finds, both from the Middle – Late Jurassic Tiaojishan Formation in the Hebei Province of China, have provided new food for thought in this debate, whilst not necessarily providing any definitive answers. The two new species, described in separate papers in last week’s issue of Nature preserve not only teeth, but skull material and post-cranial elements such as vertebrae, limb bones and even fur. Both species belong to an extinct group of mammals known as the haramiyids.
The first new species, Arboroharamiya jenkinsi, described by Zheng et al., was an omnivore or herbivore that had several adaptations for living in trees, such as elongated digits. The morphology of its caudal (tail) vertebrae also hints at it possessing a prehensile tail. It has been dated to 160 Ma.
The second new species, Megaconus mammmaliaformis, described by Zhou et al., was herbivore that lived on the ground, with its morphology indicating it had an ambulatory (walking) gait similar to that of a modern day armadillo. Megaconus was more primitive than Arboroharamiya and is also slightly older, dating to around 164 – 165 Ma.
Now, so far so good. But where these two studies provide conflicting opinions about the early mammal evolution is in their phylogenetic analyses. The Zheng et al. paper groups Arboroharamiya and all other haramiyids as the sister group to the multituberculates within Mammalia. This puts the origin of mammals at around 215 Ma, in the Late Triassic, much older than most palaeontologists would estimate, but in agreement with molecular estimates. The Zhou et al. paper on the other hand, placed Megaconus and all other haramiyids outside of Mammalia, meaning they are not closely related to multituberculates and also estimates the origin of mammals at around 180 Ma, a figure more in line with palaeontologists expectations given what fossils are currently known.
So, which tree is the correct one? Well, neither probably. There are several factors why this will most likely turn out to be the case. One is that these two phylogenies don’t contain the other new species, a potential next move for the authors of these two papers is to combine their data a produce a phylogeny with both new taxa to see where the haramiyids place. Another is that although these fossils are relatively well preserved, there is still a lot of anatomical and morphological data missing from them, with Arboroharamiya possessing less than a quarter of the 436 characters used in the Zheng et al. study and Megaconus possessing less than half of the 475 characters used in the Zhou et al. study. A third factor is that whilst these two new taxa might be relatively well preserved, the majority of other early mammal taxa are poorly preserved or are only known from teeth. More fossils of the quality of these two new specimens would help resolve the origin of the mammals.
Finally, there is also a little lesson to be learnt here about cladistics, the method by which phylogenies are now generated. Whilst this method is undoubtedly the best and most powerful tool we possess for distinguishing relationships between species at present, there are many different cladistic techniques that scientists can employ, and it will often depend which technique is used as to which phylogeny they end up obtaining. So don’t always accept the phylogenetic position of taxa just because there’s a phylogeny showing it that way, try to look at what methods they’ve used to obtain their results. Remember, good scientists will question everything!
Cifelli, R. L. & Davis, B. M. 2013. Jurassic fossils and mammalian antiquity. Nature 500, 160–161.
Zheng, X., Bi, S., Wang, X. & Meng, J. 2013. A new arboreal haramiyid shows the diversity of crown mammals in the Jurassic period. Nature 500, 199–202.
Zhou, C.-F., Wu, S., Martin, T. & Luo, Z.-X. 2013. A Jurassic mammaliaform and the earliest mammalian evolutionary adaptations. Nature 500, 163–167.
If you were to look at the diversity of life on Earth today, you could be forgiven for thinking that animals have always been around and have dominated the planet since time memorial. However, you would in fact be completely wrong! Animals have only been around for roughly 600 million years whilst life first evolved over 3.5 billion years ago and remained in single-celled form for the majority of the Earth’s history.
The period when animals rapidly diversified into the majority of extant phyla is known as the ‘Cambrian explosion’, which began approximately 545 million years ago during the Cambrian period. One particularly enigmatic example of this is the Burgess Shale, where beautifully preserved animals, some of which are unlike anything alive today, have been found.
Until the past few decades, the Burgess Shale has stood out as our best glimpse into this stage of the evolution of life on Earth. However in China, several localities (e.g. Chengjiang) have been found, producing fossils of equally exquisite detail which scientists have been excitedly studying. The advantages of localities like these is that we can decipher how living groups first evolved and what would most likely have been the ancestral state for our modern animal groups.
Two new fossil species, described this past week in the journal Nature give us just such an insight for arthropods, the group containing animals such as insects, crustaceans, centipedes, spiders and the extinct trilobites. The fossils, named as Chengjiangocaris kunmingensis and Fuxianhuia xiaoshibaensis are from a group known as the fuxianhuiids, which are regarded as representatives of early arthropods.
The fossils, which were found in a Lagerstätte (a locality of exceptional preservation) near the city of Kunming in the Yunnan province of China, have been dated to approximately 520 million years old, meaning they are from a relatively early stage of the ‘Cambrian explosion’. Previous specimens of fuxianhuiids have had their heads covered by their head shield, part of the tough exoskeleton that is synonymous with arthropods. This has meant that debate over what exactly the paired post-antennal structures in other fuxianhuiids actually represented has never had a clear resolution. Until now that it is. In a stroke of geological good fortune, numerous specimens of the two new fuxianhuiid species have experienced ‘taphonomic dissections’, where the conncective tissues of the head shield have softened before final burial allowing the head shields to rotate forwards, exposing the structures underneath and making them visible to scientists for the first time.
The fossils are so well-preserved that the functional articulation of these post-antennal structures can be explained. The limited range of movement in the limbs means that they would most likely have been used to sweep detritus into the mouth, where the food particles would then have been filtered out of it. The nerve cord is also the first documented
case of a preserved post-cephalic central nervous system in a stem group arthropod. It is simple in structure, especially compared to animals alive today (perhaps as expected).
The locality these fossils were found has just begun to be explored. With the potential for more insights into this pivotal period in the evolution of life and finds with this quality of preservation, I could very well be writing more articles on invertebrates sooner than I think!
Jie Yang, Javier Ortega-Hernández, Nicholas J. Butterfield, Xi-guang Zhang. Specialized appendages in fuxianhuiids and the head organization of early euarthropods. Nature, 2013; 494 (7438).
Oviraptorids are not your usual type of theropod dinosaur. Most people assume that all theropods had jaws bristling with teeth like steak knives ready to eviscerate the next hapless victim they came across, but Oviraptorids actually possessed short deep skulls and toothless jaws. These beaks were initially thought to have been used to crush eggs (the name Oviraptor literally means ‘egg-thief’). When the first Oviraptorid fossil was found in 1924 (Oviraptor philoceratops, Osborn, 1924) it was alongside a nest of what were thought to be Protoceratops eggs, with the Oviraptorid being assumed to be trying to grab a meal. It wasn’t until 1999 (Clarke et al. 1999) and the discovery of a closely related species (Citipati osmolskae) brooding its eggs that it was realised that Oviraptor was more likely caring for its unborn offspring rather than securing its dinner.
So what exactly did Oviraptorids eat? Were they herbivores, carnivores or omnivores? These questions are still the subject of some debate by palaeontologists. A new paper, published in the German journal Naturwissenschaften may have discovered new evidence to help sway the argument one way or another. The paper, by Lu et al., describes a new species of Oviraptorid, Yulong mini and compares it to all known Oviraptorids and other theropods.
When the authors analysed the results of their comparisons, they noticed that the hind limb proportions of Oviraptorids remain constant throughout their whole lives. This is different to most theropods, whose hind limb proportions tend to change as they mature (allometric growth). The interesting thing is that this type of growth (known as isometric) tends to be found in is herbivores. Furthermore, although the individuals found were less than a year old, their limb bones were quite well developed, suggesting they were already able to move around easily. If you couple this with the lack of adult skeletons found at the localites Yulong was discovered, there may be a case for Yunlong possessing precocial development (i.e little/no parental care, effectively born like a mini adult), something again associated with herbivores. Carnivores tend to be altricial, where they are born relatively helpless and require more care take longer to develop.
So was Yunlong and other Oviraptorids herbivorous? We still don’t know for sure. But this new evidence suggests it remains a distinct possibility.
Clark, J.M., Norell, M.A., & Chiappe, L.M. (1999). “An oviraptorid skeleton from the Late Cretaceous of Ukhaa Tolgod, Mongolia, preserved in an avianlike brooding position over an oviraptorid nest.” American Museum Novitates, 3265: 36 pp., 15 figs.; (American Museum of Natural History) New York.
Lu, J, Currie, PJ, Xu, L, Zhang, X, Pu, H & Jia, S (2013) Chicken-sized oviraptorid dinosaurs from central China and their ontogentic implications. Naturwissenschaften 100: 165-175.
Osborn, H.F. (1924). “Three new Theropoda, Protoceratops zone, central Mongolia.” American Museum Novitates, 144: 12 pp., 8 figs.; (American Museum of Natural History) New York.