I am a non-avian dinosaur palaeontologist by trade with a particular interest in their palaeoecology. This can be an endless source of both fascination and frustration. Fascination, because non-avian dinosaurs are quite unlike anything alive today, warranting some use of creative license when imagining them as living animals. Frustration, because the lack of good, extant ecological analogues frequently makes reconstruction of their ancient ecologies an almost insurmountable challenge.

The Canadian Museum of Nature where I work has a good collection of Late Cretaceous turtles. I took an interest in these some years ago because it struck me that, despite the quality of our collection, relatively few people come to study them. I thought, "Someone should work on these. Why not me?" I figured studying a new fossil group would present a fun change of pace and perhaps a more straightforward object of palaeoecological reconstruction. After all, fossil turtles are a lot like living turtles, so how hard can it be? Right?

In 2018, I took a special interest in one recently prepared fossil turtle, which I determined to be a new species of Basilemys (Mallon and Brinkman, 2018). Basilemys held my interest because, although it is a relatively common form, there has been some debate concerning the palaeohabitat of this animal and its closest relatives, the nanhsiungchelyids. Some have argued for an aquatic habitat for these animals; others, for a terrestrial one. It seems that where one comes down on the issue depends on which aspect of ecomorphology is emphasized. If it is on the flat carapace, nanhsiungchelyids must have been aquatic; if it is on the stout feet, terrestrial. This is how I came to appreciate the numerous ecomorphological proxies (e.g., skull shape, shell shape, limb proportions) that are used in turtle palaeoecology and how incongruent they can sometimes be. So much for easy answers!

The present study by Evers et al. is a response to an original piece of research by Lichtig and Lucas (2017), who claimed to be able to use simple shell measurements (carapacial doming and relative plastral width) to accurately deduce/infer the habitats of living turtles and, by extension, fossil ones. In short, they found that terrestrial turtles tend to have more domed carapaces and wider plastra, yielding some unconventional palaeoecological reconstructions of particular stem turtles. Evers et al. take issue with several aspects of this study, including issues of faulty data entry, inappropriate removal of extant taxa from the model, and insufficient accounting for phylogenetic non-independence. By correcting for these overights, they find that the model of Lichtig and Lucas (2017) performs more poorly than advertised and that the palaeoecological classification it produces should be questioned. "The map is not the territory", as Alfred Korzybski put it, and this latest study by Evers et al. serves as an important reminder of that lesson.
Still, even if Lichtig and Lucas's model is overly simplistic, it is true that aquatic turtles, on average, have lower carapaces and narrower plastra, and that they have relatively lower skulls and longer toes. Surely, there is merit in each of these anatomical proxies, even if no single one predicts ecology with total accuracy. I would love to see a model that combines them all. Until then, Evers et al. have inched us closer to knowing what turtle morphology can (and cannot) tell us about habitat.

Thanks to D. Brinkman and H. Smith for their helpful reviews of the manuscript.

References

Evers, S. W., Foth, C., Joyce, W. G., and Hermanson, G. (2024). Simple shell measurements do not consistently predict habitat in turtles: A reply to Lichtig and Lucas (2017). bioRxiv, 586561, ver. 3 peer-reviewed by PCI Paleo. https://doi.org/10.1101/2024.03.25.586561

Lichtig, A. J., and Lucas, S. G. (2017). A simple method for inferring habitats of extinct turtles. Palaeoworld, 26(3), 581–588. https://doi.org/10.1016/j.palwor.2017.02.001

Mallon, J. C., and Brinkman, D. B. (2018). Basilemys morrinensis, a new species of nanhsiungchelyid turtle from the Horseshoe Canyon Formation (Upper Cretaceous) of Alberta, Canada. Journal of Vertebrate Paleontology, 38(2), e1431922. https://doi.org/10.1080/02724634.2018.1431922

There are less and less experts on taxonomy of particular groups particularly among early career paleontologists and (paleo)biologists – this also includes ammonoid cephalopods. Techniques cannot replace this taxonomic expertise (Engel et al. 2021) but machine learning approaches can make taxonomy more efficient, reproducible as well as passing it over more sustainable. Initially ammonoid taxonomy was a black box with small differences sometimes sufficient to erect different species as well as really idiosyncratic groupings of superficially similar specimens (see De Baets et al. 2015 for a review). In the meantime, scientists have embraced more quantitative assessments of conch shape and morphology more generally (see Klug et al. 2015 for a more recent review). The approaches still rely on important but time-intensive collection work and seeing through daisy chains of more or less accessible papers and monographs without really knowing how these approaches perform (other than expert opinion). In addition, younger scientists are usually trained by more experienced scientists, but this practice is becoming more and more difficult which makes it difficult to resolve the taxonomic gap. This relates to the fact that less and less experienced researchers with this kind of expertise get employed as well as graduate students or postdocs choosing different research or job avenues after their initial training effectively leading to a leaky pipeline and taxonomic impediment.

Robust taxonomy and stratigraphy is the basis for all other studies we do as paleontologists/paleobiologists so Foxon (2021) represents the first step to use supervised and unsupervised machine-learning approaches and test their efficiency on ammonoid conch properties. This pilot study demonstrates that machine learning approaches can be reasonably accurate (60-70%) in identifying ammonoid species (Foxon, 2021) – at least similar to that in other mollusk taxa (e.g., Klinkenbuß et al. 2020) - and might also be interesting to assist in cases where more traditional methods are not feasible. Novel approaches might even allow to further approve the accuracy as has been demonstrated for other research objects like pollen (Romero et al. 2020). Further applying of machine learning approaches on larger datasets and additional morphological features (e.g., suture line) are now necessary in order to test and improve the robustness of these approaches for ammonoids as well as test their performance more broadly within paleontology.

References

De Baets K, Bert D, Hoffmann R, Monnet C, Yacobucci M, and Klug C (2015). Ammonoid intraspecific variability. In: Ammonoid Paleobiology: From anatomy to ecology. Ed. by Klug C, Korn D, De Baets K, Kruta I, and Mapes R. Vol. 43. Topics in Geobiology. Dordrecht: Springer, pp. 359–426.

Engel MS, Ceríaco LMP, Daniel GM, Dellapé PM, Löbl I, Marinov M, Reis RE, Young MT, Dubois A, Agarwal I, Lehmann A. P, Alvarado M, Alvarez N, Andreone F, Araujo-Vieira K, Ascher JS, Baêta D, Baldo D, Bandeira SA, Barden P, Barrasso DA, Bendifallah L, Bockmann FA, Böhme W, Borkent A, Brandão CRF, Busack SD, Bybee SM, Channing A, Chatzimanolis S, Christenhusz MJM, Crisci JV, D’elía G, Da Costa LM, Davis SR, De Lucena CAS, Deuve T, Fernandes Elizalde S, Faivovich J, Farooq H, Ferguson AW, Gippoliti S, Gonçalves FMP, Gonzalez VH, Greenbaum E, Hinojosa-Díaz IA, Ineich I, Jiang J, Kahono S, Kury AB, Lucinda PHF, Lynch JD, Malécot V, Marques MP, Marris JWM, Mckellar RC, Mendes LF, Nihei SS, Nishikawa K, Ohler A, Orrico VGD, Ota H, Paiva J, Parrinha D, Pauwels OSG, Pereyra MO, Pestana LB, Pinheiro PDP, Prendini L, Prokop J, Rasmussen C, Rödel MO, Rodrigues MT, Rodríguez SM, Salatnaya H, Sampaio Í, Sánchez-García A, Shebl MA, Santos BS, Solórzano-Kraemer MM, Sousa ACA, Stoev P, Teta P, Trape JF, Dos Santos CVD, Vasudevan K, Vink CJ, Vogel G, Wagner P, Wappler T, Ware JL, Wedmann S, and Zacharie CK (2021). The taxonomic impediment: a shortage of taxonomists, not the lack of technical approaches. Zoological Journal of the Linnean Society 193, 381–387. doi: 10. 1093/zoolinnean/zlab072

Foxon F (2021). Ammonoid taxonomy with supervised and unsupervised machine learning algorithms. PaleorXiv ewkx9, ver. 3, peer-reviewed by PCI Paleo. doi: 10.31233/osf.io/ewkx9

Klinkenbuß D, Metz O, Reichert J, Hauffe T, Neubauer TA, Wesselingh FP, and Wilke T (2020). Performance of 3D morphological methods in the machine learning assisted classification of closely related fossil bivalve species of the genus Dreissena. Malacologia 63, 95. doi: 10.4002/040.063.0109

Klug C, Korn D, Landman NH, Tanabe K, De Baets K, and Naglik C (2015). Ammonoid conchs. In: Ammonoid Paleobiology: From anatomy to ecology. Ed. by Klug C, Korn D, De Baets K, Kruta I, and Mapes RH. Vol. 43. Dordrecht: Springer, pp. 3–24.

Romero IC, Kong S, Fowlkes CC, Jaramillo C, Urban MA, Oboh-Ikuenobe F, D’Apolito C, and Punyasena SW (2020). Improving the taxonomy of fossil pollen using convolutional neural networks and superresolution microscopy. Proceedings of the National Academy of Sciences 117, 28496–28505. doi: 10.1073/pnas.2007324117

Bone inner structure bears a strong functional signal and can be used in paleontology to make inferences about the ecology of fossil forms. The increasing use of microtomography enables to analyze both cortical and trabecular features in three dimensions, and thus in long bones to investigate the diaphyseal and epiphyseal structures. Moreover, this can now be done through quantitative, and not only qualitative analyses. Studies focusing on the diaphyseal inner structure (cortical bone and sometimes also spongious bone) of long bones are rather numerous, but essentially based on 2D sections. It is only recently that analyses of the whole diaphyseal structure have been investigated. Studies on the trabecular architecture are much rarer.

Amson & Nyakatura (2018) propose a comparative quantitative analysis combining parameters of the epiphyseal trabecular architecture and of the diaphyseal structure, using phylogenetically informed discriminant analyses, and with the aim of inferring the lifestyle of extinct taxa. The group of interest is xenarthrans, one of the four major extant clades of placental mammals. Xenarthrans exhibit different lifestyles, from fully terrestrial to arboreal, and show various degrees of fossoriality. The authors analyzed forelimb long bones of some fossil sloths and made comparisons with several species of extant xenarthrans. The aim was notably to discuss the degree of arboreality and fossoriality of these fossil forms.

This study is among the first ones to conjointly analyze both diaphyseal and trabecular parameters to characterize lifestyles, and the first one outside of primates. No fossil form could undoubtedly be assigned to one lifestyle exhibited by extant xenarthrans, though some previous ecological hypotheses could be corroborated. This study also raised some technical challenges, linked to the sample and to the parameters studied, and thus constitutes a great step, from which to go further.

References

Amson, E., & Nyakatura, J. A. (2018). Palaeobiological inferences based on long bone epiphyseal and diaphyseal structure - the forelimb of xenarthrans (Mammalia). bioRxiv, 318121, ver. 5 peer-reviewed and recommended by PCI Paleo. doi: 10.1101/318121

Calcareous plankton gives us perhaps the most complete record of microevolutionary changes in the fossil record (e.g. Tong et al., 2018; Weinkauf et al., 2019), but this opportunity is not exploited enough, as it requires meticulous work in documenting assemblage-level variation through time. Especially in organisms such as coccolithophores, understanding the meaning of secular trends in morphology warrants an understanding of the functional biology and ecology of these organisms. Razmjooei and Thibault (2022) achieve this in their painstaking analysis of two coccolithophore lineages, Cribrosphaerella ehrenbergii and Microrhabdulus, in the Late Cretaceous of Iran. They propose two episodes of morphological change. The first one, starting around 76 Ma in the late Campanian, is marked by a sudden shift towards larger sizes of C. ehrenbergii and the appearance of a new species M. zagrosensis from M. undulatus. The second episode around 69 Ma (Maastrichtian) is inferred from a gradual size increase and morphological changes leading to probably anagenetic speciation of M. sinuosus n.sp.

The study remarkably analyzed the entire distributions of coccolith length and rod width, rather than focusing on summary statistics (De Baets et al., in press). This is important, because the range of variation determines the taxon’s evolvability with respect to the considered trait (Love et al., 2022). As the authors discuss, cell size in photosymbiotic unicellular organisms is not subject to the same constraints that will be familiar to researchers working e.g. on mammals (Niklas, 1994; Payne et al., 2009; Smith et al., 2016). Furthermore, temporal changes in size alone cannot be interpreted as evolutionary without knowledge of phenotypic plasticity and environmental clines present in the basin (Aloisi, 2015). The more important is that this study cross-tests size changes with other morphological parameters to examine whether their covariation supports inferred speciation events. The article addresses as well the effects of varying sedimentation rates (Hohmann, 2021) by, somewhat implicitly, correcting for the stratophenetic trend using an age-depth model and accounting for a hiatus. Such multifaceted approach as applied in this work is fundamental to unlock the dynamics of speciation offered by the microfossil record. 

The study highlights also the link between shifts in size and diversity. Klug et al. (2015) have previously demonstrated that these two variables are related, as higher diversity is more likely to lead to extreme values of morphological traits, but this study suggests that the relationship is more intertwined: environmentally-driven rise in morphological variability (and thus in size) can lead to diversification. It is a fantastic illustration of the complexity of morphological evolution that, if it can be evaluated in terms of mechanisms, provides an insight into the dynamics of speciation.

References

Aloisi, G. (2015). Covariation of metabolic rates and cell size in coccolithophores. Biogeosciences, 12(15), 4665–4692. doi: 10.5194/bg-12-4665-2015

De Baets, K., Jarochowska, E., Buchwald, S. Z., Klug, C., and Korn, D. (In Press). Lithology controls ammonoid size distribution. Palaios.

Hohmann, N. (2021). Incorporating information on varying sedimentation rates into palaeontological analyses. PALAIOS, 36(2), 53–67. doi: 10.2110/palo.2020.038

Klug, C., De Baets, K., Kröger, B., Bell, M. A., Korn, D., and Payne, J. L. (2015). Normal giants? Temporal and latitudinal shifts of Palaeozoic marine invertebrate gigantism and global change. Lethaia, 48(2), 267–288. doi: 10.1111/let.12104

Love, A. C., Grabowski, M., Houle, D., Liow, L. H., Porto, A., Tsuboi, M., Voje, K.L., and Hunt, G. (2022). Evolvability in the fossil record. Paleobiology, 48(2), 186–209. doi: 10.1017/pab.2021.36

Niklas, K. J. (1994). Plant allometry: The scaling of form and process. Chicago: University of Chicago Press.

Payne, J. L., Boyer, A. G., Brown, J. H., Finnegan, S., Kowalewski, M., Krause, R. A., Lyons, S.K., McClain, C.R., McShea, D.W., Novack-Gottshall, P.M., Smith, F.A., Stempien, J.A., and Wang, S. C. (2009). Two-phase increase in the maximum size of life over 3.5 billion years reflects biological innovation and environmental opportunity. Proceedings of the National Academy of Sciences, 106(1), 24–27. doi: 10.1073/pnas.0806314106

Razmjooei, M. J., and Thibault, N. (2022). Morphometric changes in two Late Cretaceous calcareous nannofossil lineages support diversification fueled by long-term climate change. PaleorXiv, nfyc9, ver. 4, peer-reviewed by PCI Paleo. doi: 10.31233/osf.io/nfyc9

Smith, F. A., Payne, J. L., Heim, N. A., Balk, M. A., Finnegan, S., Kowalewski, M., Lyons, S.K., McClain, C.R., McShea, D.W., Novack-Gottshall, P.M., Anich, P.S., and Wang, S. C. (2016). Body size evolution across the Geozoic. Annual Review of Earth and Planetary Sciences, 44(1), 523–553. doi: 10.1146/annurev-earth-060115-012147

Tong, S., Gao, K., and Hutchins, D. A. (2018). Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2. Global Change Biology, 24(7), 3055–3064. doi: 10.1111/gcb.14065

Weinkauf, M. F. G., Bonitz, F. G. W., Martini, R., and Kučera, M. (2019). An extinction event in planktonic Foraminifera preceded by stabilizing selection. PLOS ONE, 14(10), e0223490. doi: 10.1371/journal.pone.0223490

Rhinocerotoidea originated in the Lower Eocene and diversified well during the Cenozoic in Eurasia, North America and Africa. This taxon encompasses a great diversity of ecologies and body proportions and masses. Within this group, the family Rhinocerotidae, which is the only one with extant representatives, appeared in the Late Eocene (Prothero & Schoch, 1989). They were well diversified during the Early and Middle Miocene, whereas they began to decline in both diversity and geographical range after the Miocene, throughout the Pliocene and Pleistocene, in conjunction with the marked climatic changes (Cerdeño, 1998). 

In Eurasian Early and Middle Miocene fossil localities, a variety of species are often associated. Therefore, it may be quite difficult to estimate how these large herbivores cohabited and whether competition for food resources is reflected in a diversity of ecological niches. The ecologies of these large mammals are rather poorly known and the detailed study of their teeth could bring new elements of answer. Indeed, if teeth carry a strong phylogenetic signal in mammals, they are also of great interest for ecological studies, and they have the additional advantage of being often numerous in the fossil record. 

Hullot et al. (2022) analysed both dental microwear texture, as an indicator of dietary preferences, and enamel hypoplasia, to identify stress sensitivity, in a large number of rhinocerotid fossil teeth from nine Neogene (Early to Middle Miocene) localities in Europe and Pakistan. Their aim was to analyse whether fossil species diversity is associated with a diversity of ecologies, and to investigate possible ecological differences between regions and time periods in relation to climate change. Their results show clear differences in time and space between and within species, and suggest that more flexible species are less vulnerable to environmental stressors. 

Very few studies focus on the palaeocology of Miocene rhinos. This study is therefore a great contribution to the understanding of the evolution of this group.

References

Cerdeño, E. (1998). Diversity and evolutionary trends of the Family Rhinocerotidae (Perissodactyla). Palaeogeography, Palaeoclimatology, Palaeoecology, 141, 13–34. https://doi.org/10.1016/S0031-0182(98)00003-0

Hullot, M., Merceron, G., and Antoine, P.-O. (2022). Spatio-temporal diversity of dietary preferences and stress sensibilities of early and middle Miocene Rhinocerotidae from Eurasia: Impact of climate changes. BioRxiv, 490903, ver. 4 peer-reviewed by PCI Paleo. https://doi.org/10.1101/2022.05.06.490903

Prothero, D. R., and Schoch, R. M. (1989). The evolution of perissodactyls. New York: Oxford University Press.