* Phacops are a type of trilobite, an extinct marine creature. They are generally found in rocks from the Silurian and Devonian periods, spanning roughly 443 to 359 million years ago.

* Tropites are a type of ammonite, another extinct marine creature. They existed from the Triassic period (about 252 to 201 million years ago) to the Early Cretaceous period (about 145 million years ago).

The Problem:

* The Phacops fossils indicate a rock layer from the Silurian or Devonian period (443 to 359 million years ago).

* The Tropites fossils indicate a much younger rock layer from the Triassic or Early Cretaceous (252 to 145 million years ago).

Possible Explanations:

1. Misidentification or Contamination: There's a possibility that one or both of the fossils have been misidentified or that the rock layer has been contaminated with fossils from a different age.

2. Reworking: Sometimes, older fossils can be eroded from their original location and deposited in younger rock layers. This process is called reworking.

3. Faulting and Displacement: If there are geological faults in the area, rock layers can be shifted and mixed, leading to fossils from different ages appearing in the same layer.

The Plant Remains:

It's impossible to determine the age of the plant remains based on the information given. To determine its age, scientists would need to analyze the plant fossils themselves using methods like radiometric dating.

Need More Information:

To accurately assess the situation, we need more information, including:

* Location of the discovery: Knowing the geological context is crucial.

* Detailed descriptions of the fossils: Are there any other fossils present?

* The type of rock layer: What kind of rock is it (sedimentary, metamorphic, etc.)?

Let me know if you can provide more details, and we can try to narrow down the possibilities!