enter image description here i found this amongst some river rock. Please explain why or what happened to it.

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    $\begingroup$ Please read this guide and edit in as much information as you can, especially where you found it geographically. Without more information, there's no way to distinguish what we're seeing from construction adhesive. $\endgroup$ – Spencer Nov 3 '18 at 12:15

These are fossil burrows... Looks like Thalassinoides or something similar. They are made by fishes, sea anemones, and crustaceans and can occur in different shapes... elongated, rounded, ending in t-sections, straight, spiral-shaped. A scalebar would be helpful for further classification.

One version of classical Thalassinoides

Take a look at the Wikipedia page about it

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