A nearly complete, anteriorly slightly damaged cephalon with both in situ preserved librigenae is articulated with fourteen thoracic segments; left pleura of the ninth to fourteenth thoracic segments are not exposed, the pygidium is missing (Fig. 3A, B)

The central and posterior glabellar surface bears three pairs of clearly visible, rounded, white-coloured markings positioned in L1 to L3 (Figs 3; 10D[Dc1 to Dc3]); slightly larger markings are preserved also in the occipital ring (Figs 3; 10D[Dc0]) as well as in the axial part of the six anterior-most thoracic segments (Figs 3; 10D[Dt1 to Dt6]).

The central area between the white markings is widest and uncoloured in the glabella. In contrast, a quite narrow median strip is seen in the occipital ring and the six anterior thoracic segments (Fig. 3A, B); this narrow strip continues in the seventh and eight axial rings and is slightly darker in colour than the surface of the axial rings (marked only part of outline of the strip, Figs 3A, B; 10D).

The anterior-most white-coloured left and right markings bear at least three fine, sagittally oriented scratches on its surface. Up to six similarly fine but transversally oriented scratches are seen in the other more posterior cephalic and thoracic white-coloured markings (Fig. 3B). The scratches did not form as a result of mechanical cleaning of the surface and their origin remains unknown.

Incomplete, strongly flattened and considerably damaged remains of a thorax containing eleven articulated axial rings connected with three nearly complete anterior-most pleurae and small remains of other eight pleurae at the left side and ten pleurae at the right side. The posterior part of the strongly damaged cephalon is bent down at an angle of about 70 degrees. The left librigena is strongly weathered but its remains are still preserved in situ. A large part of the right librigena is broken off, and only a small part remains. The pygidium and the posterior parts of the thorax are missing.

Small, irregular, whitish spots are preserved in the occipital ring and in the posterior part of the incomplete glabella (Figs 4; 10B[Dc0 and Dc1]). The flattened, only slightly vaulted axial rings bear prominent light markings in the first to the sixth thoracic segments (Fig. 4). These markings are paired and range from small and rounded (Figs 4; 10B[Dt5r]) through ellipsoidal and sagittally prolonged (Fig. 10B[Dt4r]) to large and rounded (Fig. 10B[Dt6l]). In the first to third thoracic segments, the surface of the axial rings is barely perceptible to be darker between the light markings (Fig. 10B[G]).

Strongly damaged but originally complete articulated exoskeleton with remains of both librigenae preserved in situ (Fig. 5A). A small remnant of the in situ hypostome is seen in the left antero-lateral part of the glabella (Fig. 10A[HY]). The anterior-most part of the cephalon is broken off. The axis and fourteen pleurae are visible at the right side of the thorax, while only four pleurae are preserved at the left thoracic side; four more posterior pleurae are only partially preserved (Fig. 5A). Remains of the axis and pygidial pleural lobe are preserved on the right side (Fig. 5A).

The surface of the flattened glabella bears three white areas in L2 and L3 (Figs 5; 10A[Dc2l, Dc2r and Dc3r]), two other light, quite poorly visible areas are preserved in L1 (Figs 5; 10A[Dc1l and Dc1r]). White spots are seen also in the occipital ring (Figs 5; 10A[Dc0l and Dc0r]). Comparatively small, paired, irregularly elliptical white spots are preserved in the first to fourth axial rings (Figs 5; 10A[Dt1 to Dt6]). The colour of the surface between the paired spots and holes is identical to the colour of other parts of the thorax.

Very slightly damaged cephalon with hypostome and both librigenae preserved in situ and with eleven incomplete, slightly disarticulated and posteriorly retracted thoracic segments; the pygidium and posterior thoracic segments are missing (Fig. 6A). The U-shaped narrow groove in the middle of the glabella represents the posterior margin of the hypostome (Figs 6; 10C[HY]).

One small cavity occurs on each side of the hypostome (Figs 6; 10C[Dc3]). Two pairs of large, polygonal, light-coloured spots are situated in L1 and L2 of the glabella (Figs 6; 10C[Dc1 and Dc2]). Two other light-coloured, irregular areas are present in the occipital ring (Figs 6; 10C[Dc0]). An elliptical, sagittally prolonged grey-coloured spot is preserved in the anterior-most third of the glabella (Figs 6; 10C[E]). Seven small to large, rounded white spots are preserved in the axes of the first to fifth thoracic segments (Figs 6; 10C[Dt1 to Dt5]). The colour of the surface between all the light spots is identical to the colour of other parts of the thorax.

The more damaged specimen includes a nearly complete cephalon with both librigenae (Figs 7A; 10Ga), which is articulated with remains of four anterior thoracic segments. A partly exposed external surface of the in situ hypostome is seen in the anterior part of the glabella (Figs 7Aa; 10Ga[HY]).

The postero-lateral margins of the hypostome are bordered by a pair of yellow-brown, large, antero-laterally oriented ellipsoidal depressions in L3 (Figs 7A; 10Ga[Dc3]). In L1 and L2, two pairs of yellow-brown spots are seen in the posterior part of the glabella; these spots are shallow, transversally narrow and sagittally short (Figs 7A; 10Ga[Dc1 and Dc2]). The whole surface of the occipital ring and the axis of the anterior thoracic segment are replaced by sagittally short and transversally wide, yellow-brown cavities (Figs 7A; 10Ga[Dc0 and Dt1]). Posteriorly from the hypostome, the central posterior surface of the glabella between Dc3 and Dc0 exhibits a wide sausage-like elevation (Figs 7A; 10Ga[G]). The axis of the fourth thoracic segment as well as all the more posterior thoracic segments and the pygidium are missing.

A strongly damaged and only partly uncovered cranidium is articulated with remains of twelve thoracic segments. The external mould of the in situ preserved hypostome is exposed under the broken-off anterior half of the glabella (Figs 7B; 10Gb[HY]).

Inside L3, laterally from the in situ hypostome, rounded, yellow-brown spots are developed on both sides of the glabella (Figs 7B; 10Gb[Dc3]); two other yellow-brown, ellipsoidal depressions are present behind the postero-lateral margin of the hypostome (Figs 7B; 10Gb[Dc2]). Generally comparable, transversally slightly wider depressions are also developed in the postero-lateral surface of the glabella in L1 (Figs 7B; 10Gb[Dc1]). The occipital ring is broken off and is replaced by a tiny brown-coloured surface (Figs 7B; 10Gb[Dc0]). The axial parts of the second to fourth thoracic segments are obscured by an external surface of a remnant of an indeterminable trilobite. The axis of the fifth thoracic segment is broken off (Fig. 7B). A rectangular elevated area is clearly visible in the central glabellar surface between the posterior hypostomal margin and the anterior margin of the occipital ring (Figs 7B; 10Gb[G]).

A slightly damaged cephalon with both librigenae is preserved together with a damaged thorax consisting of eleven axial rings associated with 14 pleurae on the right side and remains of three pleurae on the left side (Fig. 8A).

A small cavity is seen in the right antero-lateral part of the conical glabella (Figs 8; 10E[Dc3]). Two much larger and deeper cavities are preserved in both L2 of the glabella (Figs 8; 10E[Dc2]). Two additional quite large cavities are developed in the postero-lateral glabellar surface of L1 (Figs 8; 10E[Dc1]). A shallow, ellipsoidal, yellow-coloured cavity is seen in the occipital ring (Figs 8; 10E[Dc0l]). Three pairs of small to large, yellow-coloured cavities are preserved in the axis of the first to third thoracic segments (Figs 8; 10E[Dt1 to Dt3]). Similarly, the axis of the fourth to sixth thoracic segments bears paired, shallow, large depressions (Figs 8; 10E[Dt4 to Dt6]). Below the well-preserved surface of the central part of the glabella, between Dc3r and Dc2l, a sausage-like transversal widening is seen (Figs 8; 10E[G]). In the area between Dc2 and middle of the posterior margin of the anterior glabellar lobe is developed a narrow bulge (Fig. 8B[arrow]). The colour of the surface between the paired cavities is identical to the colour of other parts of the thorax.

A polygonal rock sample bears the anterior part of a damaged articulated specimen (Fig. 9A[a]) preserved together with remains of articulated but incomplete thoraxes of four other specimens (Fig. 9A[b to e]). Exceptionally preserved specimen is composed of a very slightly damaged cephalon with both in situ librigenae articulated with incompletely preserved remains of the three anterior thoracic segments. Specimen a contains remains of the digestive system; specimens b to e do not show digestive-system remains.

The surface of the well-preserved conical glabella bears an anteriorly arched bulge in the frontal glabellar lobe (Figs 9A, B; 10F). Posteriorly, this bulge continues as a wide, V-shaped, sagittally prolonged and posteriorly gradually narrowing tube-like extension in the central part of the glabella (Figs 9A, B; 10F[G]). At the left side, a small depression is developed in L3 just near the tube-like extension (Figs 9; 10F[Dc3l]). Laterally from the bulge, the course of the V-shaped tube-like body becomes broader in front of Dc3. Even more anteriorly, the bulge is bluntly ended (Fig. 9B[arrow]). A pair of kidney-shaped imprints is seen at both sides of the tube-like extension in the middle of the glabella (Figs 9; 10F[Dc2]). Another pair of small rounded pits is preserved in L1 in the postero-lateral surface of the glabella (Figs 9; 10F[Dc1]). The axis of the first thoracic segment is situated below the chipped-off occipital ring and bears a transversally wide whitish spot (Figs 9; 10F[Dt1r]). A pair of small rounded, whitish spots is seen in the axis of the second, slightly clockwise-rotated thoracic segment (Figs 9; 10F[Dt2]). Another pair of similar, slightly larger spots is preserved in the axis of the third thoracic segment (Figs 9; 10F[Dt3]). The rest of the thorax of this specimen is missing.

The alimentary canal preserved in the middle and posterior parts of the glabella is associated with three pairs of large lobate imprints; the fourth pair occurs in the occipital ring. All these imprints are interpreted as paired cephalic digestive glands, designated Dc0 to Dc3 (Fig. 11). The spots or cavities developed on both sides of the gut in the axial region of the six anterior-most thoracic segments belong to partially preserved, paired thoracic digestive glands, designated Dt1 to Dt6 (Fig. 11). The number of paired digestive glands, and their morphology and positioning, are the same as the remains of the digestive system described in previous work (e.g. Chatterton et al. 1994; Lerosey-Aubril et al. 2011, 2012; Fatka et al. 2013a, b, 2015).

The body in the frontal glabellar lobe we interpret to belong to the foregut chamber. This morphology is consistent with the proposal of Lerosey-Aubril & Peel (2018: 751), who argued that the anteriormost pair of digestive glands occurs immediately posterior to the frontal glabellar lobe. These authors also hypothesised that the digestive glands belong to the midgut and the boundary between the foregut and midgut was located under the posterior margin of the frontal lobe; consequently, the part of the gut associated with paired digestive glands preserved in the cephalon and thorax corresponds to the midgut.

Following Cervellione et al. (2017) and Fatka et al. (2024), the paired digestive glands in the cephalon (Dc0 to Dc3) and in the thorax (Dt1to Dt6), are together called the “perigastric organ” (Fig. 11).

A large part of the Skryje Shales belongs to the conocoryphid biofacies of Álvaro & Vizcaïno (2003), with episodic common reworked, rarely amalgamated trilobite and brachiopod shells reflecting numerous storm disruption events. Mud deposits with articulated skeletons were accumulated from suspension. The fossil association of the Skryje Shales includes only trilobites with well-developed eyes such as Ptychoparioides and diverse paradoxidids; blind trilobites characteristic of the conocoryphid biofacies, such as the widely distributed and abundant Conocoryphe or the locally common Ctenocephalus, are entirely absent. This composition of the skeletal fauna suggests deposition within the photic zone, and the absence of bioturbation indicates poor oxidation of the sea floor.

The sandy light green greywacke at the “Jestřábí” locality from which the studied articulated specimens of Ptychoparioides ‌henkli were obtained also contains disarticulated remains of other trilobites and rare articulated eocrinoid echinoderms. This unit does not exhibit current sorting. The studied specimens come from a monospecific accumulation that contained the remains of approximately 50 articulated and disarticulated exoskeletons of adults of Ptychoparioides ‌henkli. At least two different processes could have been responsible for producing this type of accumulation.

1) The accumulation represents a mass transportation event of living individuals. The killing and mass deposition of their bodies most probably resulted from a debris flow (see Cisne 1973; Brett et al. 2012; M. Eliáš, personal communication 2008).

2) The preserved accumulation originated from a large monospecific, size-segregated assemblage of fully articulated specimens that was rapidly buried in situ. This type of accumulation was named a body cluster by Speyer & Brett (1985).

Due to apparent monospecific aggregation of size-segregated and fully articulated exoskeletons, we interpret the studied association as a body cluster of Speyer & Brett (1985).

All studied specimens of P. ‌henkli show in situ preserved librigenae, various stages of disarticulation of the thoracopygon, and partially preserved remains of the digestive system. The exoskeletons of at least three specimens (Figs 3-5) show a well-articulated thorax. In contrast, three other specimens (Figs 6; 8; 9) show evident disarticulation of the exoskeleton. The remaining two specimens (e.g. Fig. 7) are strongly damaged and it is not possible to determine the completeness of their exoskeletons.

Partially or completely preserved sets of cephalic and thoracic diverticula with remains of the gut are preserved in six specimens. Just diverticula, without remains of the gut, are seen in two specimens (CGS MV 27, Figs 5; 10A and CGS MV 28, Figs 6; 10C). The most complete remains of the digestive system are seen in specimen CGS MV 32, in which four pairs of cephalic and six pairs of thoracic diverticula are associated with gut remains and are seen in the occipital ring and in six anterior thoracic segments (Figs 3; 10D[Dc0-Dc3, Dt1-6 and G]). All ten pairs of diverticula are also preserved in the incomplete and strongly damaged specimen CGS MV 30 (Figs 8; 10E[Dc0-Dc3, Dt1-6]); however, no remains of the gut are detectable in this specimen. Only one cephalic diverticulum (Dc3l) is missing in specimen CGS MV 27 (Fig. 5; 10A). Complete sets of cephalic diverticula are preserved in three specimens: CGS MV 29a, CGS MV 29b (Figs 7; 10G) and CGS MV 31 (Figs 9; 10F). One pair of thoracic diverticula is preserved in the specimen CGS MV 29a (Figs 7; 10G) and six pairs are seen in CGS MV 26 (Figs 4; 10B).

In the cephalon, small parts of the gut are preserved in five specimens (CGS MV 26, Figs 4, 10B; CGS MV 30, Figs 8, 10E; CGS MV 31, Figs 9, 10F; CGS MV 29a and CGS MV 29b, Figs 7, 10G); gut remains in the anterior six thoracic segments are seen in one specimen (CGS MV 32, Figs 3, 10D). Gut remains in more posterior thoracic segments and in the pygidium are absent in all studied specimens.

The question of how remains of the digestive system could be preserved despite the substantial differences in the degree of exoskeletal disarticulation must be addressed. To answer this question, we compared our specimens with the results of the experimental taphonomic work of Butler et al. (2015) and Klompmaker et al. (2017, 2018).

Butler et al. (2015) concluded that endogenous gut bacteria are the main factor controlling decay in the brine shrimp Artemia ‌salina. They showed that the carcass of this shrimp was consumed rapidly by endogenous bacteria after rupture of the gut wall and that the developed biofilm mediated authigenic mineralization of soft tissues, including the gut and body cavity. In their experimental conditions, the wall of the gut failed at the mid- to hindgut junction, allowing gut-derived microbes to spread into the body, where they proliferated (see Butler et al. 2015: 7). This observation explains the preservation of cephalic and thoracic diverticula as well as the absence of the hindgut in specimens of Ptychoparioides.

Klompmaker et al. (2017) studied the decay of eight marine arthropods by means of experiments; in this work, we compare the preservation of Ptychoparioides with the degree of exoskeletal disarticulation and the completeness of the digestive system in stomatopods. In stomatopods, disintegration of the carapace commenced after approximately 50 days and the soft tissue had entirely decayed after about 100 days (Klompmaker et al. 2017: 779, figs 3c; 4). This comparison makes it possible to explain the observed exoskeletal disarticulation and partial preservation of the digestive system in Ptychoparioides as resulting from embedding earlier than 50 days after the death of the trilobites.

The occurrence of paired digestive glands implies slow digestion of ingested particles, which agrees with a hypothesised detritus-feeding habit (see Fatka & Budil 2018).