Ctena decussata IL, P, S

Mancosu, Andrea & Nebelsick, James H., 2020, Tracking biases in the regular echinoid fossil record: The case of Paracentrotus lividus in recent and fossil shallow-water, high-energy environments, Palaeontologia Electronica (a 42) 23 (2), pp. 1-35 : 10-12

Ctena decussata IL, P, S

Order Ostreida

Family Ostreidae

Ostrea sp. 20) Sciberras et al., 2009

21) Rufino et al., 2010

31, 34 22) Albano and Sabelli, 2011

2, 11, 17, 25, 31, 33, 34 23) Gutierrez-Mas, 2011 6, 10, 11, 20, 25, 38 24) Öztürk, 2011

25) Albano and Sabelli, 2012 31, 34 26) Župan et al., 2012

27) Belgacem et al., 2013

28) Sessa et al., 2013

29) Urra et al., 2013

30) Brunetti and Della Bella, 2014 31, 33 31) Öztürk et al., 2014

32) Pitacco et al., 2014

2, 3, 5, 31, 33, 34 33) Alf and Haszprunar, 2015

34) Poursanidis et al., 2016

35) Chimienti et al., 2017

36) Kersting et al., 2017

8, 17, 18, 26, 27, 31, 33, 34, 37) Morton and Puljas, 2017 40

38) Donnarumma et al., 2018

2, 13, 23 39) Giacobbe and Renda, 2018

40) Casoli et al., 2019

15, 22, 25

2, 8, 11, 17, 31, 33, 34

2, 27, 29, 31, 33, 34

2, 11, 27, 31, 33, 40

29

7, 21, 33

11, 21, 35, 38

35, 38, 39

are dominated by the presence of encrusting thalli and low protuberances.

Recent Paracentrotus lividus Remains

In Sa Mesa Longa Beach ( Figure 6A ) remains of Paracentrotus occur abundantly on and within the sediments, around rocky bottoms on which living specimens commonly occur ( Figure 6 B-H). Other echinoids are represented by very rare remains of the regular echinoid Arbacia lixula ( Figure 6I ) along with highly abundant tests of the minute clypeasteroid echinoid Echinocyamus pusillus ( Figure 6J ).

The remains of Paracentrotus from Sa Mesa Longa Beach occur in a broad spectrum of preservation ( Table 2 -Part 2 and 3; Figure 6 B-H) similar to that of the studied fossil counterparts. They mainly consist of isolated coronal elements, most of which belonging to ambulacral and interambulacral plates (see Table 2 -Part 2 and 3). Fragments consisting of several plates still sutured together are also frequent. Genital and ocular plates of the apical system were also collected, but are relatively rare. Additionally, complete specimens lack spines, the apical system and the Aristotle’s lantern. Fragmented remains representing large parts of the test are present, though rare. Intraplate fragmentation along with surface abrasion occur very commonly (see Table 2 -Part 2 and 3). Both complete denuded specimens and test fragments show encrustation by polychaete serpulids, including the genus Spirorbis and coralline algae ( Figure 6 B-E).

Numerous disarticulated elements of the Aristotleʼs lantern were recovered and include mostly hemipyramids and subordinately rotulae and teeth. Epiphyses and compasses also occur sporadically. Lantern elements are commonly affected by fragmentation and abrasion while encrustation is low and restricted to hemipyramids and teeth. Spines also abundantly occur. Their state of preservation is also similar to those in the fossil deposits ranging from pristine and complete, which were rarely found, to fragmentary and abraded. Some specimens show regenerated tips. Spine remains also show encrustation by coralline algae.

The very rarely encountered remains of Arbacia lixula consist exclusively of test fragments and isolated plates. The preservation of clypeasteroid Echinocyamus pusillus ranges from complete pristine specimens to fragmented and abraded ones.

Morphology, Test Thickness and Plate Sutures of Paracentrotus lividus

The test of Paracentrotus is moderately thick with thickness decreasing slightly from near the apex to the ambitus. The ambulacral plates are usually thicker than adjoining interambulacral plates at the ambitus. Both ambulacral and interambulacral plates are thicker at their margins. The extent of plate thickening is greater along the radial sutures, particularly along the perradial sutures between ambulacral plates, which form a radial ridge when viewed from inside the test. The peristomial margin is thick with the perignathic girdle forming a reinforcing palisade just inside the peristomial edge. Plates are interconnected along sutures by collagenous fibers, crossing plate boundaries. Galleried stereom (sensu Smith, 1980) occurs at the sutures between interambulacral plates ( Figure 7 ). This stereom, which possesses long parallel galleries running in one direction only and shows no pore alignment perpendicular to this direction, is functionally associated with long bundles of collagen fibers at the sutures (Smith, 1980).

In Paracentrotus , parallel galleries can be observed along interradial and adradial sutures between interambulacral plates (aboral to oral direction) ( Figure 7 A-B) whereas no pore alignment can be observed along latitudinal sutures in interambulacral plates ( Figure 7C ). In ambulacral plates, galleried stereom can be observed along the adoral sutures joining ambulacral and interambulacral plates, along the adradial sutures, as well as along the perradial suture joining ambulacral plates together ( Figure 7D ). Additionally, knob-like trabecular protrusions interlock with cavities formed by the opposing plate’s stereom thus interlocking the adjacent plates ( Figure 7C ). This occurs for both radial (longitudinal) sutures (adradial, interradial and perradial) and circumferential (latitudinal) sutures (adapical and adoral).

Figures

FIGURE 6. A, Panoramic view of Sa Mesa Longa Beach (Central-western Sardinia). B-H, Recent remains of Paracentrotus lividus from Sa Mesa Longa. B, Complete denuded test showing encrustation by serpulids. C, Test fragment made up of several ambulacral and interambulacral plates sutured together, showing inter- and intraplate fragmentation and encrustation by coralline algae and the polychaete Spirorbis. D, Internal view of a large test fragment affected by intraplate fragmentation and encrustation by serpulid worms. E, Two interambulacral plates showing abrasion and encrustation by Spirorbis. F, Single ambulacral plates showing fragmentation (white arrow). G, Epiphysis from the jaw apparatus showing fragmentation and abrasion. H, Madreporite from the apical system affected by fragmentation (white arrow). I, Test fragment of Arbacia lixula showing intraplate fracturing. J, Complete test of Echinocyamus pusillus. B, C, D, I, J Scale bar equals 1 cm. E–H scale bars equal 0.5 cm.

FIGURE 7. Scanning electron micrographs of suture faces in recent Paracentrotus lividus. A, Interradial suture between interambulacral plates showing galleried stereom with galleries running in aboral-oral direction. A1, Detail of the galleried stereom and collagen fibers. B, Adradial suture of interambulacral plates showing galleried stereom. B1, Detail of the parallel galleries and collagen fibers. C, Adapical suture of an interambulacral plates showing knob-like trabecular protrusions and cavities. C1, The close-up shows numerous knob-like protrusions some of which are interconnected to one another in twos, threes or more. D, Adoral suture of ambulacral plates; 1) Radial ridge at the boundary between ambulacral plates (running across perradial sutures) and 2) galleried stereom. A, B, C Scale bars equal 100 µm; A1, B1, C1 Scale bars equal 20 µm. D Scale bar equals 1 mm.