  In this paper, we analyse the causal aspects of evolving marginally trapped
surfaces in a D-dimensional spherically symmetric spacetime, sourced by perfect
fluid with a cosmological constant. The norm of the normal to the marginally
trapped tube is shown to be the product of lie derivatives of the expansion
parameter of future outgoing null rays along the incoming and outgoing null
directions. We obtain a closed form expression for this norm in terms of
principal density, pressure, areal radius and cosmological constant. For the
case of a homogeneous fluid distribution, we obtain a simple formula for
determining the causal nature of the evolving horizons. We obtain the causal
phase portraits and highlight the critical radius. We identify many solutions
where the causal signature of the marginally trapped tube or marginally
anti-trapped tube is always null despite having an evolving area. These
solutions don't comply with the standard inner and outer horizon classification
for degenerate horizons. we propose an alternate prescription for this
classification of these degenerate horizons.
