Moroccan Trilobite Scabriscutellum developmentally abnormal pygidium

[Biotalker]

Something caught my eye at the cheap table of trilobites at a Moroccan vender at a past fossil show. There were several Scabriscutellum of mediocre quality but I bought two. You can see them here. I am pretty certain these fossils are legit. One seems to have to have a clear cleft in the pygidium. There is even a small overlap of one pygidial "lobe" over the other. This is the first time I have seen such a "dramatic" mutation. I would guess that it is far more likely to be a developmental issue rather then a genetic mutation. The cleft trilobite obviously survived to its current size just fine (both approx. 4cm or 1.5 inches long). What do you think? Has anyone seen similar mutations within a species?

[Sjfriend]

Definitely looks like the trilobite grew well past damage. Can't tell if injured or defect ??

[Mediospirifer]

Oh, that's cool!

 

I have a number of brachiopods that look like that. The suspected cause (according to a paper I was shown) is a failed predation attempt. Something (possibly a cephalopod, for my brachiopods) took a bite out of the critter, the victim survived and healed. What you see is the scar.

 

It looks like there's another scar on the other side of the pygidium. Is the edge of the pygidium on that side complete (and distorted), or visibly broken? 

 

 

 

[Nanosaurus]

That is an amazing pathology!!

[Fossildude19]

Good eye - well spotted.  

[FossilDAWG]

That's remarkable!  I have never seen such an extreme example of a survived injury.  Is the irregular left side of the pygidium due to prep or preservation, or might that also be original/injury related?

 

Don

[piranha]

Interpretation of the origin of anomalies seen on trilobite exoskeletons begins to receive continuous attention from the scientific forum (see Owen 1985), simply because they are not only attractive curios but indirectly provide evidence of the structure of their body, location of internal organs and of palaeoecological conditions in which inflicted animals lived.

 

Three basic categories of anomalies can be distinguished: pathologic (deformations and neoplasms of a parasitic or deceased origin etc.), teratologic (genetic, ontogenetic, mutagenic malformations etc.) and injured specimens (healed moulting damages, healed injuries inflicted by durophagous predators etc.). However, as stated by Owen (1985), it is often very difficult to draw exact boundaries between them because what has remained for study are the more-or-less favourably preserved, sometimes incomplete tagmata in which the origin of anomaly is obscured by manyfold moulting and hence difficult to study.

 

text from:

 

Šnajdr, M. 1990

Five Extremely Malformed Scutelluid Pygidia (Styginidae, Trilobita).

Věstník Ústředního Ústavu Geologického, 65(2):115-118

 

 

Babcock, L.E. 2007
Role of Malformations in Elucidating Trilobite Paleobiology: A Historical Synthesis.
In: Fabulous Fossils: 300 Years of Worldwide Research on Trilobites.
New York State Museum, Bulletin, 507:3-19  PDF LINK

 

Owen, A.W. 1985

Trilobite Abnormalities.

Transactions of the Royal Society of Edinburgh: Earth Sciences, 76(2-3):255-272

[Biotalker]

Most agree it is a healed injury of some sort. Here is a better view of the left side of the pygidium showing clearly rounded (healed) sides as well. I don't see evidence of a doublure in this species, which would help assess the nature of the pygidial margins. Thanks for solid responses.

[Biotalker]

One other point to consider is this: compare in the first picture and see that the "nodule" that forms the base of the pygidium is clearly skewed to the left to favor the left pygidial lobe in the mutant but it is centered in the wild type trilobite. I guess in retrospect that is why I favored a development explanation. But if it was an injury, some process would have to cause a realignment of the nodule, rather than just consolidating uninjured tissue. 

[Biotalker]

57 minutes ago, piranha said:

Interpretation of the origin of anomalies seen on trilobite exoskeletons begins to receive continuous attention from the scientific forum (see Owen 1985), simply because they are not only attractive curios but indirectly provide evidence of the structure of their body, location of internal organs and of palaeoecological conditions in which inflicted animals lived.

 

Three basic categories of anomalies can be distinguished: pathologic (deformations and neoplasms of a parasitic or deceased origin etc.), teratologic (genetic, ontogenetic, mutagenic malformations etc.) and injured specimens (healed moulting damages, healed injuries inflicted by durophagous predators etc.). However, as stated by Owen (1985), it is often very difficult to draw exact boundaries between them because what has remained for study are the more-or-less favourably preserved, sometimes incomplete tagmata in which the origin of anomaly is obscured by manyfold moulting and hence difficult to study.

 

text from:

 

Šnajdr, M. 1990

Five Extremely Malformed Scutelluid Pygidia (Styginidae, Trilobita).

Věstník Ústředního Ústavu Geologického, 65(2):115-118

 

 

Babcock, L.E. 2007
Role of Malformations in Elucidating Trilobite Paleobiology: A Historical Synthesis.
In: Fabulous Fossils: 300 Years of Worldwide Research on Trilobites.
New York State Museum, Bulletin, 507:3-19  PDF LINK

 

Owen, A.W. 1985

Trilobite Abnormalities.

Transactions of the Royal Society of Edinburgh: Earth Sciences, 76(2-3):255-272

Thanks for the biting insight, Piranha