Rosa Rubicondior: Refuting Creationism - Yes, It's Another Of Those 'Non-Exitent' Transitional Fossils!

Holotype of Helmetia expansa USNM 83952, dorsal view. Cross polarized light.

Losso, S. R., Caron, J. B., & Ortega-Hernández, J. (2025).

Ancient fossil sheds big light on evolution mystery: solving a 100-year arthropod mystery | Department of Organismic and Evolutionary Biology

A fundamental problem with creationism is that it depends on wilfully ignoring the vast and ever-growing body of contrary evidence. The intellectual dishonesty required to sustain this belief system makes its adherents the subject of ridicule—not just among scientists, but even among many fellow theists. Its prominent proponents, often elevated to near-prophetic status by their followers, are notorious for misrepresenting or outright lying about scientific findings. Unsurprisingly, they are treated with contempt by the scientific community.

One of the more blatantly counterfactual claims in the creationist repertoire is the assertion that there are no transitional fossils, and no evidence supporting the evolution of species from common ancestors. This denialism is essential to preserve belief in the spontaneous, magical creation of all species a few thousand years ago, without any ancestral lineage.

Accordingly, the creationist industry will need to deploy its usual strategies of misdirection and denial in response to a fascinating Cambrian stem arthropod, first discovered in 1918 in the Burgess Shale of Canada. Initially described from a single specimen, this enigmatic fossil has now been thoroughly reclassified thanks to the work of a team of Harvard researchers led by Dr Sarah Losso, a postdoctoral fellow in the Department of Organismic and Evolutionary Biology. Their analysis, based on 36 newly examined specimens, sheds significant light on early arthropod evolution.

Their findings are detailed in an open-access paper published in the Journal of Systematic Palaeontology, and summarised in a Harvard University news article.

Helmetia expansa – Key Facts.

Taxonomy & Age

Genus & species: Helmetia expansa (Walcott, 1918).
A member of Helmetiidae, within Conciliterga, a group closely related to trilobites [1.1].
Originates from the Middle Cambrian (\~508 million years ago) Burgess Shale, Canada [2.1].

Specimens & Discovery

Holotype first described by Charles Doolittle Walcott in 1918.
Rediscribed in 2025 using 36 specimens from Smithsonian and Royal Ontario Museum [3.1, 4.1].

Size & Shape

Broad, leaf-shaped body measuring 9.2–18.3 cm (92–183 mm) [1.1].
Body divided into a trapezoidal cephalon (head), 6 thoracic segments, and a large, triangular pygidium equipped with spines [1.1].

Anatomy & Preservation

Features large anterolateral head spines, serrated exoskeletal margins, and a thin, flattened dorsal shell [5.1].
Advanced preservation reveals internal organs: gut with J‑shaped foregut, five pairs of digestive glands, neural structures, antennae, stalked lateral eyes beneath head shield, and a median anterior sclerite with medial eyes [4.1].
Biramous limbs (one branch for walking, one for gills) indicate benthic locomotion rather than pelagic drifting [4.1].

Behavioural Insights

First evidence of molting in concilitergans: two specimens captured mid-ecdysis, shedding old exoskeleton via the head region—similar to modern horseshoe crabs [4.1].
Indicates growth strategy and ontogenetic size variation.

Evolutionary Significance

Allows firm placement within Conciliterga and distinguishes between two key families: Helmetiidae (segmented body with spines) and Tegopeltidae (fused segments, no spines) [6.1].
Illuminates anatomical and behavioural evolution in early arthropods, filling a century‑old gap in Cambrian fossil record [7.1].

Ancient fossil sheds big light on evolution mystery: solving a 100-year arthropod mystery
For over a century, the Cambrian arthropod Helmetia expansa remained a mystery. Discovered by paleontologist Charles Doolittle Walcott in 1918, it was initially classified as a crustacean. Despite frequent mentions in research papers, this species has never been formally described, and only one specimen has ever been illustrated.

Now, in a new study published in the Journal of Systematic Palaeontology, Harvard researchers led by Sarah Losso, postdoctoral fellow in the Department of Organismic and Evolutionary Biology, formally describe Helmetia expansa, offering new insights into its anatomy, behavior and evolutionary relationships.
Helmetia expansa belongs to a rare group of early arthropods called concilitergans, close relatives of trilobites. Unlike trilobites, concilitergans lacked calcified exoskeletons, so their remains only fossilized under exceptional conditions—like those in the 508-million-year-old Burgess Shale of Canada, where even soft tissues like guts, legs, and gills were preserved.

Although more specimens were collected, only one individual of Helmetia expansa had been figured and no study examined additional material in detail to formally describe the species or clarify concilitergan evolution.

We need to study more than one specimen to see the species’ full range of morphology and preservation.

Sarah R. Losso, lead author
Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology
Harvard University, Cambridge, MA, USA.

The team examined 36 specimens, at the Smithsonian Institution and the Royal Ontario Museum, from the Cambrian Period of the Burgess Shale of Canada. They photographed the specimens, both wet and dry, using a polarizing filter that can better reveal subtle features and extinction phases, and compared them to related species found in the Chengjiang biota in China, and one from the early Cambrian Sirius Passet in Greenland.

Helmetia had a leaf-like exoskeleton, with some specimens preserving eyes, medial eyes, digestive systems and limbs. Early arthropods had limbs with a walking leg for locomotion and food capture, and a gill used for respiration. On the holotype, only the gills are visible – leading past researchers to believe Helmetia lacked legs and swam exclusively. But, the team found broad gills and walking legs in several specimens, showing it likely walked like trilobites.

Even more surprising were two specimens caught in the early stages of molting, a behavior never before documented in concilitergans.

Molting strategies have never been known in any concilitergan. All arthropods molt their hard exoskeletons to grow, but no one had seen this behavior before in a concilitergan because you have to catch a specimen in the act of molting, and it’s difficult to get just the right timing.

Sarah R. Losso.

The molting specimens show the new exoskeleton closer to the edge of the head, suggesting the animal exited front the front of the body – similar to horseshoe crabs, which use an anterior exit strategy, unlike most crabs that exit from the rear of the body.

The researchers also discovered a wide range of adult body sizes as Helmetia grew. While the smallest specimen was only 92 millimeters long, one exceeded over 180 millimeters.

These patterns tell us not only how these 508-million-year-old animals grew, but how big they could get.

Sarah R. Losso.

Based on updated interpretations of Helmetia expansa’s morphology, the researchers confirmed two main helmetiids groups: the Helmediidae (which includes Helmetia expansa) and is characterized by segment boundaries and side spines, and Tegopeltidae, which are marked by segment fusion and a lack of spines. The researchers also assigned Arthroaspis bergstroemi, known from Greenland since 2013, to the group Conciliterga.

Our findings give a much fuller picture of what Helmetia looked like, how it lived, and how concilitergans are related to each other, which is very important for future studies on Conciliterga and other early arthropods.

Sarah R. Losso.

Abstract
The trilobitomorphs are a megadiverse and ecologically versatile group of Paleozoic euarthropods that include the iconic trilobites, as well as non-biomineralized clades exclusively known from Konservat-Lagersttätten. The concilitergans, defined by the presence of a broad, variably effaced and flattened dorsal exoskeleton, have received comparatively little attention. This is particularly true for Helmetia expansa from the mid-Cambrian Burgess Shale (British Columbia) – the first chronologically reported species of Conciliterga, which was originally figured (based on a single specimen) by Walcott (1918). Here, we present the first comprehensive description of H. expansa based on all material available from the Burgess Shale, totalling 36 specimens housed at the Royal Ontario Museum and the Smithsonian Institution. The non-biomineralized dorsal exoskeleton is broad and flat, with a serrated margin throughout. The body consists of a cephalon with well-developed anterolateral spines, six thoracic tergites and a large pygidium bearing two pairs of lateral spines and a single terminal spine. The preserved appendages include short uniramous antennae followed by 15 pairs of homonomous and biramous limbs composed of a gracile endopodite and an exopodite with a broad lobe and thick lamellae. The digestive system consists of a straight gut tract, including a ‘J’-shaped foregut and five paired digestive glands on the anterior half of the body expressed as small and convex oblong structures with a submillimetric lamellar ultrastructure. Two specimens of H. expansa provide the first direct evidence of moulting in concilitergans. The lack of dorsal ecdysial sutures suggests a marginal moulting strategy similar to extant Xiphosura. A revised phylogeny of Trilobitomorpha supports Arthroaspis bergstroemi from the early Cambrian Sirius Passet of Greenland as the earliest branching concilitergan and prompts a new diagnosis for the clade. Our results inform the internal relationships within Conciliterga and formalize the families Helmetiidae (Helmetia, Rhombicalvaria, Haifengella and Kuamaia) and Tegopeltidae (Tegopelte, Skioldia, Saperion).

Introduction
Trilobitomorpha is a major clade within Artiopoda that includes diverse Paleozoic non-biomineralized euarthropods in addition to the iconic biomineralized trilobites (e.g. Berks et al., 2023; Edgecombe & Ramsköld, 1999; Lerosey-Aubril et al., 2017). The trilobitomorphs comprise various sub-groupings that are generally known from several Cambrian Konservat-Lagerstätten around the world, including well-defined clades such as nektaspids (e.g. Naraoia, Misszhouia; Mayers et al., 2019; Zhai et al., 2019.1), xandarellids (e.g. Xandarella, Sinoburius; Hou & Bergström, 1997; Chen et al., 2019.2) and concilitergans (e.g. Helmetia, Kuamaia; Hou & Bergström, 1997) (Fig. 1). In this context, concilitergans are of particular interest as they have been widely considered as either the sister group or at least close relatives of trilobites (Berks et al., 2023; Chen et al., 2019.2; Du et al., 2019.3; Edgecombe & Ramsköld, 1999; Mayers et al., 2019; Stein et al., 2013), although several analyses have otherwise recovered trilobites as sister to nektaspids (e.g. Legg et al., 2013.1; Lerosey-Aubril et al., 2017). Concilitergans are characterized by a broad and flattened dorsal exoskeleton, the variable effacement of the tergite boundaries in different species, and the presence of a prominent anterior sclerite accommodated in a cephalic notch in several taxa (Fig. 1). As traditionally defined, concilitergan diversity consists of eight Cambrian taxa (Table 1). However, the internal relationships within the group have never been specifically investigated. Beyond their original systematic descriptions, concilitergans as a clade have received less attention compared to nektaspids (e.g. Bond & Edgecombe, 2020; Budd, 1999.1; Mayers et al., 2019; Paterson et al., 2010; Pérez-Peris et al., 2020.1; Whittington, 1977; Zhai et al., 2019.4), xandarellids (e.g. Chen et al., 2019.2; Hou et al., 2019.5; Schmidt et al., 2021; X. Zhang et al., 2012) and even early branching trilobitomorphs or other artiopodans (e.g. Berks et al., 2023; Du et al., 2019.3; Schmidt et al., 2022; Stein & Selden, 2012.1; M. Zhang et al., 2022.1). Part of the complications for understanding concilitergan evolution is that their non-biomineralized body fossils are rare compared to other trilobitomorphs, with most described species to date being known from a limited number of figured specimens (e.g. Hou, 1987; Hou & Bergström, 1997; Whittington, 1985; Zhao et al., 2014).

Figure 1. Concilitergan diversity and relationships. A,Arthroaspis bergstroemi, MGUH 30382, from the early Cambrian (Stage 3) Sirius Passet, Greenland. B,Helmetia expansa, USNM 83952, from the mid-Cambrian (Wuliuan) Burgess Shale, Canada. C,Kuamaia lata, CJHMD 00064, from early Cambrian (Stage 3) Chengjiang, China. D,Tegopelte gigas, USNM 189201, from the mid-Cambrian (Wuliuan) Burgess Shale, Canada. E, simplified topology based on results from phylogenetic analyses herein which resolved Arthroaspis bergstroemi as an early-branching concilitergan.

Concilitergans are chronologically among the earliest non-trilobite Cambrian euarthropods to be recognized from the Burgess Shale, with Helmetia expansa as the first recorded representative of this group, having been discovered in 1917 and informally named by Walcott (1918, p. 7) with a single illustrated specimen. That specimen was re-illustrated by Resser in 1931 in a posthumous publication (Walcott, 1931, p. 38, plate 23) and designated as the holotype, but no other material was mentioned, and no formal description was provided. H. expansa has not been described since then, and only the holotype specimen (USNM 83952) has been discussed in the context of other findings (Briggs, 1994; Edgecombe & Ramsköld, 1999; Ortega-Hernández, 2015; Simonetta & Delle Cave, 1975; Walcott, 1918). This is problematic, as H. expansa remains incompletely known and yet has been featured in numerous phylogenetic analyses that explore the evolution of Trilobitomorpha and Artiopoda more broadly (Berks et al., 2023; Chen et al., 2019; Edgecombe & Ramsköld, 1999; Hou & Bergström, 1997; Hou et al., 2019; Legg et al., 2013.1; Paterson et al., 2012.2; Schmidt et al., 2022; Stein et al., 2013). In this study, we formally describe Helmetia expansa based on the holotype and previously unstudied material at both the Royal Ontario Museum and the Smithsonian Institution, report new morphological data including evidence for moulting and a digestive system with paired glands and produce a new phylogeny for Conciliterga in the broader context of Trilobitomorpha.

Figure 2. Holotype of Helmetia expansa USNM 83952, dorsal view. A, Cross polarized light. B, Low angle light. C, Magnification of A showing cephalon. D, Magnification of A showing lamellae. Abbreviations: cep, cephalon; anl, anterolateral spine; ant, antenna; asc, anterior sclerite; ey, eye; ex, exopodite; hyp, hypostome; lm, lamellae; mey, medial eyes; mg, midgut gland; opn, optical nerve; py, pygidium; rs, reniform structure; stn, sternite number; th, thorax.

Figure 6. Complete specimens of Helmetia expansa. A, ROMIP 66311, ventral view. B, ROMIP 66303, dorsal view. C, ROMIP 66309, dorsal view. D, ROMIP 65115, dorsal view. Abbreviations: an, anus; ant, antenna; asc, anterior sclerite; ey, eye; g, gut tract; mey, medial eyes; mg, midgut gland; taf, transverse anterior fold.

Losso, S. R., Caron, J. B., & Ortega-Hernández, J. (2025).
Helmetia expansa Walcott, 1918 revisited – new insights into the internal anatomy, moulting and phylogeny of Conciliterga.
Journal of Systematic Palaeontology, 23(1). https://doi.org/10.1080/14772019.2025.2468195

Copyright: © 2025 The authors.
Published by Informa UK Limited (Taylor & Francis Group). Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

If they weren’t so determined to ignore it—or so quick to invent reasons to dismiss it—this sort of evidence would be the stuff of nightmares for creationists. Not only does the fossil date to around 500 million years ago, long before they believe the universe even existed, but it also provides compelling evidence of a stem group at the point of divergence leading to trilobites and other arthropods.

Creationists often misrepresent the Cambrian as a time when multicellular animals appeared suddenly, fully formed, and without ancestors. However, fossils such as Helmetia expansa clearly demonstrate the existence of transitional, stem-group arthropods in the early Cambrian. These forms predate the more familiar, derived organisms like trilobites and show the gradual emergence of complexity.

The Cambrian was not a singular, explosive event in which all major body plans appeared simultaneously, but rather a prolonged evolutionary process lasting roughly 10 to 25 million years. It unfolded in stages, each characterised by distinct faunas and clear evidence of progression from simpler to more complex forms. Fossils like Helmetia are vital pieces of this evolutionary puzzle, and their existence thoroughly undermines creationist narratives.

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