Scabriscutellum Tails

[Huntonia]

Ever since I first got my Scabriscutellum sp. I've felt like something was off with the pydium but could never place it. That is until today. I realised that the pydium on my trilobite was inverted as opposed to most pictures I've seen. By that I mean the lines along the pydium (sorry I don't know the technical term, if anyone wants to enlighten me that would be appreciated) which appear to be raised in most photos are depressed. Here's a picture of a "normal" scabriscutellum. (Hopefully you can tell what I'm on about.) 

As you can see raised lines along the pydium. However here's a picture of my scabriscutellum.

As you can see quite the opposite. I'm curious is this some sort of pathology, differing species or sign of fakery? Here's where it gets interesting, circled in red are a few of several discolored spots where the shell seems to be missing. When I first noticed these I thought perhaps plastic, that the ends of the spines had broken off and replaced with painted plastic. However a hot needle test proves these spots are not plastic. Also interesting, there are what look like air bubbles in the matrix which I am 100% positive is real rock and not resin. (Result of volcanic activity maybe?) Today though I also found pictures of another scabriscutellum with the weird tail and it also has the discoloration. 

I'm honestly a bit confused and a lot curious any opinions are appreciated. 

[JohnBrewer]

@piranha

[piranha]

The pygidium is not inverted, these are two different species.  The difference is the width of the pygidial ribs and interpleural furrows. 

With regard to the color change, it is not an unusual occurrence to see multiple color gradients on underlying layers of the exoskeleton.

 

[Huntonia]

Good to know. I figured probably just a difference in species but the thing that made me skeptical is that all of the obviously fake scabriscutellum casts I've seen have the wider pygidial ribs. (Probably easier to cast). Any idea about the air bubbles? My best guess is at some point millions of years ago the matrix was exposed to extreme heat, I would assume from a nearby underwater volcano. Is there any documentation of something like that?

[FossilDAWG]

I don't think they are air bubbles.  They look like minor damage from a pin vise or something like that during prepping.  For one thing, they are very uniform in size whereas bubbles would be variable.

 

Don

[Huntonia]

I believe you are correct. I was curious because the back of the fossil has quite a few similar pock marks in varying sizes but I think that is simply because it's a somewhat porous rock. The marks around the trilobite are so uniform that they must be from the preparation. (The prep of this bug was pretty rough, the main reason it was in my budget to begin with!) Thanks for the help!