Introduction

Echinoderms are a diverse phylum of marine invertebrates with a wide array of body plans, autecologies, and a long, rich fossil record. The five extant classes of echinoderms are characterised by a pentameral (or pseudo-pentameral) radial symmetry, a water vascular system, which is used for feeding or locomotion, a mesodermal skeleton of numerous plates of calcite stereom, and a unique type of connective tissue, Mutable Collagenous Tissue (MCT). One or more of these traits are hypothesised to be missing from various extinct clades complicating phylogenetic analyses1. The fossil record offers firm evidence that the most of the central characteristics of echinoderms (stereom, water vascular system, pentameral symmetry) evolved in the Cambrian2,3,4, but “the sole derived character unambiguously shared by all known extinct and extant echinoderms is the stereom endoskeleton”1.

Many animals possess connective tissue but Mutable Collagenous Tissue (MCT) found in extant echinoderms can alter its mechanical properties on very short time scales5. MCT has the capacity to alter its mechanical properties within a timescale of seconds to minutes under the direct control of the nervous system (summary of early literature6, summary of recent MCT literature and potential biomedical applications5,7).

Extant echinoderm classes use MCT for different functions ‒ control of the stiffness of the body wall (holothurians), automising body parts (holothurians, ophiuroids, crinoids), and control of spine-posture (echinoids, asteroids)8. Using MCT, passive suspension feeding echinoderms, for example crinoids, can maintain their filtration fan for prolonged periods of time for food collection in varying currents with minimal energy expenditure9. MCT is found in the arms, cirri, and stalks of modern crinoids and is critical to their ability to respond to predation pressure and habitat selection in addition to acquiring food9.

MCT has been inferred in fossil crinoids and other extinct Palaeozoic clades. Sessile Early and Middle Cambrian echinoderms attached themselves to the substrate using an organic “glue”, hypothesised to be MCT, in contrast to later sessile echinoderms that cemented themselves to the seafloor using calcite10. The majority of inferences of MCT in fossil echinoderms involve the crinoid stem11,12,13,14,15 specifically the “broken-stick” model of crinoid stem taphonomy11, and suggests that stem autotomy in Palaeozoic crinoids may have been pervasive16. MCT may have allowed calceocrinids, highly modified Palaeozoic crinoids, to maintain a prostrate posture along the sediment/water interface17. Based on uniquely preserved fossils, we present evidence for the presence of MCT in an extinct echinoderm class, the Blastoidea, Palaeozoic echinoderms most closely allied with the Crinoidea among the extant classes.

The blastoid theca

A typical blastoid theca is composed of three basal plates, five radials, four deltoids plus a variable number of anal deltoids, and five ambulacra composed of lancets, numerous side plates, and outer side plates. Ambulacral, peristomal, and anal areas were covered by numerous small polygonal plates, which are often not preserved. Thecal plates are fused together to form the theca which articulates with the stem and brachioles. The total number of ossicles in a typical blastoid is ∼190,00018. In contrast to the primary thecal plates, the vast majority of the plates are very small.

Brachioles are the feeding appendages of blastoids and are composed of a thin biserial row of plates, longer than wide, which articulate with the adjoining brachiolar plate19. Although rarely preserved, fossilised brachioles most often reflect the trauma position, a configuration attributed to rapid burial in high velocity currents20. Very rarely, blastoids preserve brachioles which give insight into their role as the primary feeding apparatus. Studies of the architecture of blastoid brachiolar plate articulations are lacking. However, brachiolar articulations in rhombiferan blastozoans lack stereomic structures that would indicate the presence of muscular articulations and support the hypothesis that rhombiferan brachioles contained only ligamentary articulations21.

Most blastoids were sessile marine organisms with the theca attached to the seafloor with a stem 20–30 cm long. When feeding, blastoids arrayed the brachioles in a three-dimensional feeding fan modelled on the feeding posture of modern stemmed crinoids. A few blastoid genera were eleutherozoic and sat directly on the seafloor. A stem is either vestigial or missing in these blastoids. Eleutherozoic blastoids often illustrate thecal asymmetry associated with their contact with the seafloor.

Wirtenbach blastoids

Devonian blastoids with brachioles preserved have been found in Nümbrecht-Wirtenbach in the Oberbergisches Land (eastern Rhenish Slate Mountains) of Germany. Blastoids from Wirtenbach mostly are preserved as internal molds and casts in a fine-grained argillaceous siltstone (Figs. 1 and 2). Over thirty flattened individuals are known in varying degrees of exposure on the bedding planes. All appear to be the same species ‒ Dissimiloblastus inequalis22 ‒ but their taxonomic affinity is unclear. They are unusual in having excellent preservation of the brachioles, which are fanned out above the oral summit of the theca (Figs. 1 and 2). Brachioles are interpreted to have formed a feeding filtration fan modeled on modern crinoids23. Blastoid workers have hypothesised the brachioles formed a filtration fan with a three-dimensional aspect depending on the length of the ambulacra23.

Fig. 1: The eleutherozoic blastoid Dissimiloblastus inequalis Bohatý et al., 2024, possibly indicating the occurrence of brachiolar MCT in the rigid state.
figure 1

a Specimen with preserved theca and brachioles from Wirtenbach, Germany. The longer and thinner adoral brachioles were arrayed in rigid flattened filtration fan. They give the appearance of having been in a rigid state possibly indicating the presence of MCT in the stiffened state (orange arrowheads with asterisks). The shorter and wider aboral brachioles were recurved and arrayed in a snowshoe configuration, indicating MCT in the stiffened state (red arrowheads with asterisks). b Detail view of the theca, showing the recurved brachioles. Scale bars: a = 7.5 mm, b = 5.0 mm. See Supplementary Appendix for specimen repository and details of the stratigraphy and locality.

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Fig. 2: The eleutherozoic blastoid Dissimiloblastus inequalis Bohatý et al., 2024, possibly indicating the occurrence of brachiolar MCT in the transition stage from the rigid to the compliant state.
figure 2

a Partly preserved theca and brachioles from Wirtenbach, Germany. The aboral brachioles are not preserved. The longer and thinner adoral brachioles are preserved as a flattened fan, possibly showing a transition stage from the rigid (orange arrowhead with asterisk) to the destiffened state (orange arrowheads). b False colour illustration of the same specimen. Scale bar = 5.0 mm. See Supplementary Appendix for specimen repository and details of the stratigraphy and locality.

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The lack of a stem in the Wirtenbach blastoids suggests an eleutherozoic life habitat in contrast to most blastoids which were attached to the bottom with a stem (Figs. 3–5).

Fig. 3: Slab with two individuals of Lotusoblastus medusa (Jaekel, 1895), associated with crinoids.
figure 3

a The neotype [Lm(n)] and the smaller specimen (Lm) likely had the rigid brachioles (arrowheads with asterisks) splayed in a feeding fan when they were killed and rapidly buried together with the crinoids (c). The gently curving nature of some of the brachioles (arrowheads) indicate the beginning of the transition from a feeding posture to a trauma posture and the possible softening of MCT from the stiffened to the more compliant mode. b Radiograph of the slab, showing detached stems displaying the “broken-stick” configuration (blue arrowheads). (Photo by Mike Reich, Braunschweig, Germany; radiograph by Wilhelm Stürmer, Erlangen, Germany; with permission of the Bavarian State Collection for Palaeontology and Geology of the State Natural History Collections of Bavaria, Munich, Germany). Scale bars = 10.0 mm. See Supplementary Appendix for specimen repository and details of the stratigraphy and locality.

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Fig. 4: Lotusoblastus medusa (Jaekel, 1895), possibly indicating the occurrence of brachiolar MCT in the locked or rigid state.
figure 4

a Specimen with preserved theca, stem, and brachioles arrayed in a fan that may be indicative of brachiolar MCT in the locked or rigid state, some show gentle curvature at the distal ends (arrowheads). The fan could represent a collapsed feeding fan. b Enlarged detail of the theca and of the proximal arms, whitened with ammonium chloride. Scale bars = 10 mm. See Supplementary Appendix for specimen repository and details of the stratigraphy and locality.

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Fig. 5: Pentremitella osoleae Lehmann, 1949, possibly indicating the occurrence of brachiolar MCT in the rigid and compliant states.
figure 5

a Specimen with preserved theca, brachioles and part of the stem, showing two arms with brachiolar MCT in the rigid state (arrowheads with asterisks). b Radiograph of the specimen (by Wilhelm Stürmer, Erlangen, Germany). Scale bars = 10 mm. See Supplementary Appendix for specimen repository and details of the stratigraphy and locality.

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Brachioles in the Wirtenbach blastoids are unique as they have two different widths (Fig. 1). Brachioles are predominantly long and thin, about twice the length of the theca, and oriented in a rigid fan (Fig. 1). In life, the filtration fan would have been three dimensional23, but is preserved as a flattened fan. Other brachioles are shorter, thicker, and recurved. As preserved, they are restricted to the lateral margins of the aboral part of the theca. These brachioles have complex curves, are oriented 180° from the longer brachioles, and may have formed a snowshoe-like structure supporting the eleutherozoic theca on a fine-grained substrate. MCT in its stiffened state is the most viable mechanism by which a string of tiny, thin calcite plates (the brachioles) could form long rigid structures and simultaneously form tightly curved flattened structures not only in a single individual, but in a single morphological structure, the ambulacrum.

Figure 2 shows another specimen from Wirtenbach with long brachioles forming a fan. Individual brachioles are either straight rigid structures or gently curving. This blastoid illustrates the transition from a feeding-posture of the brachioles to a trauma posture20 when the MCT was beginning to destiffen as the current swept over the animal and buried it rapidly. Snowshoe brachioles are not preserved.

Hunsrück blastoids

The Devonian Hunsrück Slate in Germany is world famous for exquisitely preserved fossils often retaining evidence of soft tissue through pyritisation. An autochthonous taphocenosis of the blastoid Lotusoblastus medusa, from the Hunsrück Slate at Bundenbach (southern Rhenish Slate Mountains) was described more than 125 years ago24. A second completely preserved blastoid from the Hunsrück Slate, Pentremitella osoleae, was described 75 years ago25.

One of the Hunsrück slabs has two specimens of Lotusoblastus medusa (Fig. 3). One specimen is a theca with a short incomplete stem and brachioles. It is partially obscured by a more complete specimen in the foreground. This specimen has an incomplete holomeric stem about 80 mm long composed of alternating thicker and thinner columnals. The blastoid has a broad cone-shaped theca with protruding basalia and brachioles that are about 20 mm long. As preserved, the brachioles are rigid and straight, forming a fan (Fig. 3a). Figure 3b is a radiograph of the same slab showing the two blastoid specimens. Thicker and thinner columnals in the stems can be seen clearly. The longer stem has gentle arcs and is detached from the theca just below the cicatrix. Three or four columnals appear to remain attached to the theca. The shorter stem is also detached from the theca just below the cicatrix and displays the “broken-stick” configuration described for fossil crinoids11. Many more brachioles in both individuals are visible in the radiograph than on the slab itself. Brachioles mostly are rigid and straight, but some show gentle curvature at the distal ends (Fig. 3). Figure 4 shows another theca and stem with rigid brachioles arrayed in a fan likely formed by brachiolar MCT in the locked or rigid state. Some brachioles show gentle curvature at the distal ends.

Columnals in the stems of the specimens of Lotusoblastus medusa were likely connected with MCT. The arcs in the long stem and the “broken-stick” configuration of the shorter stem are related to the presence of MCT, but the evidence is not conclusive. In contrast, the rigid, straight posture of most brachioles in Figs. 3 and 4 suggests a compelling, albeit indirect, indication of MCT in the stiffened state.

These individuals likely had the brachioles splayed in a feeding fan when they were killed and rapidly buried. Gently curving brachioles indicate the transition from a feeding posture to a trauma posture and the softening of MCT from the stiffened to the more compliant mode.

Figure 5a shows a specimen of Pentremitella osoleae also from the Hunsrück Slate with its oral surface oriented toward the upper right with the basals and stem toward the lower left. Thecal detail is largely obscured by the mass of curved brachioles, which appear swept toward the aboral end of the theca. In the radiograph (Fig. 5b) the theca is elongate with long ambulacra, which appear to be set within the ambulacral sinus. A narrow stem shows a sharp flexure or break just below the cicatrix. Many more fine and hair-like brachioles than are visible in the radiograph on the slab. Individual brachiolar plates can be seen, but not in enough detail to establish their biserial nature. Most of the brachioles are arced toward the aboral end of the theca as if swept in a current, but a few are not. They are rigid and protruding straight from the oral surface (Fig. 5a, b).

The majority of brachioles in the specimen of Pentremitella osoleae show MCT in the compliant state. Delicate brachioles are not organised into a feeding fan but are swept toward the aboral end of the specimen presumably by currents. However, a few brachioles at the oral end show the rigid orientation with MCT in the stiffened state.

Combining observations of the two Hunsrück blastoid species, we can hypothesise that the stem columnals had ligamentary connections, probably of MCT origin. Evidence in the brachioles is more conclusive. Specimens of Lotusoblastus medusa show the delicate brachioles preserved as rigidly straight with MCT in the stiffened state. The Pentremitella osoleae specimen shows the compliant state of MCT with the brachioles preserved as current-swept limp hair-like structures. Comparisons of the specimens show these two states, but each specimen also shows individual brachioles in each of the two states.

Results and discussion

Extant clades of echinoderms possess MCT, a unique form of connective tissue that can rapidly change its tensile character under direct control of the nervous system. MCT in extinct clades of echinoderms has been hypothesised, but direct evidence is lacking because soft tissue preservation in echinoderms is extremely rare. Muscles or ligamentary connective tissue in echinoderms can be inferred from the morphology of the stereomic microstructure. Direct evidence of MCT in fossil echinoderms requires not only the presence of the soft tissue, but an ability to demonstrate that the tissue could change its tensile properties. Details of the stereomic microstructure or evidence of soft tissue is not possible in our fossil material due to the preservation (hollow molds and pyritisation). Nevertheless, by interpreting the preserved postures of the brachioles in our specimens, we can obtain compelling circumstantial evidence of MCT in blastoids.

Blastoids with preserved brachioles are rare and are often found in the postmortem trauma posture. Rigid brachioles forming a filtration fan in the blastoids from the Hunsrück imply that they were preserved with MCT in its stiffened state. Other blastoids from the Hunsrück are preserved with limp brachioles implying MCT in its compliant state. Neither of the configurations reflect the postmortem trauma posture.

Brachioles in two very different configurations within individual blastoids from Wirtenbach indicate that individual brachioles within the same ambulacrum could perform different functions utilising MCT. Aboral brachioles were arrayed in a snowshoe configuration to stabilise the eleutherozoic theca on a soft, perhaps mobile, substrate, while adoral brachioles were arrayed in rigid filtration fan for feeding. Within the same ambulacrum, stiffened MCT could transform individual brachioles into straight, rigid elements of the filtration fan or tightly curved snowshoes depending on the brachiole’s function. Other specimens from both localities appear to show rigid brachioles implying MCT in a stiffened state while others seem to show MCT transitioning to the compliant state. Morphological evidence of innervation of the brachiolar system in other blastoids23 implies that each brachiole possessed a separate brachiolar nerve.

Our conclusion is that MCT, acting under direct control of the nervous system, gave blastoids the ability to form the brachioles into a filtration fan for feeding. Eleutherozoic blastoids could modify the function of some brachioles to aid in substrate stabilisation while others formed a filtration fan for feeding. Most blastoids are found in marine environments with energy dominated by unidirectional currents. In these environments, rigid brachioles would produce an optimised feeding fan. Sophisticated control of brachioles by MCT produces the possibility that blastoids could have employed a fundamentally different feeding fan configuration in shallow water environments dominated by bidirectional wave energy opening these environments to colonisation. Other brachiole-bearing blastozoans likely possessed MCT, although preserved morphological data are very scarce. One particularly innovative recent study21 reinterpreted the mode of life of a rhombiferan blastozoan from an erect sessile suspension feeder to a mobile benthic detritovore with two modified brachioles at the anterior end being propelled by a muscular stem that pushed the animal forward.

Finally, the blastoids from the Hunsrück and from Wirtenbach give us important taphonomic information about the burial of the individuals. The sedimentation events that buried the individuals were so rapid that the MCT either did not have time to destiffen so the animal could transition between a feeding to a trauma posture or was just beginning the process. Given the known transition time of MCT, this suggests these individuals were buried in seconds to minutes.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.