Non-technical Summary
Zooplankton are a crucial part of the modern ocean ecosystem, and multiple lineages of arthropods have convergently evolved to occupy this niche. But assessing whether an extinct species was pelagic is difficult and relies on its being found in different environments, across a broad geographic range and morphology similar to that of a modern planktonic organism. Adult pelagic trilobites are thought to have gone extinct at the end of the Ordovician Period; however, some authors have suggested that the unusual “bubble-headed” morphology of Deiphon may be an adaptation for this lifestyle. This interpretation has been contentious, with other researchers suggesting a benthic lifestyle. The sedimentological and paleogeographic information for Deiphon is sparse, because it is not an abundant fossil, making morphology the most informative proxy for understanding lifestyle. Here, we use 3D modeling of Deiphon in conjunction with information about its occurrence and computational fluid dynamics to test the hypothesis of its lifestyle. The glabellar bubble has been proposed to contain either low-density fluids such as lipids to aid in buoyancy or an expanded gut. We show that the presence of the glabellar bubble increases buoyancy of the body regardless of material inside it. Density estimates of Deiphon show the importance of exoskeleton thickness, with the two thinnest treatments resulting in specific gravities similar to those of modern plankton. The most likely thickness of exoskeleton and lipid-filled glabellar bubble suggests Deiphon was an armored hoverer planktonic organism with adaptations to decrease density and increase surface area to slow sinking in the water. Our work shows an adult pelagic trilobite during the Silurian Period and highlights the need for similar work to understand the paleoecology of other species.
Introduction
Zooplankton are an important part of the modern water column, occupying low trophic levels in marine food webs and forming an integral part of the biological pump (e.g., Turner Reference Turner2015; Steinberg and Landry Reference Steinberg and Landry2017). To remain in the water column, zooplankton employ a range of strategies to maintain their buoyancy, including reducing their density through incorporating light materials such as lipids into their bodies and generating lift through swimming (e.g., Alexander Reference Alexander1990; Vogel Reference Vogel1996; Campbell and Dower Reference Campbell and Dower2003). Zooplankton strategies can be divided into several categories: gelatinous floaters; muscle swimmers; and armored hoverers (Tsukamoto et al. Reference Tsukamoto, Yamada, Okamura, Kaneko, Tanaka, Miller, Horie, Mikawa, Utoh and Tanaka2009). We follow traditional pelagic terminology herein (e.g., Fortey Reference Fortey1985), but Rigby and Milsom (Reference Rigby and Milsom2000) noted planktonic categories include animals that can swim against the prevailing current, which would not be considered plankton in the strict terms often used by paleontologists.
Modern plankton use different strategies to remain in the water column. Gelatinous floaters, with a large size range from 1 to 2 mm in the cnidarian Muggiaea to >100 mm in eel larvae, maintain neutral buoyancy with the seawater surrounding them (Tsukamoto et al. Reference Tsukamoto, Yamada, Okamura, Kaneko, Tanaka, Miller, Horie, Mikawa, Utoh and Tanaka2009). Muscle swimmers, which can also be very small (1–2 mm, such as copepods) and do not reach similar lengths as gelatinous floaters (maximum sizes around 50 mm, such as lobster larvae), have a higher density than their surrounding water, using swimming to generate lift and to maintain their position in the water column (Tsukamoto et al. Reference Tsukamoto, Yamada, Okamura, Kaneko, Tanaka, Miller, Horie, Mikawa, Utoh and Tanaka2009). Some muscle swimmers are vertical migrants, moving from deeper areas to surface waters to feed during the night (see Bianchi and Mislan Reference Bianchi and Mislan2016). Armored hoverers generally have the smallest body sizes (from 1 to 25 mm, rarely up to 50 mm), and often have mineralized exoskeletons (e.g., pteropods; Hunt et al. Reference Hunt, Pakhomov, Hosie, Siegel, Ward and Bernard2008). These animals have the greatest difference in density with the surrounding water and use swimming appendages to counteract their downward sinking. They sink slowly, due to their small size, meaning that they experience seawater as a viscous fluid, and some have large surface areas to provide additional drag to further reduce sinking speeds (Tsukamoto et al. Reference Tsukamoto, Yamada, Okamura, Kaneko, Tanaka, Miller, Horie, Mikawa, Utoh and Tanaka2009).
Many euarthropod groups have entered the pelagic realm over the Phanerozoic and are an integral part of the modern zooplankton (Perrier et al. Reference Perrier, Williams and Siveter2015). Inference of a pelagic mode of life in fossil zooplankton requires combining evidence from sedimentology (the animal is found across a range of sediment types), geography (the animal is found across a wide area), and morphology (the animal shares morphological features important for a pelagic mode of life with modern analogues) (e.g., Fortey Reference Fortey1985). Functional morphology is an important line of evidence in determining the lifestyle of trilobites, such as the fusiform body with enlarged globular eyes in Opipeuter and Carolinites (Fortey Reference Fortey1985). During the Paleozoic, examples of euarthropod muscle swimmers include Isoxys longissimus, using its carapace shape to generate lift (Pates et al. Reference Pates, Daley, Legg and Rahman2021) and large suspension-feeding radiodonts such as Aegirocassis (Perrier et al. Reference Perrier, Williams and Siveter2015; Van Roy et al. Reference Van Roy, Daley and Briggs2015), and examples of euarthropod armored hoverers include trilobite larvae, whose spherical shape and elongate spines are interpreted to have slowed sinking speeds (Laibl et al. Reference Laibl, Saleh and Pérez-Peris2023). Adult trilobites can be considered as both muscle swimmers and armored hoverers (“well streamlined” and “poorly streamlined” groups of Fortey Reference Fortey1985). Well streamlined trilobites are generally larger (up to 100 mm) than the poorly streamlined groups (20–40 mm) and are interpreted to have generated lift through swimming, while the poorly streamlined groups may have utilized elongate genal spines to maintain stability (Fortey Reference Fortey1985) and, by analogy with armored hoverers, slow their rate of sinking.
In the modern literature, pelagic trilobites are considered to be restricted to the late Cambrian and Ordovician periods (e.g., Fortey Reference Fortey1985; Adrain et al. Reference Adrain, Edgecombe, Fortey, Owen and McCormick2004). Repeated evolution of a pelagic mode of life in trilobites from this time period allowed them to take advantage of food sources and habitats and made them a key link in the early Paleozoic biological pump as zooplankton (Perrier et al. Reference Perrier, Williams and Siveter2015). Computational fluid dynamics (CFD) analyses have been conducted on several trilobite species showing a range of lifestyles, including the role of enrollment in swimming (e.g., Microparia speciosa; Esteve and López-Pachón Reference Esteve and López-Pachón2023), queuing behavior (e.g., Ampyx priscus; Trenchard et al. Reference Trenchard, Brett and Perc2017; Song et al. Reference Song, Song, Rahman and Chu2021; Wang et al. Reference Wang, Edgecombe and Hou2024; Trenchard et al. Reference Trenchard, Brett and Perc2025), nektobenthic swimming (e.g., Hypodicranotus striatus; Shiino et al. Reference Shiino, Kuwazuru, Suzuki and Ono2012, Reference Shiino, Kuwazuru, Suzuki, Ono and Masuda2014), hopping (e.g., Placoparia cambriensis; Esteve et al. Reference Esteve, López, Ramírez and Gómez2021), and demonstrating sensory function of the anterior glabellae (Gómez-Rodríguez et al. Reference Gómez-Rodríguez, López-Pachón and Esteve2025).
While the literature since the 1980s considers adult trilobites living in the water column to be restricted to the late Cambrian and Ordovician periods (e.g., Fortey Reference Fortey1985; Adrain et al. Reference Adrain, Edgecombe, Fortey, Owen and McCormick2004), some historic studies from the early 1900s suggested that some post-Ordovician bubble-headed trilobites, including Deiphon, lived in the water column (Fig. 1). The glabellar bubble of this animal was inferred to have been filled with a low-density material (Dollo Reference Dollo1910; Staff and Reck Reference Staff and Reck1911; Ruedemann Reference Ruedemann1934; Whittard Reference Whittard1934), such as lipids known from across modern arthropod groups (e.g., Campbell and Dower Reference Campbell and Dower2003). Giant postembryonic stages of the trilobites Hydrocephalus and Eccaparadoxides may have stored lipids beneath the highly inflated glabella, similar to modern crustaceans (Anger Reference Anger2001; Laibl et al. Reference Laibl, Esteve and Fatka2017). Further morphological support comes from the reduced body size relative to the head, suggested as a way to increase the surface area compared to the weight with the development of spines, and thus slow down sinking (Whittard Reference Whittard1934). Thus, this historically inferred lifestyle is comparable to that of armored hoverers in the modern oceans. Deiphon was reinterpreted as a benthic animal following measurement of its exoskeleton thickness at the base of its genal spines (0.30–0.35 mm), which was considered too thick to facilitate low enough buoyancy for a pelagic lifestyle (Holloway and Campbell Reference Holloway and Campbell1974). These authors also considered it unlikely that the bubble contained lipids, as direct evidence for this would not be preserved in fossils (Holloway and Campbell Reference Holloway and Campbell1974). No direct evidence for lipid storage within the expanded glabellae has been found. As the bubble in Deiphon is an expansion of the glabella, it is possible this reflects an enlargement of the gut or crop (Fortey and Owens Reference Fortey and Owens1997), although the digestive system is not known in this taxon. There is a broad disparity in the cephalic and thoracic morphology across the polyphyletic grouping of trilobites considered to have “bubble-head” morphologies, and it cannot be assumed that all lived in similar ways (Fortey and Owens Reference Fortey and Owens1997). The olenid Parabalinella bolbifrons exhibits an inflated preglabellar field (Fortey and Owens Reference Fortey and Owens1997), which may be a sexually dimorphic trait to create a brood pouch in adult females (Fortey and Hughes Reference Fortey and Hughes1998). Further progress in interpreting thin-section slides and 3D micro-CT scans have allowed us to gain a much better understanding of trilobite morphologies in 3D, and thus allow us to more accurately estimate their densities (Losso et al. Reference Losso, Affatato, Nanglu and Ortega-Hernández2023; El Albani et al. Reference El Albani, Mazurier, Edgecombe, Azizi, Bakhouch, Berks and Bouougri2024; Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024). Indeed, trilobite exoskeleton thickness varies significantly across the body and is often thickest near the doublure (Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024), making it likely the exoskeletal thickness of Deiphon was much less than 0.3 mm for most of its body.
Deiphon from the Ordovician of Laurentia and Avalonia with a highly inflated glabella. A–C, Deiphon longifrons (USNM PAL 255910). A, Dorsal view. B, Anterior view. C, Lateral view. D–F, Deiphon dikella (CAMSM A.3507). D, Dorsal view. E, Anterior view. F, Lateral view. G, H, hypostome of Deiphon longifrons (USNM PAL 405426). G, Ventral view. H, Anterior view.
Figure 1. Long description
The image consists of eight panels labeled A through H.
* Panels A, B, and C show Deiphon longifrons (U S N M PAL 255910) with a 5 mm scale bar. A is a dorsal view showing a large, spherical, bulbous glabella protruding from the head. B is an anterior view showing the glabella centered between two small, lateral eye stalks. C is a lateral view showing the glabella’s significant forward projection.
* Panels D, E, and F show Deiphon dikella (C A M S M A dot 3507) with a 10 mm scale bar. D is a dorsal view showing the glabella and the segmented thorax and pygidium. E is an anterior view, and F is a lateral view, both highlighting the inflated glabella relative to the flatter body.
* Panels G and H show the hypostome of Deiphon longifrons (U S N M PAL 405426) with a 1 mm scale bar. G is a ventral view showing a shield-like structure with a central oval protrusion. H is an anterior view showing the low profile of the hypostome structure.
Here, in light of the limited sedimentological and geographic information available for Deiphon, we combine volume and density estimates with 3D CFD to evaluate its lifestyle. We test the hypothesis that Deiphon lived as an armored hoverer in the Silurian oceans, providing a robust estimate of its density and assessing its stability in the water column.
Methods
Occurrence
Deiphon occurrences were downloaded from the Paleobiology Database (PBDB, www.paleobiodb.org) on 28 March 2025 and again on 14 April 2026 to determine the range of lithologies from which Deiphon has been recovered. The search term ‘Deiphon’ was used in the “Taxon or Taxa to Include” search box, using the latest identification, all taxonomic resolution and applying no modifiers or filters (search URL: http://paleobiodb.org/data1.2/occs/list.txt?datainfo&rowcount&base_name=deiphon&limit=100). All occurrences were vetted with the peer-reviewed literature (Supplementary Table 1). Lithology data from “lithology 1” and “lithology 2” and depositional environmental data from the “environment” were simplified following Bault et al. (Reference Bault, Balseiro, Monnet and Crônier2022), in order to link lithologies to water depth and distance from shore. Information about lithology and environment was checked with the published literature, and references are included in Supplementary Table 1.
Model Creation
We used photogrammetry of Deiphon dikella specimen CAMSM A.3507, housed at the Sedgwick Museum of Earth Sciences (Cambridge, U.K.; CAMSM), to produce a 3D model (Fig. 1D–F) using Agisoft Metashape 1 (www.agisoft.com). A 3D reconstruction of the exoskeleton of D. dikella (Fig. 2A) was created based on CAMSM A.3507 using Shapr3D (https://www.shapr3d.com). To test the impact of the glabellar bubble on the buoyancy of Deiphon, two models were created, one with a highly inflated bubble and the other with a glabella similar to Ceraurus pleurexanthemus, selected as a closely related species of similar size. Non-biomineralized tissues based on closely related other cheirurid trilobites housed at the Museum of Comparative Zoology at Harvard University (Cambridge, Mass., U.S.A.; MCZ) including Ceraurus pleurexanthemus (MCZ:IP:110933 and MCZ:IP:112018) and Anacheirurus adserai (MCZ:IP:202508) were added to both models, also using Shapr3D.
Volume estimates of exoskeleton and non-biomineralized tissues in trilobites and horseshoe crabs. A–E, Volume estimates of Deiphon dikella based on A3507. A, Model of exoskeleton morphology. B, Internal volume of glabellar bubble. C, Volume of body estimated as a cylinder with a diameter the width of the axial lobe. D, Volume of the gut tract estimated as cylinder occupying 20% of the axial lobe width. E, Volume estimate of biramous appendage with wedge-shaped protopodite, cylindrical podomeres and cuboid exopodite. F, Pie chart showing volume of each body part for Deiphon. G–I, A 3D reconstruction of Limulus polyphemus MCZ:IP:41112. G, Dorsal view of exoskeleton. H, Ventral view of cephalothoracic appendages. I, Ventral view of gills. J, Ventral view of body lumen. K, Pie chart showing volume of each body part for Limulus.
Figure 2. Long description
The diagram is divided into two main sections.
Top section (A through F) focuses on Deiphon dikella:
* A: A grey 3 D model of the exoskeleton with a large spherical glabella and long lateral spines.
* B: A pink sphere representing the internal glabellar bubble volume.
* C: A pink cylinder representing the estimated body volume.
* D: A thin purple cylinder representing the gut tract volume.
* E: Yellow geometric shapes (wedges, cylinders, and cuboids) representing biramous appendage components.
* F: A pie chart for Deiphon showing volume percentages: bubble at 41 percent, limbs at 35 percent, body at 13 percent, and exo at 11 percent.
Bottom section (G through K) focuses on Limulus polyphemus:
* G: A grey dorsal view of the horseshoe crab exoskeleton.
* H: A brown 3 D reconstruction of the ventral cephalothoracic appendages.
* I: A brown 3 D reconstruction of the ventral gills.
* J: A pink 3 D model of the internal body lumen.
* K: A pie chart for Limulus showing volume percentages: exo at 40 percent, body at 26 percent, legs at 22 percent, and gill at 12 percent.
A legend at the bottom identifies color-coded categories: grey for Exoskeleton, orange for Appendages, pink for Body, and purple for Gut.
Volume Estimates
It is important to estimate density of Deiphon based on the individual properties and volumes of each tissue. In the modern Atlantic horseshoe crab Limulus polyphemus, the exoskeleton occupies the largest volume (40%), compared with the body (26%), endopodites (22%), and gills (12%) (Fig. 2G–K). The surface area of the exoskeleton without the inflated glabella was estimated by finding the area of librigena, genal spine, half of the thorax, and pygidium in ImageJ (Schneider et al. Reference Schneider, Rasband and Eliceiri2012) and multiplying by two. Deiphon has an originally poorly vaulted exoskeleton excluding the glabella (Fig. 1D–F). The exoskeletal thickness of Deiphon is unknown for the majority of the body, and so four thicknesses were used in order to explore the importance of this feature for the overall density of Deiphon. Four values of exoskeleton thickness were used to capture the possible range, 0.008 mm, 0.1 mm, 0.175 mm, and 0.3 mm, drawn from comparisons with closely related cheirurid C. pleurexanthemus and trilobites from the Ordovician of Spitsbergen, which show a broad range in exoskeletal thicknesses across Trilobita (Fortey and Wilmot Reference Fortey and Wilmot1991; Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024). The volume of the exoskeleton without the inflated glabella was calculated by multiplying the area by each potential thickness (Fig. 2, Supplementary Table 1). The inflated glabella exoskeleton volume was found by creating internal spheres by subtracting the exoskeleton thickness from the measured size (Fig. 2, Supplementary Table 1). The volume of the inner sphere of the glabella was calculated by removing the exoskeleton thickness from the radius of the larger sphere (Fig. 2). With an uninflated glabella, an exoskeleton thickness of 0.3 mm would result in a structure completely filled with calcite with no space to be filled with lipids, seawater, or sediment. To account for this, the exoskeleton thickness for the glabella is adjusted to 0.25 mm, and only one data point is shown, as changing the material inside has no impact on specific gravity (SG). A complete sphere, rather than a hemisphere, was used for this volume, as the hypostome is also highly vaulted in Deiphon, creating a nearly spherical shape internally (Fig. 1 G,H).
The volume of non-biomineralized tissues was based on comparison with C. pleurexanthemus, in which the body cavity occupies a cylindrical space under the axial lobe that is as deep as it is wide (Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024). A thin membrane extends from the body cavity to the doublure, but is only as thick as the exoskeleton, so it was not included in our volume estimates. The axial lobe of A3507 is ca. 2.5 mm across and only slightly tapers posteriorly along the ca. 10 mm length, so a cylinder with a radius of 1.25 mm was used to estimate the volume of the body (Fig. 2). The gut was estimated to occupy 20% of the width of the axial lobe based on the cheirurid A. adserai MCZ:IP:202508 from the Fezouata Shale. The gut was modeled as a cylinder with a radius of 0.5 mm and length of 10 mm (Fig. 2, Supplementary Table 1).
The volumes of appendages were estimated based on the two cheirurids with known preserved limb morphology, A. adserai and C. pleurexanthemus (Pérez-Peris et al. Reference Pérez-Peris, Laibl, Vidal and Daley2021; Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024). Podomere numbering follows recent studies of extant arthropods showing a unified framework (Bruce and Patel Reference Bruce and Patel2020a, Reference Bruce and Patel2020b; Bruce Reference Bruce2021). The protopodite is wedge shaped in cross section, being sagittally broadest dorsally and tapering ventrally (Losso et al. Reference Losso, Affatato, Nanglu and Ortega-Hernández2023; Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024). A rectangular wedge was used to estimate the volume of the protopodite, 1.14 mm in width, 1.99 mm in height, and 3.19 mm in length (Fig. 2, Supplementary Table 1). Podomere dimensions vary between trilobite species (Pérez-Peris et al. Reference Pérez-Peris, Laibl, Vidal and Daley2021; Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2022, Reference Losso and Ortega-Hernández2024), but can be estimated as cylinders with decreasing diameters distally (Fig. 2). The proximal three podomeres were based on measurements from C. pleurexanthemus (MCZ:IP:110933 and MCZ:IP:112018; Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024), whereas the distal podomeres were determined by the proportions seen in A. adserai (Pérez-Peris et al. Reference Pérez-Peris, Laibl, Vidal and Daley2021; Supplementary Table 1). The exopodite was estimated as a cuboid the size of the exopodite articles, because the volume of the lamellae is miniscule. The length (3.62 mm) and width (1.16 mm) of the cuboid were based on A. adserai (Pérez-Peris et al. Reference Pérez-Peris, Laibl, Vidal and Daley2021) and the height (0.13 mm) on C. pleurexanthemus (Losso and Ortega-Hernández Reference Losso and Ortega-Hernández2024; Supplementary Table 1). Trilobite appendages decrease in size along the axis, with the posteriormost appendages of Olenoides serratus being 60% the volume of the anteriormost (Supplementary Table 1). Resulting estimates of volume of each structure are similar between Deiphon without an inflated glabella, C. pleurexanthemus, and A. adserai (Supplementary Fig. 1). O. serratus has a proportionally smaller body and larger appendages but is less closely related to Deiphon (Supplementary Fig. 1). L. polyphemus, has a substantially larger proportion of its body as exoskeleton than the trilobites measured in this study (Supplementary Fig. 1).
Density Estimates
The densities of the individual body parts (exoskeleton, interior of bubble, and soft anatomy) were required to estimate the density of Deiphon. For the exoskeleton, the density of calcite was used (2.711 g/cm3) (DeFoe and Compton Reference DeFoe and Compton1925). For the interior of the bubble, both lipids (0.91 g/cm3) (Lee and Joye Reference Lee and Joye2006) and seawater (1.03 g/cm3; from Engineering Tool Box, www.engineeringtoolbox.com) were used. For the gut, the density of seawater of 1.03 g/cm3 was used. Additionally, another set of calculations were produced using a sediment-filled gut with a density of 1.6 g/cm3 (Tenzer and Gladkikh Reference Tenzer and Gladkikh2014) to replicate a deposit-feeding strategy. While suitable for benthic animals feeding on the sediment (the alternative hypothesis for Deiphon), it also provides a higher density than likely gut contents for pelagic animals (water or soft tissues). It is unlikely that an animal that spent its adult life in the water column would have a sediment-filled gut.
For non-biomineralized tissues, comparisons to the density of the soft parts of modern crustaceans were used. These were calculated from the density and mass of types of material that make up the composition of arthropod soft tissues following processing to find their nutritional content: water, protein, lipid, and ash. The mass of water, protein (calculated as the nitrogen content multiplied by 6.25), lipid, and ash (density 0.61 g/cm3) for 13 shrimps, lobsters, and prawns were collected from the USDA website (fdc.nal.usda.gov; links for each individual entry given in the Supplementary Table 2). For older entries, no ash amount was given, and so the residual mass from each sample was used as the mass of ash. The mass of each material type (water, protein, lipid, ash) was converted to a volume using the known density of each material type (water: 1 g/cm3; lipids: 0.91 g/cm3; and proteins: 1.45 g/cm3) (values from Moya-Laraño et al. Reference Moya-Laraño, Macías-Ordóñez, Blanckenhorn and Fernández-Montraveta2008).
The sum of all the masses was used to calculate the total mass for each sample, and the sum of all the volumes was used to calculate the total volume for the sample. Equation 1 was then rearranged to find the overall density for the sample. The mean density across all 13 samples (1.026 g/cm3) was used as the density for the soft parts of Deiphon (Supplementary Table 2).
The estimated volumes for each body part (exoskeleton, bubble interior, gut, and soft parts) were multiplied by their estimated densities and divided by the total volume to give an overall density for the Deiphon model. The SG was calculated using eq. (2), for comparison to extant pelagic and planktonic animals.
Computational Fluid Dynamics
A 3D model of Deiphon based on A3507 was made in Shapr3D and exported as an .stl type. Mesh creation, geometry setup, and fluid simulations were performed using ANSYS Workbench and DesignModeler, Meshing, and Fluent components, all version 2024 R2 (https://www.ansys.com).
Models were imported into the ANSYS SpaceClaim environment, shrinkwrapped, and converted to solid objects of the proprietary ANSYS filetype .scdoc. SpaceClaim models were then imported into the ANSYS Workbench DesignModeler, and scaled to 0.02 m long, comparable to the length of fossil specimens (e.g., Fig. 1). A virtual flume tank of dimensions 0.05 × 0.05 × 0.4 m was constructed using sketch planes that were then converted to a solid and extruded in the Z dimension to create a 3D cuboid (Fig. 3A). These dimensions were chosen to be large enough so that the fluid flow would not interact with the margins of the tank, as the model was positioned 0.025 m from the sides of the tank, 0.1 m from the inlet and 0.3 m from the outlet. Sketch planes and the extrude function were also used to create a smaller cuboid 0.02 × 0.02 × 0.4 m inside the tank for the creation of a volume with smaller mesh elements during the meshing step.
Computational fluid dynamics (CFD) simulation results of prone Deiphon in water column. A, Geometry of virtual flume tank with dimensions, body of influence, and position of model. Blue arrows show inlet. B, Velocity field for CFD simulation using medium mesh (m/s). C, Pressure contours for CFD simulation using medium mesh (Pa). D, Magnification of velocity field around body (m/s). E, Magnification of pressure contours around body (Pa).
Figure 3. Long description
Panel A shows a 3D wireframe of a virtual flume tank measuring 0.4 m in length, 0.05 m in height, and 0.05 m in width. A smaller inner box, the body of influence, measures 0.4 m by 0.02 m by 0.02 m. Blue arrows on the right indicate the inlet, and a purple arrow on the left indicates the outlet. A small trilobite model is positioned in the right half of the tank.
Panel B displays the velocity field in meters per second using a teal-to-white gradient. The flow moves from right to left, with a uniform field of 0.075 m/s interrupted by the model, creating a wake. A color scale on the left ranges from 0 to 0.15 m/s.
Panel C shows pressure contours in Pascals. The field is mostly neutral grey, with a concentrated blue high-pressure zone at the front of the model and a red low-pressure zone behind it. The scale ranges from negative 6.49 to positive 5.01 Pa.
Panel D is a magnification of the velocity field around the body. Vector arrows show fluid accelerating over the dorsal surface and a turbulent wake forming behind the posterior, marked with a purple star.
Panel E is a magnification of the pressure contours, showing concentric isolines radiating from the head and body, with the highest pressure gradient localized at the leading edge of the cephalon.
CFD results are impacted by the mesh design, mesh convergence procedures, and boundary layer resolution, which can in turn impact interpretations of locomotion and lifestyle (López-Pachón and Marcé-Nogué Reference López-Pachón and Marcé-Nogué2025). To ensure accuracy of the results, a series of meshing controls were applied to create coarse, medium, and fine meshes, with the number of elements ranging from ~750,000 to ~3.1 million (Table 1). An Element size control was applied to the whole mesh to set the maximum size for a mesh element. The size varied across the coarse, medium, and fine meshes (Table 1). A Patch Conforming Method was applied to the whole geometry, to create a tetrahedral shape for mesh elements. A Body Sizing was applied to the smaller cuboid around the Deiphon model, extending to the anterior and posterior of the virtual flume tank, to set the maximum size for a mesh element close to the model and in its wake. Again, a range of sizes were applied, for the coarse, medium, and fine meshes respectively (Table 1). Face sizing was applied to the surface of the Deiphon model to capture the details of the model by setting a minimum face size for elements touching the model. Three different sizes were used, one each for coarse, medium, and fine meshes (Table 1). An Inflation layer was also applied to the surface of Deiphon model, to create small elements that capture the boundary layer close to the model. Twenty layers were used with the Smooth Transition inflation option, with a transition ratio of 0.272 and growth rate of 1.2.
Mesh sizes in computational fluid dynamics (CFD) simulations
Table 1. Long description
The table consists of four columns: an unlabeled parameter column, Coarse, Medium, and Fine.
* Element size: Coarse is 2 times 10 super minus 2 m; Medium is 1 times 10 super minus 2 m; Fine is 9 times 10 super minus 3 m.
* Body sizing: Coarse is 5.5 times 10 super minus 3 m; Medium is 2.5 times 10 super minus 3 m; Fine is 1.5 times 10 super minus 3 m.
* Face sizing: Coarse is 1.25 times 10 super minus 3 m; Medium is 5 times 10 super minus 4 m; Fine is 4 times 10 super minus 4 m.
* Total number of elements: Coarse is 749,795; Medium is 1,361,016; Fine is 3,092,750.
Fluid flow simulations were performed using ANSYS Fluent. The pressure-based solver with absolute velocity formation and steady state were used, with the viscous realizable k-epsilon turbulent solver, with enhanced wall treatment. The k-epsilon model is a two-equation model, based on transport equations for the turbulent kinetic energy (k) and dissipation rate (epsilon) (Launder and Spalding Reference Launder and Spalding1974). The realizable k-epsilon model differs from the original by modifying the transport equation for the dissipation rate and providing an alternative formula for the turbulent viscosity (Shih et al. Reference Shih, Liou, Shabbir, Yang and Zhu1995). Further details on the mathematics behind the models and their implementation in ANSYS is provided in the software documentation (ANSYS Academic Fluent, release 2024 R2)
The fluid was set to have the properties of water-liquid in the Fluent Database (density: 998.2 kg/m3; dynamic viscosity: 0.001003 kg/ms). The inlet velocity was set to 0.1 m/s, simulating the swimming speed for Deiphon, and the pressure outlet was set to zero gauge pressure.
Simulations were run for all three mesh types to ensure that interpretations were mesh independent. At least 100 iterations were run, and more iterations were run until the solution was converged if necessary. Simulations were treated as converged if residuals of all six equations (continuity, x-velocity, y-velocity, z-velocity, k, and epsilon) were all below 1 × 10−6. Although not interpreted, values for drag and lift force acting on the model were also collected to confirm that these were also stable and unchanged across at least 50 iterations for a simulation deemed converged.
The wall Y+, contours of pressure, and vectors of velocity were visualized for the surface of the Deiphon model (wall Y+) and along the z-axis through the midline of the Deiphon model (pressure, velocity) to ensure the Y+ was low enough for the k-epsilon model to provide reasonable results for the boundary layer, and to visualize the impact of the Deiphon model on the flow of water around the model.
Results
Occurrence
Specimens assigned to the genus Deiphon have been recovered from a broad range of environments, including shallow-marine, reefal, slope/basin, and the outer platform (Fig. 4A). There are slightly more occurrences of the genus in deeper water environments than shallow ones (Fig. 4A). Sixteen of the occurrences on the PBDB are only identified as marine. Deiphon specimens are found in nine different lithologies (Fig. 4B). The most common lithology is limestone, followed by shale. There are five occurrences each of Deiphon in dolomite, lime mudstone, and grainstone, three each in marl and sandstone, and two each within mudstone and siltstone (Fig. 4B). Deiphon globifrons is known from shallow and outer platform environments, while other species are restricted to a single environment (Supplementary Table 3).
Occurrence of Deiphon. A, Occurrence of Deiphon by depositional environment. B, Occurrence of Deiphon by lithology.
Figure 4. Long description
Panel A is a bar chart titled A. The Y-axis is labeled count with increments of 2.5 from 0.0 to 12.5. The X-axis lists depositional environments. From left to right, the bars show: shallow at 8, reefal at 8, slope forward slash basin at 10, outer platform at 12, and marine at 12. The bars transition in color from light cyan on the left to dark blue on the right.
Panel B is a bar chart titled B. The Y-axis is labeled count with increments of 5 from 0 to 20. The X-axis lists lithologies. From left to right, the bars show: limestone at 21, dolomite at 4, lime mudstone at 5, marl at 3, grainstone at 4, sandstone at 3, shale at 7, mudstone at 2, and siltstone at 1. The bars are colored in various shades of gray, tan, and brown.
Buoyancy in Deiphon
The SG of Deiphon is largely controlled by the estimated exoskeleton thickness, material within the glabellar bubble, and presence/absence of the bubble (Fig. 5). Filling the bubble with lipids results in a 0.06–0.08 SG decrease across the different exoskeleton thickness treatment groups. The thickest exoskeleton estimate, 0.3 mm, results in SG ranging from 1.284 to 1.327, with seawater resulting in the highest value (Fig. 5, Supplementary Table 4). The medium exoskeleton (0.175 mm) results in a more buoyant body with SG ranging from 1.163 to 1.212 (Fig. 3, Supplementary Table 4). The exoskeleton of 0.1 mm based on Ceraurus pleurexanthemus results in a body with SG ranging from 1.084 to 1.138 (Fig. 5, Supplementary Table 4). The thinnest exoskeleton (0.008 mm) leads to the most buoyant body, SG 0.981–1.040, with the lipid-filled bubble being more buoyant than surface seawater at 10°C (Fig. 5, Supplementary Table 4). Within the thick and medium exoskeleton treatment groups, the absence of a glabellar bubble results in higher SG (Fig. 5, Supplementary Table 4). The impact of the bubble is more nuanced with the thinnest exoskeleton. The lipid-filled bubble with the thinnest exoskeleton results in positive buoyancy compared with seawater (Fig. 5). The seawater-filled glabella results in similar neutral SG with or without a glabellar bubble (Fig. 5, Supplementary Table 4). In all cases, a sediment-filled glabellar bubble drastically increases the SG of Deiphon (Supplementary Fig. 2).
Buoyancy of Deiphon with bubble-shaped glabella and without.
Figure 5. Long description
A scatter plot with a vertical Y axis representing Specific gravity, ranging from 1.0 at the top to 1.5 at the bottom. The background is a gradient from light green at the top to deep blue at the bottom.
Data points are represented by trilobite icons of varying colors, descending diagonally from the top-left (low specific gravity) to the bottom-right (high specific gravity).
Two horizontal dashed lines near the top mark reference points. The upper line is labeled 10 degrees Celsius, 10 per mille, surface. The lower line is labeled 10 degrees Celsius, 10 per mille, 100 m.
Below the plot are four legend boxes categorized by shell thickness:
1. Thin 0.008 mm: Yellow and orange icons. Bubble variants use Lipid (pale yellow) or Salt water (bright yellow). No bubble variants use Lipid (light orange) or Salt water (dark orange).
2. Ceraurus 0.1 mm: Green icons. Bubble variants use Lipid (light green) or Salt water (mint). No bubble variants use Lipid (forest green) or Salt water (dark teal).
3. Medium 0.175 mm: Purple icons. Bubble variants use Lipid (light purple) or Salt water (magenta). No bubble variants use Lipid (dark purple) or Salt water (deep indigo).
4. Thick 0.3 mm: Red and brown icons. Bubble variants use Lipid (pink) or Salt water (rose). No bubble variant is shown as a single dark brown square.
The thick and medium exoskeletons without a glabellar bubble result in specific gravities that are lower than those of modern plankton (Figs. 5, 6). The groups with the two thinnest exoskeletons and glabellar bubble filled with lipids all fall within or near SG range of modern plankton (Figs. 5, 6). Thick and medium exoskeletons with a glabellar bubble fall within the range of armored hoverers, but those organisms are much smaller (Figs. 5, 6). The impact of seawater or sediment filling the gut is minimal (Supplementary Fig. 2), resulting in similar SG in all cases, because the volume of the gut is so small compared with the volume of the trilobite.
Specific gravity (SG)compared with body length of planktonic metazoans. Planktonic data were digitized from Tsukamoto et al. (Reference Tsukamoto, Yamada, Okamura, Kaneko, Tanaka, Miller, Horie, Mikawa, Utoh and Tanaka2009) and benthic data were added from Spaargaren (Reference Spaargaren1979). Only values for Deiphon with 0.1-mm-thick exoskeleton are shown, as this is the most accurate (see discussion).
Figure 6. Long description
A scatter plot with a horizontal X axis representing Body length in m m from 0 to 225 and a vertical Y axis representing Specific gravity S G from 1.0 at the top to 1.4 at the bottom.
Data points are categorized by color and shape:
* Gelatinous floaters (light blue circles) are clustered at the top, maintaining a low S G near 1.02 across lengths from 0 to 210 m m.
* Muscle swimmers (medium blue circles) cluster between 1.04 and 1.08 S G, mostly under 50 m m in length.
* Armored hoverers (dark blue circles) show a vertical distribution at very small body lengths, with S G values ranging from 1.05 down to 1.41.
* Benthic organisms (grey circles) are scattered across the middle and lower sections, generally between 1.1 and 1.3 S G, with lengths ranging from 50 to 220 m m.
* Deiphon with lipids is represented by a single pink square at approximately 35 m m length and 1.08 S G.
* Deiphon with seawater is represented by a single pink triangle at approximately 35 m m length and 1.13 S G.
Stability in Fluid Flow
In all cases, there was no interaction between diverted fluid flow and the edges of the fluid domain, meaning that the domain was large enough to avoid edge effects. Wall Y+ values below 1 indicate the turbulent layer around the object is captured within the number of cells, leading to accurate results. Wall Y+ values for all three mesh types were <1 across the surface of the Deiphon model (Supplementary Fig. 3). Results were consistent across the three mesh types (coarse, medium, fine), and so interpretations can be considered mesh and domain size independent.
The medium mesh type is visualized in Figure 3 (coarse and fine meshes in Supplementary Fig. 4). Flow is initially uninterrupted, but the fluid velocity slows as it encounters the anterior margin of Deiphon (Fig. 3B,D). Fluid flow remains slow near the surface of the model, particularly posterior to the glabellar bubble. The field velocities show a small vortex immediately behind the glabella and a wake less than the length of body, with flow recombining ~15 mm behind the posteriormost point of the Deiphon model (Fig. 3D). No vortexes form below or behind the body.
A similar profile can be seen when visualizing the pressure contours. An area of high pressure forms in front of the body, at the tall anterior edge of the glabellar bubble (Fig. 3C,E). A low-pressure region forms dorsal to the glabella and body. This ranges from near the dorsal margin of the bubble and extends over the first third of the thorax (Fig. 3E). Another smaller low-pressure region forms in a similar position ventral to the cephalon but does not extend below the thorax (Fig. 3E).
Discussion
Estimating the Density of Trilobites: A New Approach
The density of a body will depend on the constituent parts, both the materials of each and the proportions within the body. Previously, the density of modern marine benthic crustaceans has been used for trilobites (1.14 g/cm3; Wang et al. Reference Wang, Edgecombe and Hou2024), which assumes a homogenous body that may not accurately reflect the same proportions of tissue types and exoskeleton in the extinct organism. This can be seen in the comparison between Limulus and trilobites, with very different proportions of the body made up of exoskeleton (Supplementary Fig. 1). Trilobites are composed of several different materials, the high-density calcite exoskeleton (0.0027 g/cm3), the body (0.001 g/cm3), and the gut, which could be filled with various materials (Supplementary Table 2).
The density of Deiphon is controlled by exoskeletal thickness, resulting in specific gravities (SG) ranging from less than sea water to similar as planktonic animals (Fig. 6). Even small increases in exoskeletal calcite thickness have dramatic impacts on density. The impacts on density caused by increases in exoskeletal thickness would force trade-offs between being heavily armored for defense or lightweight for planktonic lifestyles. The bubble dramatically changes the volume of the body, and proportions of the body composed of different materials, occupying over half of volume of the individual (Supplementary Fig. 1); thus the presence of and material within the bubble both impact SG. Only the thinnest exoskeleton treatment of Deiphon results in a neutrally buoyant body, suggesting this would rarely occur in other trilobites lacking specialized morphology. The exoskeleton thickness at the base of Deiphon’s genal spine was reported as 0.30–0.35 mm (Holloway and Campbell Reference Holloway and Campbell1974), much thicker than the range (0.07–0.13 mm) reported from Spitsbergen trilobites across a range of environments (Fortey and Wilmot Reference Fortey and Wilmot1991). However, the exoskeletal thickness has not been investigated over the rest of the body of Deiphon. If the thickness of 0.3 mm was maintained across the entire body, it would significantly weigh the body down, indicating a benthic lifestyle (Fig. 5). However, a thickness of 0.3 mm is unlikely, as the exoskeletal thickness was likely thicker at the base of the genal spine than across most of the exoskeleton. Support for this inference comes from the closely related Ceraurus pleurexanthemus, which has a much thicker exoskeleton (0.3 mm) close to the doublure, where it interacts with sediment, than for the rest of the body (0.1 mm). Thus, it is reasonable to consider Deiphon having an exoskeletal thickness similar to C. pleurexanthemus at 0.1 mm or at least that the exoskeleton was thinner at other areas of the body. The exoskeletal thickness is a controlling factor on buoyancy in a specimen of this size; thus, it is crucial this is accurately estimated. A broader sampling of exoskeletal thickness in trilobites with regard to size, clade, and environment would create more robust studies. We interpret the exoskeleton of Deiphon as likely being similar to that of C. pleurexanthemus, a close relative, which also falls within the range reported by Fortey and Wilmot (Reference Fortey and Wilmot1991). Non-biomineralized tissues are not known in Deiphon; thus, there is no direct evidence for material occupying the glabellar bubble. There are two competing theories for material filling the bubble, either the expanded gut (Fortey and Owens Reference Fortey and Owens1997) or a low-density material such as lipids (Dollo Reference Dollo1910; Staff and Reck Reference Staff and Reck1911; Ruedemann Reference Ruedemann1934; Whittard Reference Whittard1934).
An expansion of the gut in Deiphon to occupy the entire glabella would likely reflect a highly enlarged crop, which in modern crustaceans is used to masticate food (Ceccaldi Reference Ceccaldi1989; Rupert and Barnes Reference Rupert and Barnes1994; Lerosey-Aubril et al. Reference Lerosey-Aubril, Hegna and Olive2011; Hopkins et al. Reference Hopkins, Chen, Hu and Zhang2017). Although crops have evolved in trilobites multiple times, convincing evidence for them is not widespread among different taxa, and a crop reported in the cheirurid C. pleurexanthemus has since been regarded as unlikely to represent a preserved digestive system (Lerosey-Aubril et al. Reference Lerosey-Aubril, Hegna and Olive2011). The functional implications of such an enlarged crop are unclear, as it may make mastication difficult. The ventral side of the glabellar bubble in Deiphon is unknown, but evidence of muscle attachment would support the presence of a crop (Fortey and Owens Reference Fortey and Owens1997; Lerosey-Aubril et al. Reference Lerosey-Aubril, Hegna and Olive2011).
Modern plankton often store lipids within the body both as energy storage and to regulate buoyancy (Lee and Hirota Reference Lee and Hirota1973; Childress and Nygaard Reference Childress and Nygaard1974; Sargent and Falk-Petersen Reference Sargent and Falk-Petersen1988). Unlike in modern copepods, in which the lipids are easily compressed at depth (Campbell and Dower Reference Campbell and Dower2003), the rigid calcite exoskeleton of Deiphon would prevent this. The storage of lipids could have aided Deiphon in surviving long periods without food or dispersal and in increasing buoyancy. We regard lipid storage as the more likely function of the glabellar bubble, as a crop of this size would struggle to masticate food, because the lumen would be very large. If this were the case, the glabella would function similarly to pelagic lecithotrophic trilobite larvae (Laibl et al. Reference Laibl, Esteve and Fatka2017). Lipid storage to increase buoyancy has been demonstrated in macroplanktonic decapods (Childress and Nygaard Reference Childress and Nygaard1974; Lawrence Reference Lawrence1976; Lee et al. Reference Lee, Hagen and Kattner2006). Mesoplanktonic copepods such as Calanus store lipids in oil sacs, which may be located near the digestive system, which produces some of the molecules (Lee et al. Reference Lee, Hagen and Kattner2006). Lipid storage in copepods also provide energy storage for periods of starvation, and lipids are found in the highest proportions in high-latitude, deep-dwelling species (Cavallo and Peck Reference Cavallo and Peck2020). If the highly inflated glabellae of deiphonines did store lipids, it may have served multiple purposes.
This approach of estimating density and SG coupled with CFD for Deiphon could be applied to other trilobite taxa with unusual morphologies. CFD is useful in determining locomotion and environmental interactions (Esteve et al. Reference Esteve, López, Ramírez and Gómez2021; López-Pachón and Marcé-Nogué Reference López-Pachón and Marcé-Nogué2025), but it is important to account for the SG of the organism, as this will impact its ability to swim. This methodology can be applied more broadly to other euarthropod groups to investigate lifestyles.
A Pelagic Lifestyle for Deiphon?
Fortey (Reference Fortey1985) suggested three criteria to infer a planktonic lifestyle for an extinct organism: (1) analogy with other animals, (2) functional morphology, and (3) independent geologic evidence. The limited number of formations with specimens of Deiphon, only 16 occurrences in the PBDB, makes it challenging to use sedimentological and paleobiogeographic evidence to assess the likelihood of a planktonic lifestyle. Planktonic animals are expected to occur in a broad range of environments because they lived in the water column above a range of sediment types, sinking to the seafloor after depth. With limited numbers of individual specimens at the species level, this cannot be assessed for most species. Deiphonines are distributed broadly through the low to intermediate latitudes (Pérez-Peris et al. Reference Pérez-Peris, Adrain and Daley2024). Deiphon is known from a wide range of environments and lithologies, suggesting a pelagic lifestyle that facilitated dispersal (Fortey Reference Fortey1985). However, only one species, Deiphon globifrons is known from shallow and outer platform environments, while Deiphon americanus occurs within one formation, with limited occurrences (Supplementary Table 3). Considering this limited sedimentological and paleobiogeographic information available for Deiphon, as it is not a highly abundant taxon, the functional morphology is the strongest evidence to evaluate its lifestyle.
The buoyancy of adult trilobites has not previously been estimated. Morphological features hypothesized to increase buoyancy across trilobites and closely related groups include elongate spines and a highly vaulted exoskeleton in larval trilobites (Speyer and Chatterton Reference Speyer and Chatterton1989; Park and Choi Reference Park and Choi2010; Laibl et al. Reference Laibl, Saleh and Pérez-Peris2023) and the thin exoskeleton of the nektaspid Naraoia (Haug and Haug Reference Haug and Haug2016). The primary control on trilobite buoyancy comes from the exoskeletal thickness (Fig. 4). In addition, the lower SG for Deiphon models with bubbles compared to those without, for all exoskeleton thicknesses considered, demonstrates that the presence of the highly inflated glabella increases the buoyancy of Deiphon regardless of the infilling fluid, providing a secondary control on buoyancy for bubble-headed trilobites.
For trilobites traditionally considered pelagic, the exoskeletal thicknesses likely varied depending on whether they were muscle swimmers or armored hoverers, as these require different buoyancies. The well-streamlined muscle swimmers require lower SG than armored hoverers and thus are expected to have thinner exoskeletons leading to lower overall densities. Deiphon’s estimated SG with the 0.1-mm-thick exoskeleton of 1.084–1.138, combined with its reduced body with large spines, supports an inference of the armored hoverer planktonic mode. Modern animals such as Gigantocypris, a modern deep-sea giant copepod, has a nearly spherical body, possible analogous to the large bubble head of Deiphon, and swims slowly by “dragging” itself through water at low Reynolds numbers, using its second antennae (Davenport Reference Davenport1990). We speculate that Deiphon may have used its limbs sparingly to provide thrust and lift, counteracting its slow sinking. The crucial role of exoskeletal thickness on potential lifestyle highlights the need for further study in more clades and how it changes across the body. Considering the range of different morphologies and pelagic strategies available in the modern ocean, this model-based approach to estimate the densities of trilobites may facilitate recognition of additional trilobite armored hoverers from other clades and/or later in the Paleozoic Era.
Pelagic Trilobites in the Silurian?
The possibility that trilobites like Deiphon occupied roles as armored hoverers in the water column after the Late Ordovician Mass Extinction (LOME), would increase the range of known lifestyles and ecological roles for members of this diverse euarthropod clade and would represent the first recognized adult pelagic trilobites from the Silurian Period. However, as Deiphon is part of a clade of cheirurids that emerged in the Early Ordovician, this raises the question of whether deiphonines survived in the pelagic realm through the LOME, or alternatively, if Deiphon was the first member of this group to live in the water column during the Silurian.
The early deiphonines from the Floian Stage of the Ordovician Period, such as Cyrtometopus, do not exhibit highly inflated glabellae and closely resemble other cheirurids (Pärnaste Reference Pärnaste2003; Pérez-Peris et al. Reference Pérez-Peris, Adrain and Daley2024), making it unlikely that these animals were pelagic. In contrast, later-diverging members of the paraphyletic grade ‘Cyrtometopinae’ (sensu Pérez-Peris et al. Reference Pérez-Peris, Adrain and Daley2024), such as Actinopeltis, and Ordovician members of deiphonines such as Mainbrookia and Sphaerocoryphe, do exhibit highly inflated glabellae. As it is unlikely that there was a change in the material filling the glabella within this clade, this raises the possibility deiphonines were an Ordovician–Silurian tribe of armored hovering trilobites all with lipid storage. The smaller size of these earlier-diverging deiphonines means that a broader range of densities is available for them to have used an armored hoverer lifestyle (Fig. 6), and so the overall size of the bubble relative to body size was smaller and their body did not show any reductions for saving mass such as the reduced thoracic pleurae of Deiphon. Interpreted in this way, these smaller deiphonines would have been part of the mesozooplankton, alongside abundant pelagic trilobite larvae (Laibl et al. Reference Laibl, Saleh and Pérez-Peris2023). We can hypothesize that the increase in bubble size relative to body length in Deiphon, alongside the reduced nature of the thoracic pleurae, may have been part of a trade-off facilitating lower a density and thus larger size in this genus, while retaining a position within the pelagic realm, but as a member of the macrozooplankton rather than mesozooplankton. Adult trilobites traditionally interpreted as pelagic, such as the well-streamlined (e.g., Degamella and Parabarrandi) and poorly streamlined (e.g., Carolinites and Opipeuter), did not survive the LOME, and are restricted to the late Cambrian and Ordovician periods (Fortey Reference Fortey1985; Adrain et al. Reference Adrain, Edgecombe, Fortey, Owen and McCormick2004). The LOME selected against agnostids and other pelagic species as their ranges contracted with glaciation (Sheehan Reference Sheehan2001), and pelagic trilobite larvae were also severely impacted (Speyer and Chatterton Reference Speyer and Chatterton1989; Laibl et al. Reference Laibl, Saleh and Pérez-Peris2023). However, some post-Ordovician trilobite groups retained pelagic larvae (Laibl et al. Reference Laibl, Saleh and Pérez-Peris2023), and, if the glabellar bubble is interpreted to have been filled with lipids, then pelagic adult deiphonine mesozooplankton also survived the LOME, later diversifying into the macrozooplankton with Deiphon, which could grow up to at least 3 cm long (Fig. 1).
Acknowledgments
We thank M. Riley (Sedgwick Museum, Cambridge) for access to the collections and loan of camera equipment for photography of Deiphon dikelus. We thank P. Affatato (Columbia University) for providing the micro-CT scan of Limulus polyphemus, MCZ:IP:41112; G. Hunt (Smithsonian Institution) and C. C. Labandeira (Smithsonian Institution) for facilitating access to specimens; F. Pérez-Peris (Harvard University) for discussion of early deiphonines; W. C. Weyland (Harvard University) for assistance with Blender. S.P. acknowledges funding from a NERC Independent Research Fellowship NE/X017745/2. We thank M. Hopkins (editor) and J. Esteve and L. Laibl (reviewers) for their helpful comments to improve this work.
Competing Interests
The authors declare no competing interests.
Data Accessibility Statement
R code and spreadsheets are provided as electronic Supplementary Material. The 3D models and ANSYS archived projects are available through Zenodo: https://doi.org/10.5281/zenodo.22042443.
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