This is Mazon Monday post #65. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Another shrimp is up for this week's Mazon Monday. We have Paleocaris typus. P. typus was described way back in 1865 by Meek and Worthen. Fielding Bradford Meek (1817 - 1876) was an American geologist and paleontologist, who specialized in invertebrates. He had many contributions to science, including Paleaontology of California. You can find his papers at the Smithsonian Institution Archives. Amos Henry Worthen (1813 - 1888) was an American paleontologist from Illinois. He was the first curator of the Illinois State Museum.
P. typus was a syncarid shrimp. It is often mistaken for Acanthotelson stimpsonii, also a syncarid shrimp, which we spotlighted back in Mazon Monday #52. They are both common in the Braidwood Fauna. The major distinguishing characteristic between the two, are their tails. P. typus has a proper telson, while A. stimpsonii has a spiky tail.
This shrimp appeared in "ESCONI Keys to Mazon Creek Animals" from 1989, "Richardson's Guide to the Fossil Fauna of Mazon Creek", and "The Mazon Creek Fossil Fauna". "Creature Corner" even provided comparisons between the various species of syncarid shrimp found in the Mazon Creek biota. Excerpts are shown below.
"Creature Corner" on syncarids.
Super Order Syncarid Shrimp
Malacostracan (soft-shelled) crustaceans of the super order Syncarida are found in Mazon Creek concretions. These slender-bodied, short, perhaps one and a half inch long shrimp were fresh to brackish water inhabitants. This fact has made the concretions containing Palaeocaris typus or Acanthotelson stimpsoni, which constitute 98% of the fresh water malacostracans found, invaluable in plotting the ancient deltaic system.
Ferns, plant fragments, insects, etc. could have blown or floated out into the Pennsylvanian marine waters. Not so, with the Syncarid shrimp.
Concretions containing Syncarid shrimp are being used to plot riverbeds, lagoons, bays and coastal swamps. The paleogeographer is reconstructing this long-buried tropical coastline, near the Equator at that time, with the aid of these concretions.
From "ESCONI Keys to Mazon Creek Animals"... with excellent drawings by Don Auler.
P. typus was first described in 1865 from specimens found along Mazon Creek. This shrimp and A. stimpsoni are common in the Braidwood Fauna, but uncommon in the Essex. It is of moderate size. Cephalon is short and smooth with short rostrum, a prominent eye notch, and stalked eyes. It has long antennae. The first thoracic segment is quite short and the sixth is larger than others.. Last five body segments are progressively smaller giving the abdomen a tapered look. The telson is oval; uropods are flap-like, oval, hairy, and distinctly longer than telson. The first thoracopod is short; the others long and adapted for walking. Last five appendages (pleopods) have the appearance of long ringed bristles. These appendages are seldom found well preserved.
This filter feeder is one of the smallest shrimp in the Essex Fauna, hence is often overlooked. Body preservation is good, leg preservation is not, due to fragility and small size. Oval carapace, half the length of the animal, is squarish in pro file, bearing prominent optic notch. The antennae are short. The first abdominal segment is relatively large and bulbous, the following segments are flexed down in a sinuous "S" curve, hence its generic name. This odd configuration is an aid in field identification. Elements of the tail-fan were sharply tapered. It lacked pleopods.
NOTE: A. stimpsoni and P. typus are the two most common shrimp found in the Braidwood Fauna.
From "Richardson's Guide to the Fossil Fauna of Mazon Creek", chapter 12 "Crustaceans by Fredrich R. Schram, W. D. Ian Rolfe, and Andrew A. Hay
Palaeocaris typus Meek and Worthen, 1865 Figures 12.10, 12.11
Mandibles massive, head-to-thorax ratio 1:4, antennal scale longer than the two peduncular segments of the antennal flagellum or whiplike branch; first thoracic segment greatly reduced, sixth thoracic segment longer dorsally than any other segment, exopod branches of the thoracic limbs flaplike; abdominal limbs are multisegmented; uropods very setose (hairy), with a two-segmented outer branch or exopod, exopod longer than the endopod, which in turn is longer than the telson, outer margin of exopod with slightly developed and widely placed spines; the telson is ovoid but wider at its base than near its tip.
This is the second most common crustacean from the classic Braidwood localities, although rare and poorly preserved in the Essex fauna.
The reconstruction of Brooks (1962) had the abdominal limbs as flaplike appendages; however, these are now known to be rather delicate and multisegmented.
From "The Mazon Creek Fossil Fauna", by Jack Wittry
Palaeocaris typus, another syncarid, was first described in 1865 from specimens found in concretions along the Mazon River. The cephalon is short and smooth with a short rostrum, a prominent eye notch, and stalked eyes. It has long antennae. The first thoracic segment is quite short and the sixth is larger than the others. The last five body segments are progressively smaller, giving the pleon (abdomen) a tapered look. The telson is softly triangular and elongate. The uropods are flap-like, oval, hairy, and distinctly longer than the telson. The first thoracopod is short and the others long and adapted for walking. The last five appendages (pleopods) have the appearance of long, ringed bristles. These appendages are seldom found well-preserved.
Acanthotelson stimpsoni and P. typus are the two most common syncarid shrimp found in the Braidwood Fauna.
Specimens
From "The Mazon Creek Fossil Fauna"
