Trilobites are arthropods that became extinct at the greatest marine mass extinction over 250 Ma ago. Because of their often bizarre forms, their great diversity and disparity of shapes, they have attracted the interest of researchers and laypersons alike. Due to their calcified exoskeleton, their remains are quite abundant in many marine strata. One particularly interesting aspect, however, is the fossilization of various molting stages. This allows the reconstruction of both juvenile strategies (lecitotrophic versus planktotrophic) and the entire life history of at least some well-documented taxa (e.g., Hughes 2003, 2007; Laibl 2017). For example, life history of trilobites is, based on certain morphological changes, classically subdivided in the three phases protaspis (hatchling, one dorsal shield with few segments with no articulation between), meraspis (juvenile, two and more shields connected by articulations) and holaspis (when the terminal number of thoracic segments is reached). At most molting events, a new skeletal element is added (only in the holaspis, the number of thoracic segments does not change). Nevertheless, many trilobites are known mainly from late meraspid and holaspid stages, because the dorsal shields of the first ontogenetic stages are usually very small and thus often either dissolved or overlooked. An improved understanding of trilobite ontogeny could thus help filling in these gaps in fossil preservation and subsequently, to better understand evolutionary pathways. This is where this paper comes in. In a very clever approach, the New-York-based researcher Melanie Hopkins modeled the growth of these segmented animals (Hopkins 2020). Previous growth models of invertebrates focused on, e.g., mollusks, whose shells grow by accretion. Modelling arthropod ontogeny represented a challenge, which is now overcome by Hopkins' brilliant paper. Her generative growth model is based on empirical data of *Aulacopleura koninckii* (Barrande, 1846). Hong et al. (2014) and Hughes et al. (2017) documented the ontogeny of this 429 Ma old trilobite species in great detail. In the Silurian of the Barrandian region (Czech Republic), this species is locally very common and all growth stages are well known. I could imagine that the paper of Hughes et al. (2017) planted the seed into Melanie Hopkins’ mind to approach trilobite development in general in a quantitative way with a mathematical approach comparable to the mollusk-research by, e.g., David Raup (1961, 1966) and George McGhee (2015). Hopkins’ growth model requires “a minimum of nine parameters […] to model basic trilobite growth and segmentation, and three additional parameters […] to allow a transition to a new growth gradient for the trunk region during ontogeny” (Hopkins 2020: p. 21). It is now possible to play with parameters such as protaspid size, segment dimensions, segment numbers, etc., in order to estimate changes in body size or morphology. Furthermore, the model could be applied to similarly organized arthropod exoskeletons like many early Cambrian arthropods (e.g., marellomorphs) or even crustaceans (e.g., conchostracans or copepods). Of great interest could also be to assess influences of environmental changes on arthropod ontogeny. Also, her work will help to reconstruct unknown developmental information missing from trilobite species (and possibly other arthropods) and also to explore their morphospace. **References** Barrande, J. (1846). Notice préliminaire sur le système Silurien et les trilobites de Bohême. Leipzig: Hirschfield. Hong, P. S., Hughes, N. C., & Sheets, H. D. (2014). Size, shape, and systematics of the Silurian trilobite *Aulacopleura koninckii*. Journal of Paleontology, 88(6), 1120–1138. doi: [ 10.1666/13-142](https://dx.doi.org/ 10.1666/13-142) Hopkins, M. J. (2020). A simple generative model of trilobite segmentation and growth. PaleorXiv, version 3, peer-reviewed by PCI Paleo. doi: [ 10.31233/osf.io/zt642](https://dx.doi.org/ 10.31233/osf.io/zt642) Hughes, N. C. (2003). Trilobite tagmosis and body patterning from morphological and developmental perspectives. Integrative and Comparative Biology, 43(1), 185–206. doi: [ 10.1093/icb/43.1.185](https://dx.doi.org/ 10.1093/icb/43.1.185) Hughes, N. C. (2007). The evolution of trilobite body patterning. Annual Review of Earth and Planetary Sciences, 35(1), 401–434. doi: [ 10.1146/annurev.earth.35.031306.140258](https://dx.doi.org/ 10.1146/annurev.earth.35.031306.140258) Hughes, N. C., Hong, P. S., Hou, J., & Fusco, G. (2017). The development of the Silurian trilobite *Aulacopleura koninckii* reconstructed by applying inferred growth and segmentation dynamics: A case study in paleo-evo-devo. Frontiers in Ecology and Evolution, 5, 00037. doi: [ 10.3389/fevo.2017.00037](https://dx.doi.org/ 10.3389/fevo.2017.00037) Laibl, L. (2017). Patterns in Palaeontology: The development of trilobites. Palaeontology Online, 7(10), 1–9. McGhee, G. R. (2015). Limits in the evolution of biological form: a theoretical morphologic perspective. Interface Focus, 5(6), 20150034. doi: [ 10.1098/rsfs.2015.0034](https://dx.doi.org/ 10.1098/rsfs.2015.0034) Raup, D. M. (1961). The geometry of coiling in gastropods. Proceedings of the National Academy of Sciences, 47(4), 602–609. doi: [ 10.1073/pnas.47.4.602](https://dx.doi.org/ 10.1073/pnas.47.4.602) Raup, D. M. (1966). Geometric analysis of shell coiling: general problems. Journal of Paleontology, 40, 1178–1190.

In their analysis, Pallas and colleagues identify 32 postcranial elements from the Plio-Pleistocene collections of the Lower Omo Valley, Ethiopia as colobine (Pallas et al., 2024). This is a valuable contribution towards understanding colobine evolution, Plio-Pleistocene environments of the Turkana Basin, Kenya and Ethiopia, and how the many large-bodied catarrhines, including at least three hominins, four colobines, and three papionins, all with body masses over 30 Kg shared this ecosystem.

Today, colobine monkeys have greater diversity in Asia than in Africa, where they are represented by three small to medium-sized forms: olive, red, and black and white colobus (Grubb et al., 2003; Roos and Zinner, 2022). In the Pliocene and Pleistocene, however, they were significantly more diverse, with at least four additional large-bodied genera that varied considerably in body size, and as evidenced by multiple proxies, their preferred habitats, diets, and locomotor behaviors (Frost et al., 2022 and references therein). The highly fossiliferous sediments of the Shungura and Usno Formations in the Lower Omo Valley span the period from 3.75 to 1.0 Ma (Heinzelin, 1983; McDougall et al., 2012; Kidane et al., 2014) and have contributed greatly to understanding human and mammalian evolution during the African Plio-Pleistocene (Howell and Coppens, 1974; Boisserie et al., 2008), including the enigmatic large-bodied colobines (Leakey, 1982; 1987). Despite large samples of postcranial material from the Lower Omo Valley (Eck, 1977), most of our knowledge of fossil colobine postcrania is based on a relatively few associated skeletons from other eastern African sites (Birchette, 1982; Frost and Delson, 2002; Jablonski et al., 2008; Anderson, 2021). This is because the vast majority of postcrania from the Lower Omo Valley are not directly associated with taxonomically diagnostic elements.

Based on qualitative and quantitative comparison with an extensive database of extant cercopithecoid postcrania, Pallas et al. (2024) identify 32 long bones of the fore- and hindlimbs as colobine. These range in age from approximately 3.3 to 1.1 Ma. They made their identifications using a combination of body mass estimation and comparison with associated skeletons of Plio-Pleistocene and extant taxa. In this way, they tentatively allocate some of the larger material dated to 3.3. to 2.0 Ma to taxa previously recognized from craniodental remains, especially Rhinocolobus cf. turkanaensis and Paracolobus cf. mutiwa; and the smaller ca. 1.1 Ma to cf. Colobus. Interestingly, they also identify several specimens, especially from Members B and C, that are unlikely to represent taxa previously described for the Lower Omo Valley and make a possible link to Cercopithecoides meaveae, otherwise only known from the Afar Region, Ethiopia (Frost and Delson, 2002).

Based on these identifications, Pallas et al. (2024) hypothesize that Rhinocolobus may have been adapted to more suspensory postures compared to Cercopithecoides and Paracolobus which are estimated to have been more terrestrial. Additionally, they suggest that the possibly semi-terrestrial Paracolobus mutiwa may show adaptations for vertical climbing. These are novel observations, and if they are correct give further clues as to how these primates seemingly managed to co-exist in the same area for nearly a million years (Leakey, 1982; 1987; Jablonski et al., 2008). Better understanding the locomotor and positional behaviors of these taxa will also make them more useful in reconstructions of the paleoenvironments represented by the Shungura and Usno Formations.

References

Anderson, M. (2021). An assessment of the postcranial skeleton of the Paracolobus mutiwa (Primates: Colobinae) specimen KNM-WT 16827 from Lomekwi, West Turkana, Kenya. Journal of Human Evolution, 156, 103012. https://doi.org/10.1016/j.jhevol.2021.103012

Birchette, M. G. (1982). The postcranial skeleton of Paracolobus chemeroni [Unpublished PhD thesis]. Harvard University.

Boisserie, J.-R., Guy, F., Delagnes, A., Hlukso, L. J., Bibi, F., Beyene, Y., and Guillemot, C. (2008). New palaeoanthropological research in the Plio-Pleistocene Omo Group, Lower Omo Valley, SNNPR (Southern Nations, Nationalities and People Regions), Ethiopia. Comptes Rendus Palevol, 7(7), 429–439. https://doi.org/10.1016/j.crpv.2008.07.010

Eck, G. (1977). Diversity and frequency distribution of Omo Group Cercopithecoidea. Journal of Human Evolution, 6(1), 55–63. https://doi.org/10.1016/S0047-2484(77)80041-9

Frost, S. R., and Delson, E. (2002). Fossil Cercopithecidae from the Hadar Formation and surrounding areas of the Afar Depression, Ethiopia. Journal of Human Evolution, 43(5), 687–748. https://doi.org/10.1006/jhev.2002.0603

Frost, S. R., Gilbert, C. C., and Nakatsukasa, M. (2022). The colobine fossil record. In I. Matsuda, C. C. Grueter, and J. A. Teichroeb (Eds.), The Colobines: Natural History, Behaviour and Ecological Diversity. Cambridge University Press. Pp. 13–31. https://doi.org/10.1017/9781108347150

Grubb, P., Butynski, T. M., Oates, J. F., Bearder, S. K., Disotell, T. R., Groves, C. P., and Struhsaker, T. T. (2003). Assessment of the diversity of African primates. International Journal of Primatology, 24(6), 1301–1357. https://doi.org/10.1023/B:IJOP.0000005994.86792.b9

Heinzelin, J. de. (1983). The Omo Group. Archives of the International Omo Research Expedition. Volume 85. Annales du Musée Royal de l’Afrique Centrale, série 8, Sciences géologiques, Tervuren, 388 p.

Howell, F. C., and Coppens, Y. (1974). Inventory of remains of Hominidae from Pliocene/Pleistocene formations of the lower Omo basin, Ethiopia (1967–1972). American Journal of Physical Anthropology, 40(1), 1–16. https://doi.org/10.1002/ajpa.1330400102

Jablonski, N. G., Leakey, M. G., Ward, C. V., and Antón, M. (2008). Systematic paleontology of the large colobines. In N. G. Jablonski and M. G. Leakey (Eds.), Koobi Fora Research Project Volume 6: The Fossil Monkeys. California Academy of Sciences. Pp. 31–102.

Kidane, T., Brown, F. H., and Kidney, C. (2014). Magnetostratigraphy of the fossil-rich Shungura Formation, southwest Ethiopia. Journal of African Earth Sciences, 97, 207–223. https://doi.org/10.1016/j.jafrearsci.2014.05.005

Leakey, M. G. (1982). Extinct large colobines from the Plio‐Pleistocene of Africa. American Journal of Physical Anthropology, 58(2), 153–172. https://doi.org/10.1002/ajpa.1330580207

Leakey, M. G. (1987). Colobinae (Mammalia, Primates) from the Omo Valley, Ethiopia. In Y. Coppens and F. C. Howell (Eds.), Les faunes Plio-Pléistocènes de la Basse Vallée de l’Omo (Ethiopie). Tome 3, Cercopithecidae de la Formation de Shungura. CNRS, Paris, pp. 148-169.

McDougall, I., Brown, F. H., Vasconcelos, P. M., Cohen, B. E., Thiede, D. S., and Buchanan, M. J. (2012). New single crystal 40Ar/39Ar ages improve time scale for deposition of the Omo Group, Omo–Turkana Basin, East Africa. Journal of the Geological Society, 169(2), 213–226. https://doi.org/10.1144/0016-76492010-188

Pallas, L., Daver, G., Merceron, G., and Boisserie, J.-R. (2024). Postcranial anatomy of the long bones of colobines (Mammalia, Primates) from the Plio-Pleistocene Omo Group deposits (Shungura Formation and Usno Formation, 1967-2018 field campaigns, Lower Omo Valley, Ethiopia). PaleorXiv, bwegt, ver. 8, peer-reviewed by PCI Paleo. https://doi.org/10.31233/osf.io/bwegt

Roos, C., and Zinner, D. (2022). Molecular phylogeny and phylogeography of colobines. In I. Matsuda, C. C. Grueter, and J. A. Teichroeb (Eds.), The Colobines: Natural History, Behaviour and Ecological Diversity. Cambridge University Press. Pp. 32-43. https://doi.org/10.1017/9781108347150

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.

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

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