Digging into the Mesozoic fossil record, literally and figuratively.

Research in the Makovicky lab fits broadly into, and spans across, three main areas: 1) increasing our knowledge of past biodiversity 2) understanding how that biodiversity responded to changes in the geosphere and biosphere, and 3) investigating how the fossil record is formed. Our principal model system for  investigating these topics is the rich fossil record of dinosaurs, whose  235 million year reign spanned across geological events including continental rifting, extreme sea level changes, and two mass extinctions, thus providing rich data for investigating the links between the bio- and geospheres. 

Accurate recognition and description of fossil species (i.e. taxonomy) is the foundation for all paleontological research, and increasing our knowledge of past diversity by collecting more fossils remains the best way to test macroevolutionary and paleobiological hypotheses. In that spirit, the Makovicky lab is heavily focused on discovery-driven research and fieldwork around the globe (Figure 1) organized around two central themes.  The first theme addresses the unevenness of the global knowledge of Mesozoic diversity. While northern hemisphere continents are well sampled for Mesozoic fossils, the record of the southern hemisphere remains more poorly known. In response, Makovicky has built a longstanding collaborative fieldwork program in South America, a developing one in South Africa, and have also worked Antarctica prior to coming to the University of Minnesota in 2019. All of these projects have added new and important contributions to the fossil record of Gondwana. 

The second theme centers on the ‘mid’-Cretaceous of North America, a period of tremendous geologic change on our continent marked by the Sevier orogeny grading into the Laramide, emplacement of the Western Interior Seaway, and the formation of Beringia and a land bridge to Asia. Recent fieldwork by the lab in the Western Interior Basin was part of a larger NSF-funded collaboration to look at paleoenvironmental ‘transects’ from basins arrayed from Montana to Texas. Collaborating with a large team of paleontologists, geochemists, and stratigraphers, Makovicky and lab members contributed to collection of new fossils, as well as copious chronostratigraphic and chemostratigraphic data. Current research related to this project aims to revise absolute ages and depositional rates across the various formations and sub-basins and then using that as a framework to examine biotic turnover through time and space and its potential links to climate shifts. With the mid-Cretaceous representing a extreme climatic ‘hothouse’, the findings of this research may hold insights with regard to the effects of predicted climate change. This project also underscores the unevenness in our knowledge of Cretaceous ecosystems. While the  Western Interior  Basin is among the best studied foreland basin systems worldwide due to the availability of outcrop, the Cretaceous of eastern North America is far less known.  Fortunately, for almost a decade, our lab has been working a uniquely rich fossil site in the Ozark foothills that is producing some of the most complete dinosaurs (Fig. 2C) as well a rich fauna of aquatic vertebrates allowing us to plumb the evolutionary impact of the WIS dividing the continent and its biota for up to 30 million years.

Arguably, the most visible of these field programs is the twenty-plus year collaboration with Argentine paleontologists, excavating fossils from the Neuguén Basin of northern Patagonia, often with the participation of Makovicky lab students (fig. 3). This area is one of the richest Cretaceous foreland basins in South America in terms of its fossils, and joint Argentine-US teams have collected both very large as well as tiny dinosaurs there that not only add to our knowledge of dinosaur diversity, but also yield important new evolutionary insights. For example, the ~5 ton carcharodontosaurid Meraxes (fig. 2A) provided insights into how large carnivorous dinosaurs grew to their large sizes while shortening their forelimbs, while the diminutive 1.5 lb Alnashetri (fig. 2B) tells a complicated tale about the evolution of miniaturization and how the biogeography of the group it belongs to was shaped by plate tectonics. This study was recently published in the journal Nature.

Students in the Makovicky lab are actively involved in the process of uncovering and documenting past biodiversity, through participation in fieldwork and in their research (Fig. 3), which includes unique opportunities to work with rare Antarctic fossils. Lynnea Jackson (MS 2024) completed a Masters describing a new species of sauropodomorph dinosaur and is now readying it for publication, while completing her PhD at the University of Michigan.  Cameron Shepard (MS 2027) is studying the skull and brain anatomy of the peculiar crested theropod, Cryolophosaurus, which is among the earliest large dinosaurian carnivores.  

Collecting new and exciting fossils in only half of the story, however. Another critical requirement is being able to confidently distinguish valid species in the fossil record. Many species experience marked changes in their anatomy as they grew form from juveniles into adults. For fossil species, this can pose a challenge because two different looking fossils might represent growth stages of a single species, or two different species (or both), potentially confounding species counts and distributions. Minyoung Son (PhD 2027) is tackling this thorny question for his dissertation. He has developed an innovative graphic method for comparing when in growth different traits develop or change across a sample of related species, allowing him to tell apart members of different species throughout their life cycle, not just when they are fully grown. He is applying this method to small ceratopsians (=horned dinosaurs) known from very complete growth series. One of these, Psittacosaurus, is both the most speciose and most abundant non-avian dinosaur genus, with up to 10 named species, some of which were reported to be coexisting in the same environments (sympatric).  Minyoung has determined that the genus Psittacosaurus is currently oversplit with a number of named species inaccurately based on juvenile specimens of other species, and that there is no evidence for sympatry, thus altering our interpretation of both the geographic and stratigraphic distributions and inferred ecology of these dinosaurs. The applicability of this approach reaches beyond dinosaurs, and may prove useful to a wide range of species in the fossil record.

One of the ‘Big Questions’ in paleontology is determining to what degree the evolution of species and communities is driven by abiotic factors such as climate, geography, and sea-level changes, or by biotic interactions with other species such as predation, competition, and disease. As noted above, dinosaurs evolved on Pangea and subsequently began diversifying on continents that rifted apart over the course of the Mesozoic. Early dinosaur groups are therefore expected to be more widespread given the lack of oceanic barriers, whereas the distribution of later living groups is expected to be shaped by the pattern of rifting and increasing isolation (dubbed ‘vicariance’) between continents with a greater degree of endemism.  The unique marsupial fauna of Australia is a prime example of the latter phenomenon.  Research by Makovicky and collaborators suggests that dinosaurs largely follow these expectations: Early Jurassic dinosaurs like Cryolophosaurus and the new sauropdomorph from Antarctica are closely related to species from other continents because Pangea acted as a biogeographic ‘superhighway’. The Cretaceous dinosaur discoveries from Africa and South America exhibit more vicariant patterns, with species like Meraxes belonging to groups with a more restricted Gondwanan distribution and a high degree of endemism. This also seems to be the case for dinosaur faunas on either side of the WIS, which acted as a barrier for dispersal and gene flow for terrestrial clades for much of the Cretaceous.

While the influence on continental scale plate tectonics on dinosaur evolution is fairly obvious, the effects of how biotic interactions shaped dinosaur diversity are harder to parse, yet no less interesting. Viktor Radermacher (PhD 2026), who will be defending his PhD this semester, is investigating one such case. Hadrosaurid, or duck-billed dinosaurs, are dominant herbivores of the latest Cretaceous environments of Asia and North America, and are characterized by a unique grinding dentition with up to five replacement teeth in each socket (fig. 4e). Large hadrosaurids can have over 500 teeth at any time forming a ‘dental battery’ with which they are hypothesized to have outcompeted other herbivores at the close of the Mesozoic. Such hypotheses are difficult to test, and require evidence that species or groups actually competed ecologically in addition to demonstrating concurrent shifts in spatiotemporal distribution patterns between. In order to test the hadrosaur dominance hypothesis, Vik developed a new metric for measuring tooth wear in herbivorous dinosaurs as a functional proxy for their feeding capacity. Using scans of complete jaws (fig. 5) he calculated values for individual teeth and whole toothrows in a dozen species and then used phylogenetic methods to extrapolate ranges of values for unsampled species based on their relationships to the sampled ones. These data were then subjected to a battery of statistical approaches. Vik’s results suggest a more complex evolutionary scenario, in which hadrosaurs could have outcompeted their less toothy relatives in western North America, but the evidence is less compelling for other landmasses. Moreover, the western North American result is actually driven by the combination of hadrosaurs appearing alongside large ceratopsid (horned) dinosaurs, another group of large herbivores with  advanced dental magazine (fig. 5c), and together squeezing out the former’s more primitive relatives. Interestingly, while hadrosaurids and ceratopsids together could have displaced other large herbivores with more primitive dentitions, their dental perform so differently with respect to the measured variables that it is unlikely they competed, a fact bolstered by recent isotope work that also suggest these groups  consumed different plant resources.

Although it is our only direct source of information of past life, the fossil record is undeniably affected by biases. One strong bias is exerted by depositional rates, as shown by the collaborative WIB project described above. Through dense sampling of zircons, project PIs have demonstrated that geographically disjunct formations that were traditionally correlated based on biostratigraphy, actually show relatively little temporal overlap. Instead, the overarching pattern is one of pulsed intervals with high depositional rates interspersed with long hiatuses unrecognized by prior studies. Not surprisingly, the richest fossil assemblages derive from the units with highest depositional rates. On the one hand these results undermine the utility of previous biostratigraphic correlations, but on the other they are yielding important insights into evolutionary rates in dinosaurs. For example, small herbivorous thescelosaurine dinosaurs (Fig 2D) previously considered to represent a single species from two different formations, are now recognized as two separate species separated by a million years and a handful of subtle but consistent anatomical features. This observation, and other like it, provides an estimate for ‘background’ rates of evolution, and indicate that dinosaur faunas of the WIB were marked by long lived lineages exhibiting relatively slow evolutionary change. This in turn suggests that major faunal turnover such as the one observed at the Early to Late Cretaceous boundary were rare and  due to a combination of extinction and immigration tied to abiotic drivers, rather than evolutionary acceleration across lineages. 

Another source of bias is associated with the processes that affect an organism between it death and its recovery as a fossil, such as disarticulation, scavenging, transport, winnowing, burial and diagenesis. Michael Chiappone (PhD 2027) is leading an innovative project taking advantage of the facilities and expertise at St Anthony Falls laboratory to quantify transport and burial of bones in fluvial environments, as the majority of terrestrial fossil assemblages form in those depositional systems. The results of his first set of experiments on bone transport were just published in the journal Paleobiology, and are highlighted separately in this newsletter. The main takeaway from those experiments is that transport is largely controlled by shape and density of the bones as well as by the type of flow (steady versus unsteady). Building of that initial work, Michael has designed a set of experiments that are allowing him to develop  predictive models for readily a bone can initiate transport under known flow and bedform conditions, and he will then move on to investigating the dynamics of burial and entry into the fossil record. The ultimate goal here is to develop a set of tools and criteria that can be applied at fossil sites to infer how they formed, why some elements or species might be missing or overrepresented at a particular site, and predict what else might found and whether to therefore invest more resources toward excavation.

A third source of bias is introduce by diagenesis. In the emerging field of ‘molecular paleontology’, paleontologists and geochemists are investigating what parts of organisms beside their hard parts are preserved in the fossil record, but a critical consideration is parsing what geochemical alterations may have occurred after burial. This is particularly germane to recent studies that make biological inferences about fossils based on elemental mapping using X-ray fluorescence (XRF). Together with researchers at the Field Museum and the Advanced Photon Source at Argonne National Labs, Makovicky is using synchrotron XRF to investigate the elemental signatures of plant, invertebrate and vertebrate fossils (Fig. 5) from several famous fossil sites with exceptional preservation of soft tissues. The principal takeaway from this study is that each site is dominated by its own diagenetic geochemical signature such that a plant and a fish from one locality will have more similar values for the principal elements, than they do with a fish or plant form another site. Within sites, however, plants will differ from invertebrates and vertebrates and they do so consistently across sites. While these results cast doubts on using concentrations of certain elements as proxies for biological compounds, this study will provide a useful reference dataset and guide future research in this developing field.

Research in the Makovicky lab brings together a global network of collaborations and fieldwork with a wide range of lab techniques ranging from traditional anatomical observation, through scanning fossils with instruments ranging from cell phones to synchotrons, to full scale hydrodynamic experiments in order to address the evolution and fossilization of vertebrates at geological time scales. Watch this space for more research news!