The evolution of the hominid clavicle has not been studied in depth by paleoanthropologists given its high morphological variability and the scarcity of complete diagnosable specimens. A nearly complete Nacholapithecus clavicle from Kenya (Senut et al. 2004) together with a fragment from Ardipithecus from the Afar region of Ethiopia (Lovejoy et al. 2009) complete our knowledge of the Miocene record. The Australopithecus collection of clavicles from Eastern and South African Plio-Pleistocene sites is slightly more abundant but mostly represented by fragmentary specimens. The number of fossil clavicles increases for the genus Homo from more recent sites and thus our potential knowledge about the shoulder evolution. 

In their new contribution, Taylor et al. (2023) present a detailed analysis of OH 89, a ~1.8-million-year-old partial hominin clavicle recovered from Olduvai Gorge (Tanzania). The work goes over previous studies which included clavicles found in the hominid fossil record. The text is accompanied by useful tables of data and a series of excellent photographs. It is a great opportunity to learn its role in the evolution of the hominid shoulder gird as clavicles are relatively poorly preserved in the fossil record compared to other long bones. The study compares the specimen OH 89 with five other hominid clavicles and a sample of 25 modern clavicles, 30 Gorilla, 31 Pan and 7 Papio. The authors propose a new methodology for measuring clavicular curvature using measurements of sternal and acromial curvature, from which an overall curvature measurement is calculated. The study of OH 89 provides good evidence about the hominid who lived 1.8 million years ago in the Olduvai Gorge region. This time period is especially relevant because it can help to understand the morphological changes that occurred between Australopithecus and the appearance of Homo. The authors conclude that OH 89 is the largest of the hominid clavicles included in the analysis. Although they are not able to assign this partial element to species level, this clavicle from Olduvai is at the larger end of the variation observed in Homo sapiens and show similarities to modern humans, especially when analysing the estimated sinusoidal curvature.

References

Lovejoy, C. O., Suwa, G., Simpson, S. W., Matternes, J. H., and White, T. D. (2009). The Great Divides: Ardipithecus ramidus peveals the postcrania of our last common ancestors with African apes. Science, 326(5949), 73–106. https://doi.org/10.1126/science.1175833

Senut, B., Nakatsukasa, M., Kunimatsu, Y., Nakano, Y., Takano, T., Tsujikawa, H., Shimizu, D., Kagaya, M., and Ishida, H. (2004). Preliminary analysis of Nacholapithecus scapula and clavicle from Nachola, Kenya. Primates, 45(2), 97–104. https://doi.org/10.1007/s10329-003-0073-5

Taylor, C., Masao, F., Njau, J. K., Songita, A. V., and Hlusko, L. J. (2023). OH 89: A newly described ∼1.8-million-year-old hominid clavicle from Olduvai Gorge. bioRxiv, 526656, ver. 6 peer-reviewed by PCI Paleo. https://doi.org/10.1101/2023.02.02.526656

The Baenidae form a diverse extinct clade of exclusively North American paracryptodiran turtles known from the Early Cretaceous to the Eocene (Hay, 1908; Gaffney, 1972; Joyce and Lyson, 2015). Their fossil record was recently extended down to the Berriasian-Valanginian (Joyce et al. 2020), but the group probably originates in the Late Jurassic because it is usually retrieved as the sister group of Pleurosternidae in phylogenetic analyses. However, baenids only became abundant during the Late Cretaceous, when they are restricted in distribution to the western United States, Alberta and Saskatchewan (Joyce and Lyson, 2015).

During the Campanian, baenids are abundant in the northern (Alberta, Montana) and southern (Texas, New Mexico, Utah) parts of their range, but in the middle part of this range they are mostly represented by poorly diagnosable shell fragments. In their new contribution, Wu et al. (2023) describe a new articulated baenid specimen from the Campanian Mesaverde Formation of Wyoming. Despite its poor preservation, they are able to confidently assign this partial shell to Neurankylus sp., hence definitively confirming the presence of baenids and Neurankylus in this formation. Incidentally, this new specimen was found in a non-fluvial depositional environment, which would also confirm the interpretation of Neurankylus as a pond turtle (Hutchinson and Archibald, 1986; Sullivan et al., 1988; Wu et al., 2023; see also comments from the second reviewer).

The study of Wu et al. (2023) also includes a detailed account of the state of the fossil when it was discovered and the subsequent extraction and preparation procedures followed by the team. This may seem excessive or out of place to some, but I agree with the authors that such information, when available, should be more commonly integrated into scientific articles describing new fossil specimens. Preparation and restoration can have a significant impact on the perceived morphology. This must be taken into account when working with fossil specimens. The chemicals or products used to treat, prepare, or consolidate the specimens are also important information for long-term curation. Therefore, it is important that such information is recorded and made available for researchers, curators, and preparators.

References

Gaffney, E. S. (1972). The systematics of the North American family Baenidae (Reptilia, Cryptodira). Bulletin of the American Museum of Natural History, 147(5), 241–320.

Hay, O. P. (1908). The Fossil Turtles of North America. Carnegie Institution of Washington, Washington, D.C. https://doi.org/10.5962/bhl.title.12500

Hutchison, J. H., and Archibald, J. D. (1986). Diversity of turtles across the Cretaceous/Tertiary boundary in Northeastern Montana. Palaeogeography, Palaeoclimatology, Palaeoecology, 55(1), 1–22. https://doi.org/10.1016/0031-0182(86)90133-1

Joyce, W. G., and Lyson, T. R. (2015). A review of the fossil record of turtles of the clade Baenidae. Bulletin of the Peabody Museum of Natural History, 56(2), 147–183. https://doi.org/10.3374/014.058.0105

Joyce, W. G., Rollot, Y., and Cifelli, R. L. (2020). A new species of baenid turtle from the Early Cretaceous Lakota Formation of South Dakota. Fossil Record, 23(1), 1–13. https://doi.org/10.5194/fr-23-1-2020

Sullivan, R. M., Lucas, S. G., Hunt, A. P., and Fritts, T. H. (1988). Color pattern on the selmacryptodiran turtle Neurankylus from the Early Paleocene (Puercan) of the San Juan Basin, New Mexico. Contributions in Science, 401, 1–9. https://doi.org/10.5962/p.241286

Wu, K. Y., Heuck, J., Varriale, F. J., and Farke, A. (2023). A baenid turtle shell from the Mesaverde Formation (Campanian, Late Cretaceous) of Park County, Wyoming, USA. PaleorXiv, uk3ac, ver. 5, peer-reviewed and recommended by Peer Community In Paleontology. https://doi.org/10.31233/osf.io/uk3ac

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

Arthropods constitute 85% of all described animal species on Earth (Brusca et al., 2016), being the most successful animal phylum at present. This phylum did not even have a shabby beginning. The fossil record shows that they were dominating the sea even in the early Cambrian (Caron and Jackson, 2008; Zhao et al., 2014; Fu et al., 2019). This planet is and has been indeed dominated by arthropods. Within the context of the Big Bang of animal evolution known as the Cambrian Explosion, delving into the origin of arthropods has been one of the all-time fascinating research themes in paleontology, and discussions on the ‘origin’ and ‘early evolution’ of arthropods from the paleontological perspective have not been infrequent (e.g., Budd and Telford, 2009; Edgecombe and Legg, 2014; Daley et al., 2018; Edgecombe, 2020).

In this context, Aria (2021) provides an interesting integration of his view on arthropod origin and early evolution. Based on well-preserved Burgess Shale materials, together with other impressive researches mainly with Jean-Bernard Caron (e.g., Aria and Caron, 2015; Caron and Aria, 2017), Cédric Aria made his name known with papers searching for the stem-groups of the two major euarthropod lineages, the Mandibulata and the Chelicerata (Aria and Caron, 2017, 2019), for which (and for his subsequent researches) assembling a large dataset for arthropod phylogeny was inevitable. Subsequently, there have been several major discoveries which could improve our understanding on early arthropod evolution (e.g., Lev and Chipman, 2020; Liu et al., 2020; Zeng et al., 2020). For Cédric Aria, therefore, this timely presentation of his own perspective on the origin of early evolution of arthropods may have been preordain.

This review stands out because it discusses almost all aspects of arthropod origin and early evolution that can be possibly covered by paleontology (many, if not all, of which are still controversial). Some of his views may be considered a brave attempt. For instance, based on the widespread occurrences of suspension feeders, Aria (2021) proposes the early Cambrian “planktonic revolution,” which has been associated rather with the Great Ordovician Biodiversification Event (Servais et al., 2016). Given the presence of lophotrochozoans and echinoderms in which the presence of planktonic larvae was likely to have been one of the most inclusive features, the “early Cambrian planktonic revolution” might be plausible, but I wonder if including arthropods into the “earlier revolution” can be readily acceptable. Arthropods arose from direct-developing ancestors, and crustaceans (the main group with planktonic larvae) are pretty much derived in the arthropod phylogenetic tree. Trilobites, inarguably the best-studied Cambrian arthropods, for example, began with direct-developing benthic protaspides in the early Cambrian and Miaolingian. The first hint of planktonic protaspides appeared in the Furongian, and it was not until the Tremadocian when such indirect developing protaspides began to be widespread (Park and Kihm, 2015), which complies well with the onset of the ‘Ordovician Planktonic Revolution’ in the Furongian, as suggested by Servais et al. (2016).

But in general, this review presents well-organized views worth hearing, and since many of the points are subject to debate, this review could be a friendly manual for those who have similar views, while it could form a fresh ground to attack for those who have disparate views. One of the endless debates in arthropod research comes from the arthropod head problem (Budd, 2002), which centers on the homology of frontal-most appendages in radiodonts, megacheirans, chelicerates, and mandibulates, as well as on the hypostome-labrum complex. Based on recent interesting discoveries of early arthropods from the Chengjiang biota (Aria, 2020; Zeng et al., 2020), Aria (2021) pertinently coined a term ‘cheirae’ for the frontalmost prehensile appendages of radiodonts and megacheirans, implying homology of them. I assume that not all researchers would agree with this though, as well as with his model of hypostome/labrum complex evolution. Notorious disaccords have also occurred around the phylogenetic positions of early arthropods, such as isoxyids, megacheirans, fuxianhuiids, and artiopodans; different research groups have invariably come up with different topologies. Cédric Aria has presented his own topologies (Aria, 2019, 2020), and figure 2 of Aria (2021) summarizes his perspective very well in combination with major character evolutions.

What makes this review especially interesting is a courageous discussion about the origin of biramous appendage, a subject that has been only briefly discussed or overlooked in recent literature. In the current mainstream of this debate lies the concept of ‘gilled lobopodians’ (Budd, 1998), which was complicated by the weird two separate rows of lateral flaps of Aegirocassis (Van Roy et al., 2015). Aria (2021) adds an interesting alternative scenario to this: biramicity originated from the split of main limb axis, as seen in the isoxyid Surusicaris (Aria and Caron, 2015). The figure 3d of his review, therefore, is worth giving thoughts for any arthropodologists who are interested in the origin of arthropod legs.

It is true that our understanding of the origin and early evolution of arthropods is still in a mist. Nevertheless, we have recently seen advancements, such as those aided by new types of analysis (Liu et al., 2020), the discoveries of new early arthropods with unexpected morphologies (Aria et al., 2020; Zeng et al., 2020), and the groundbreaking Evo-Devo research (Lev and Chipman, 2020). We will keep jousting on various aspects of the origin and early evolution of arthropods, but for some aspects we are seemingly heading for the final, as implicitly alluded in Aria (2021).

References

Aria C (2019). Reviewing the bases for a nomenclatural uniformization of the highest taxonomic levels in arthropods. Geological Magazine 156, 1463–1468.

Aria C (2020). Macroevolutionary patterns of body plan canalization in euarthropods. Paleobiology 46, 569–593. doi: 10.1017/pab.2020.36

Aria C (2021). The origin and early evolution of arthropods. PaleorXiv, 4zmey, ver. 4, peer-reviewed by PCI Paleo. doi: 10.31233/osf.io/4zmey

Aria C and Caron JB (2015). Cephalic and limb anatomy of a new isoxyid from the Burgess Shale and the role of "stem bivalved arthropods" in the disparity of the frontalmost appendage. PLOS ONE 10, e0124979. doi: 10.1371/journal.pone.0124979

Aria C and Caron JB (2017). Burgess Shale fossils illustrate the origin of the mandibulate body plan. Nature 545, 89–92.

Aria C and Caron JB (2019). A middle Cambrian arthropod with chelicerae and proto-book gills. Nature 573, 586–589. doi: 10.1038/s41586-019-1525-4

Aria C, Zhao F, Zeng H, Guo J, and Zhu M (2020). Fossils from South China redefine the ancestral euarthropod body plan. BMC Evolutionary Biology 20, 4.

Brusca RC, Moore W, and Shuster SM (2016). Invertebrates. Third edition. Sunderland, Massachusetts U.S.A: Sinauer Associates. isbn: 978-1-60535-375-3

Budd GE (1998). Stem-group arthropods from the Lower Cambrian Sirius Passet fauna of North Greenland. In: Arthropod Relationships. Ed. by Fortey RA and Thomas RH. London, UK: Chapman & Hall, pp. 125–138.

Budd GE (2002). A palaeontological solution to the arthropod head problem. Nature 417, 271–275. doi: 10.1038/417271a

Budd GE and Telford MJ (2009). The origin and evolution of arthropods. Nature 457, 812–817. doi: 10.1038/Nature07890

Caron JB and Aria C (2017). Cambrian suspension-feeding lobopodians and the early radiation of panarthropods. BMC Evolutionary Biology 17, 29. doi: 10.1186/s12862-016-0858-y

Caron JB and Jackson DA (2008). Paleoecology of the Greater Phyllopod Bed community, Burgess Shale. Palaeogeography, Palaeoclimatology, Palaeoecology 258, 222–256. doi: 10.1016/j.palaeo.2007.05.023

Daley AC, Antcliffe JB, Drage HB, and Pates S (2018). Early fossil record of Euarthropoda and the Cambrian Explosion. Proceedings of the National Academy of Sciences of the United States of America 115, 5323–5331. doi: 10.1073/pnas.1719962115

Edgecombe GD (2020). Arthropod origins: Integrating paleontological and molecular evidence. Annual Review of Ecology, Evolution, and Systematics 51, 1–25. doi: 10.1146/annurev-ecolsys-011720-124437

Edgecombe GD and Legg DA (2014). Origins and early evolution of arthropods. Palaeontology 57, 457–468.

Fu D, Tong G, Dai T, Liu W, Yang Y, Zhang Y, Cui L, Li L, Yun H, Wu Y, Sun A, Liu C, Pei W, Gaines RR, and Zhang X (2019). The Qingjiang biota—A Burgess Shale–type fossil Lagerstätte from the early Cambrian of South China. Science 363, 1338–1342. doi: 10.1126/science.aau8800

Lev O and Chipman AD (2020). Development of the pre-gnathal segments of the insect head indicates they are not serial homologues of trunk segments. bioRxiv, 2020.09.16.299289. doi: 10.1101/2020.09.16.299289

Liu Y, Ortega-Hernández J, Zhai D, and Hou X (2020). A reduced labrum in a Cambrian great-appendage euarthropod. Current Biology 30, 3057–3061.e2. doi: 10.1016/j.cub.2020.05.085

Park TYS and Kihm JH (2015). Post-embryonic development of the Early Ordovician (ca. 480 Ma) trilobite Apatokephalus latilimbatus Peng, 1990 and the evolution of metamorphosis. Evolution & Development 17, 289–301. doi: 10.1111/ede.12138

Servais T, Perrier V, Danelian T, Klug C, Martin R, Munnecke A, Nowak H, Nützel A, Vandenbroucke TR, Williams M, and Rasmussen CM (2016). The onset of the ‘Ordovician Plankton Revolution’ in the late Cambrian. Palaeogeography, Palaeoclimatology, Palaeoecology 458, 12–28. doi: 10.1016/j.palaeo.2015.11.003

Van Roy P, Daley AC, and Briggs DEG (2015). Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps. Nature 522, 77–80. doi: 10.1038/nature14256

Zeng H, Zhao F, Niu K, Zhu M, and Huang D (2020). An early Cambrian euarthropod with radiodont-like raptorial appendages. Nature 588, 101–105. doi: 10.1038/s41586-020-2883-7

Zhao F, Caron JB, Bottjer DJ, Hu S, Yin Z, and Zhu M (2014). Diversity and species abundance patterns of the Early Cambrian (Series 2, Stage 3) Chengjiang Biota from China. Paleobiology 40, 50–69. doi: 10.1666/12056

Today’s birds are divided into two deeply divergent and historically well-documented groups: Palaeognathae and Neognathae. Palaeognaths include both the flight-capable tinamous as well as the flightless ratites (ostriches, rheas, kiwis, cassowaries, and kin). Neognaths include all other modern birds, ranging from sparrows to penguins to hummingbirds. The clade names refer to the anatomy of the palate, with the “old jaws” (palaeognaths) originally thought to more closely resemble an ancestral reptilian condition and the “new jaws” (neognaths) showing a uniquely modified bony configuration. This particularly manifests in the pterygoid-palatine complex (PPC) in the palate, formed from pairs of pterygoids and palatines alongside a single midline vomer. In palaeognaths, the vomer is comparatively large and the pterygoid and palatine are relatively tightly connected. The PPC is more mobile in neognaths, with a variably shaped vomer, which is sometimes even absent. Although both groups of birds show cranial kinesis, neognaths exhibit a much more pronounced degree of kinesis versus palaeognaths, due in part to the tighter nature of the palaeognath pterygoid/palatine interfaces.

A previous paper (Hu et al. 2019) used 3D geometric morphometrics to compare the shape of the vomer across neognaths and palaeognaths. Among other findings, this work suggested that each clade had a distinct vomer morphology, with palaeognaths more similar to the ancestral condition (i.e., that of non-avian dinosaurs). This observation was extended to support inferences of limited vs. less limited cranial kinesis in various extinct species, based in part on observations of vomer shape. A new preprint by Plateau and Foth (2021) presents a reanalysis of Hu et al.’s data, specifically focusing on allometric effects. In short, the new analysis looks at how size correlates (or doesn't correlate) with vomer shape. 

Plateau and Foth (2021) found that when size effects are included, differences between palaeognaths and neognaths are less than the “raw” (uncorrected) shape data suggest. It is much harder to tell bird groups apart! Certainly, there are still some general differences, but some separations in morphospace close up when allometry—the interrelationship between shape and size—is considered. Plateau and Foth (2021) use this finding to suggest that 1) vomer shape alone is not a completely reliable proxy for inferring the phylogenetic affinities of a particular bird; and 2) the vomer is only one small component of the cranial kinetic system, and thus its shape is of limited utility for inferring cranial kinesis capabilities when considered independently from the rest of the relevant skull bones.

References

Hu, H., Sansalone, G., Wroe, S., McDonald, P. G., O’Connor, J. K., Li, Z., Xu, X., & Zhou, Z. (2019). Evolution of the vomer and its implications for cranial kinesis in Paraves. Proceedings of the National Academy of Sciences, 116(39), 19571–19578. doi: 10.1073/pnas.1907754116

Plateau, O., & Foth, C. (2021). The impact of allometry on vomer shape and its implications for the taxonomy and cranial kinesis of crown-group birds. BioRxiv, 184101, ver. 3 peer-reviewed by PCI Paleo. doi: 10.1101/2020.07.02.184101