trilobite exoskeletons

[Walt]

The great looking trilobite @Greg.Wood prepped and posted has raised questions for me.  

Where did the hard, calcitic, exoskeleton end and the soft under body begin?  How was this transition accomplished with a hard mineral?

In the below picture of Greg's bug, you will see the segments appear quite flexible. 

I realize there has been some compression when it was buried, but if the exoskeleton is as rigid as presented in the literature, shouldn't it have broken off rather than bent?  The circled area on the left shows a "rind" on the segment.  Is this part harder than the rest of the segment? (circled area on the right)

Hope someone can point me to some literature.  Thanks! 

@piranha @Kane

[Wendell Ricketts]

A good place to start in this inquiry is perhaps simply with living marine arthropods such as lobsters, crabs, shrimp, ispods, and on and on. It seems to me that all of your questions can be answered by observing their anatomy. There’s also a great deal of basic info here: https://www.trilobites.info.

[piranha]

The circled (?) is called an articulating facet.  Although the parts are rigid, the trilobite exoskeleton is comprised of many articulating devices that enabled enrollment, exuviation, flexure, movement.

[piranha]

48 minutes ago, Walt said:

...Where did the hard, calcitic, exoskeleton end and the soft under body begin?  How was this transition accomplished with a hard mineral?

 

 

All trilobites have points of muscle attachment (apodemes) on the ventral exoskeleton:

 

from Treatise (O) 1997:

 

Apodeme. Process presumably for attachment of muscles or ligaments, formed by inbending or thickening of dorsal exoskeleton on internal side of exoskeleton (syn., appendifer).

 

The form of muscle scars and of invaginations of the exoskeleton (furrows, pits, and apodemes) in and adjacent to the axial region of the exoskeleton (Fig. 8, 10, 20, 26, 51) has been revealed more clearly by silicified specimens (Fig. 26.1, 38–39). These attachment areas, symmetrically arranged in the axial region, rarely median, were for muscles extrinsic to the limbs and other organs of the body and for muscles and ligaments that extended the body and maintained its form.

[Walt]

16 minutes ago, piranha said:

All trilobites have points of muscle attachment (apodemes) on the ventral exoskeleton:

Thanks!  That led me to a paper in your open access forum that I believe will help me quite a bit.  

Walt

 

Ortega-Hernández, J., & Brena, C. (2012)
Ancestral patterning of tergite formation in a centipede suggests derived mode of trunk segmentation in trilobites.    
Plos One, 7(12):1-19 

[doushantuo]

wilmobiomechatrilobvol33_part4_pp749-768.pdf

N.V.Wilmot:

Biomechanics of trilobite exoskeletons/Palaeontology,v.33-pt 4/1990

1,46 Mb

 

leroseyenrollmeturkpaleoyer-18-3-5-0801-5.pdf

R.Lerosey-Aubril and Lucia Angolini:

Permian trilobites from Antalaya Province,Turkey,and enrollment in Late Paleozoic trilobites/Turkish journal of Earth Science 18/2009,p 427-448

about 0,5 Mb

 

I believe the enrollment of Placoparia is pretty well studied,as such things go

[Walt]

Thanks, all!