trilobite eyes on hypostoma

[trilobite tim]

Hello,

Anyone have any info on trilobites with eyes located on the hypostoma.  I have an old textbook (1935!) that states "A few trilobites appear to have lacked eyes, and in some two small compound eyes are present on the ventral hypostoma."  No other info is provided!!!!!  I would like to read more about this.

 

Thanks, Tim

[Tidgy's Dad]

Maybe..........

https://books.google.co.ma/books?id=LalmQ4346O0C&pg=PA60&lpg=PA60&dq=hypostome+eyes&source=bl&ots=32YE3OygNv&sig=NPVc8eLOhxKAa5RYDrKRFRwP4Xs&hl=en&sa=X&ved=0ahUKEwiSvIuC17XaAhWI7RQKHRSiA0gQ6AEIXDAI#v=onepage&q=hypostome eyes&f=false

And go back to the 1st mention of eyes and hypostome. (this is the third). 

[piranha]

from the Treatise on Invertebrate Paleontology, Trilobita Part O 1959:

 

MACULAE

Maculae are absent in the hypostomata of many trilobites, whereas in others they consist of a pair of smooth rounded or elongate protuberances visible on the lateral or posterolateral areas of the median body (Fig. 68 ).  In a few species the outer surface of the maculae is reticulate or bears closely set, regularly arranged tubercles resembling the lenses of the dorsal eyes.  Usually only a small portion of the macular surface is reticulate or faceted, the remainder being smooth. Usually only a small portion of the macular surface is reticulate or faceted, the remainder being smooth.  The significance of the maculae and of their reticulate or faceted surface, is still a matter of subjective interpretation.  LINDSTRÖM (1901), HANSTRØM (1926), and HUPÉ (1953) regard the maculae as true ventral eyes, whereas other paleontologists such as RAYMOND (1920), STØRMER (1949), and WHITTINGTON & EVITT (1953) prefer to regard them as places of muscle attachment. 

 

The probability that the maculae represent places of muscle attachment seems rather small.  This is indicated by several features: (1) the mineralized integument of the maculae is much thinner than that of the remainder of the hypostoma, as pointed out by LINDSTRÖM and amply verified by WHITTINGTON & EVITT (1953) on silicified material: (2) the places of muscle attachment in the cephala of trilobites lacking apodemes invariably have a smooth surface, differing markedly in this respect from maculae with a reticulate or faceted surface; and (3) in some species of trilobites having the hypostoma firmly welded to the rostral plate (as Holmia kjerulfi, Redlichia noetlingi, Paradoxides davidis, Fieldaspis furcata) smooth maculae are well developed.  In the last-named trilobites it is evident that, if the maculae represent places of muscle attachment, the muscles would function as for movements of the hypostoma alone. They may have been expansor muscles of the stomach, attached to the ventral wall of the subglabellar proventriculum, but it seems strange that such powerful muscles (judging by the size of the maculae) should have been needed to produce slight expansion of the soft-tissued stomach.

 

The available evidence seems to favor the view that the maculae had a visual function.  This is especially true for the reticulate and faceted maculae present in such genera as Scutellum, Lichas, and Illaenus.  The smooth maculae seem best regarded as "degenerate" ventral eyes that have lost their lenses.  Sections normal to the surface of the maculae show that in those of "reticulate" type the macula consists of numerous prismatic bodies very similar to the prismatic lenses of the dorsal holochroal eyes.  According to LINDSTRÖM this is true of the maculae observed in several asaphids and in some species of Illaenus.  The "faceted" maculae show an aggregate of "globular" lenses resembling biconvex lenses of the dorsal eyes. These are especially well developed in some species of Scutellum and Lichas (Fig. 68B-F).  The presence of ventral eyes in some trilobites would certainly not constitute a unique feature among the Arthropoda, as similar organs are known among the Chelicerata and Myriapoda (HANSTRØM, 1926).

 

[piranha]

Seilacher had an interesting alternative suggestion:

 

Seilacher, A., & Gishlick, A.D. (2015)

Morphodynamics.

CRC Press, 514 pp.

 

HEARING?

Maculae are hill-like elevations that resemble the eye hills by having a gentle slope on the rear side and a steep face in front.  Seen from below, the two maculae look like the streamlined headlights of a sports car.  As these structures caused extra costs in molting, they must have had an important function.  A clue is the front of a macula.  In contrast to a visual surface, it is slightly concave, like a satellite TV antenna, and is perforated by many pores, through which some kind of sensory cells reached the surface of the exoskeleton.  Being fixed at a given distance (at least during molting stage), these organs allowed the trilobite to locate the source of signals within the sediment, whether the smell of buried food or the vibrations caused by the burrowing activities of a neighbor.  In any case, the maculae allowed the trilobite to stereoscopically “see” its environment within the sediment.  It would be interesting to know what other infaunal creatures developed this sensory ability.  Trilobites belonging to different clades had maculae.  At the time when these creatures were thought to have swum belly-up (as the larvae of Triops and Limulus do), maculae were considered as a ventral visual organ (ocelli).  An extensive study by Gustaf Lindström showed that the morphological details, however, did not fit this interpretation as maculae had no functional lenses.  He therefore considered them as rudimentary organs that had a visual function earlier in ontogeny.  Alternative functions should be considered, based on the assumption that most adult trilobites had a benthic lifestyle in which the hypostome faced or even touched the sediment.  In general, maculae share the following features: 1. They are round in outline.  2. They are flat or slightly concave, rather than convex, even if placed on elevations.  3. They are smooth, with terrace lines contouring them like frames.  4. They correspond to depressions on the inside of the hypostome, making them considerably thinner than surrounding parts of the mineralized cuticle.

These features are characteristic of drums that may have different functions in different animals.  In the ears of vertebrates and insects, drums function as receptors for sounds that are taken up and analyzed inside by sensory cells.  Elaborate transfer mechanisms enhance this process.  Paired and widely distanced arrangement of the ears (with a stiff, but non-conducting connection of the receptors) enables stereoscopic hearing and thereby recognition of the direction from which the sound is coming.  All these functions make use of the ability of a membrane to vibrate; but only the last model appears feasible for trilobite maculae.  The ear hypothesis is supported by the behavior of the maculae when they become elevated above (or rather below) the surface of the hypostome.  As in the eye hills on top of the cephalon, the receptor is tilted in the direction of the most relevant signals.  In the maculae this is frontward rather than sideways.  As a more important difference to the eyes, however, the sensory surface is not convex, but remains flat.  A strong argument for a sensory function of the maculae is that they are tilted in a forward direction in many species (arrows).  The same is true for drum structures on the dorsal side of the fixed cheeks (lunettes).  They are placed between eyes and midline and tilted into a horizontal position.  An important argument against the ear hypothesis is that in trilobites this drum was mineralized.  As their whole carapace derives from a horny cuticle, why would it be made stiff at a place where protection was no issue?  We have no answer to this and can only mention that the microstructure of the maculae differs from the surrounding carapace.  This is expressed by the commonly whitish color and could mean that the ability to vibrate was increased by this structure.  Because of their ability to vibrate, drums can produce sounds as well as receive them.  But why should they be placed on the ventral side of the body?  Here we must remember that sound may actually spread better and faster within the sediment, than in open water.  On land, some snakes use this phenomenon by listening through the soil.  Why shouldn’t trilobites have done the same for early warning, as well as for communication?

[Malcolmt]

Wow ... learn something new everyday.....

[Raggedy Man]

41 minutes ago, Malcolmt said:

Wow ... learn something new everyday.....

I know right? Wow

[doushantuo]

LVE=left ventral eye in the right part of the pic below(Kwanyinaspis)

 

 

[Shamalama]

Would make some sense when you think about it. The main body has everything set for upwards viewing for defense as well as prey spotting. You'd want to be able to see what's beneath you as well if you did any swimming above the seafloor for the same reasons.

[ynot]

Could they have been electric field or infrared detection organs for prey detection?

 

Thanks for bringing this strange adaptation to light.

 

[trilobite tim]

Thanks to all for the info provided.  It was a great help.  Next trip to the House Range I will make a major effort to collect more hypostoma.  That will call for a decent magnifying glass and lots of looking.  Just what I like!  I have two or three so far, from past trips.

 

Tim