
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
Given an ellipse $C$: $\dfrac {x^{2}}{a^{2}}+ \dfrac {y^{2}}{b^{2}}=1(a > b > 0)$ with eccentricity $\dfrac {\sqrt {3}}{2}$, $F_{1}$ and $F_{2}$ are the left and right foci of the ellipse, respectively, and $M$ is any point on the ellipse other than the endpoints of the major axis, and the perimeter of $\triangle MF_{1}F_{2}$ is $4+2 \sqrt {3}$.  
$(1)$ Find the equation of ellipse $C$;  
$(2)$ A line $l$ passes through point $D(0,-2)$ and intersects ellipse $C$ at points $A$ and $B$, point $N$ satisfies $\overrightarrow{ON}= \overrightarrow{OA}+ \overrightarrow{OB}$ (where $O$ is the origin), find the maximum area of quadrilateral $OANB$ and the equation of line $l$ at this time.

Present the answer in LaTex format: \boxed{Your answer}
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