River Monsters in Mackay?

Soft muddy banks are like a canvas that records the animals of the intertidal forest.  Incredibly delicate feeding patterns from fiddler crabs and fin prints from mudskippers can be perfectly preserved.  And so can the prints of much larger and more mysterious creatures.

There is no guide to animal tracks in the mangroves, and perhaps it is time to make one, but first I need to figure out what creatures are making the marks.

In the brackish headwaters of a small river near Mackay, I noticed that the tidal riverbanks were perforated with triangular pits that were the size of a man’s fist.  There were no footprints associated with the pits so this rules out birds and mud crabs.  The pits were too delicate for crocodiles.  That leaves only fish, rays and turtles.  The mud is not dug out; rather a face has been pushed into the mud judging by the raised lip of displaced mud around the pit.  A mouthful of mud seems to be gulped in.  Plugs of mud with mouth prints could be found scattered around beside the holes.  Some were ribbed, suggesting the creature’s mouth had a ribbed roof or that the mud was pushed out past grooves in the creature’s mouth.  Other plugs were curved and smooth.  The plugs of mud were as big as a cast from a human mouth would be.  

Each pit is about 10 cm long and are similar to coffee cup in volume

A 'mouth' print in the discarded plug of mud.

A smooth plug, which might represent the other lower surface of the plug.

Rarely some very unusual fin prints were found near the triangular pits. The tracks were about half a metre wide.  I suspect that these are from a different creature.

Mysterious track - sample 1 (click to enlarge)

Mysterious track - sample 2

The stream was too narrow to turn the boat and there was a good sized crocodile slide just metres away, so I did not retrieve samples.  The place was heavily pitted.  Fiddler crabs were the only obvious source of food on the riverbank.  They were present in extraordinary density and could barely be bothered to hide when I approached. 

Crocodile slide with polished belly patch and scratches from a largish croc.

The density of the pits in this are is similar to the density of fiddler crabs

Since then, I have started to see the triangular pits elsewhere.  On the tidal flats, there are also signs that fiddler crabs are on the menu.  Holes of a slightly different shape are present along with long cylindrical plugs of mud.

Round holes with fang grooves and long plugs of mud 

Whatever makes these strange marks is likely to be very hard to observe.  It is difficult to sneak up on animals in shallow waters, and the turbidity of the water makes using underwater cameras a challenge.  If you know which creature makes these tracks, please leave a comment.